Yes you can use cheap D flip/flop logic circuits as nice one-shot pulse generators. This schematic shows how the popular CD4013 and the CD74HC74 can be used to generate pulses ranging from nanoseconds to seconds.
Click on Drawing Below to view PDF version of Schematic

D-FLIP/FLOP ONE SHOT CIRCUITS

Description:

This circuit allows audio monitoring of a remote location and so doubles as a room monitor or baby alarm. It can be run from a 12 Volt battery or mains power supply as shown below. Interconnect uses 3 wires, so multi core cable can be used; alarm or telephone cable is suitable just leave any extra wires free.

Notes

From the right hand side, T1 is the mains transformer. The primary matches your local electric supply, secondary rating 12V, 1 amp. The bridge rectifiers are rated 50V peak at 1 amp or can be made from four 1N4001 rectifiers. C3 smooths the supply and C4 decouples any high frequency noise. The audio amplifier is built around a single LM380. This IC will deliver up to 2 watts into an 8 ohm speaker. C1 provides extra supply decoupling from mains hum, whilst the zobel network formed by R2 and C5 prevent a wideband oscillation at 10MHz. (Authors note: I have once experienced the 10MHz wideband oscillation. This was on an LM380 amplifier without the zobel network; when oscillation occurs, the audio is distorted and current consumption is excessive.)
Input Circuit
The power supply and amplifier are connected by cable to a remote location (the room you want to monitor for activity). The input circuit is built around an electret mic insert and amplifier and is connected by three wires. Alarm or telephone cable can be used, if the cable has more than three cores, just leave the unused wires floating.
The electret mic is amplified by a two stage, direct coupled preamp comprising two BC549C transistors. Electret mics have two terminals, older types may have three terminals,see this page in the practical section for more detail.
Gain is controlled by the 10k preset, once set for a particular room environment listening volume is controlled at the remote location using the 10k potentiometer. The preamp power supply is decoupled by the 1k and 200u capacitor, the first stage is run at a low collector current to ensure a high signal to noise ratio. The second stage is an emitter follower which ensures a low output impedance for driving long cables. As the output impedance is low, hum and noise pickup are minimum and therefore screened cable is not required.

Room Monitor

The Circuit

The Wart Zapper uses a single CMOS 7555 oscillator (IC1), for dual purposes, as follows:

First, it pumps up a standard voltage tripler circuit, represented by the capacitor-diode network to the right of IC1 in the circuit diagram. This takes the voltage up to about 25V, if not a little more. The purpose of increasing the voltage is to overcome the resistance of the skin. According to the well known formula I=V/R, if V (voltage) is increased, while R (resistance -- in this case skin resistance) remains the same, I (current) increases proportionately.

Second, the oscillator switches power MOSFET TR1 at the required frequency, to pulse the raised voltage through the skin by means of two electrodes. One of these electrodes is positive (+25V -- called the dispersive electrode, and marked D. This may either be a metal grip held in the hand, or a metal plate applied to a large(ish) area of skin near a wart. The other electrode is negative (0V -- called the active electrode, and marked A). This is a sharp(ish) metal point which is used for direct contact with the wart. The 470k potentiometer VR1 is inserted into the dispersive electrode's lead to prevent the possibility of a brief electrical jolt at switch-on, or on first applying the active electrode to a wart.

After much experimentation, I settled on a 25V 21kHz square wave (the circuit will approach this to within about 10%), applied to a wart for five minutes. I found that pulses of a minimum 1mW power passing through the wart internally were required to achieve any effect, and that 3mW-6mW pulses were adequate (compare this with the approximately 2W required to illuminate a pocket torch)!

Current across the probes is limited by R3 to less than 3mA, to protect the circuit if these should be short-circuited. One needs also to factor in the conductivity of the flesh, which rarely falls below about 200k -- therefore little more than 100µA, or at most about 200µA, would course through the wart itself.

Zener diode ZD1, together with LED D1 and resistor R1, serve as a simple "battery low" indicator. LED D1 will normally glow dimly, and thismust be agreen LED -- it is chosen for its so-called forward voltage drop, which differs from that of other coloured LEDs. If this LED goes out, then the battery is flat, and needs to be replaced. C1 serves as a supply decoupling capacitor, and S1 as an on-off switch.

Construction

The Wart Zapper (see Fig.2) is built on a printed circuit board (PCB) measuring approximately 60mm x 44mm (2.5" x 1.8"). The prototype used a case measuring approximately 100mm x 60mm x 22mm (4" x 2.5" x 1") externally.

Begin by soldering the six solder pins to the PCB. Solder the four resistors, the six capacitors (observing the polarity of electrolytic C1), the Zener diode, the five remaining diodes (including LED D1), and power MOSFET TR1. Then solder the battery leads as shown. The positive lead is taken via switch S1. Be sure to connect the leads the right way round, since a mistake here could destroy the circuit.

Fix the PCB to the bottom of the case, perhaps with some epoxy glue. A hole is prepared in the case for LED D1, which may be wired directly to the PCB, depending on the layout of the case. The cathode (k) of D1 is identified with a "flat" on the side of its encapsulation. Mount on-off switch S1 on the case.

Attach a long, plastic sheathed wire to the dispersive electrode (a metal grip or metal plate), and pass this wire through a hole in the case. Make sure that there is sound electrical contact between the wire and the metal grip or plate. Take the free end of this wire to 470k potentiometer VR1, and wire the potentiometer to the PCB as shown. If the potentiometer is viewed from underneath with the terminal pins facing towards you, the two terminal pins on the right need to be wired to each other.

Then attach a long, plastic insulated wire to the active electrode (a sharp pin -- but not too sharp -- the end may be filed flat), and pass this wire through a hole in the case, soldering it also to the PCB as shown. The pin should be inserted in a suitable plastic shaft so that it is not directly touched when treating a wart. Finally, insert and solder IC1 on the PCB, observing anti-static precautions (touch your body to ground before handling, e.g. to a metal tap).

In Use

Removing warts has never been much fun, and the use of the Wart Zapper is likely to be painful -- but only briefly, and not too much (as hinted at in the constructor's letter above).

Considerable experimentation preceded the development of this circuit, and, as mentioned, the results gave me a new respect for the potential risks of electricity, however small the voltages and currents that are applied. Skin resistance can vary between about 100k and 10M, depending on the day and the situation. Therefore, to ensure consistency of results, skin resistance needs to be kept relatively low. Use a little skin moisturiser where the skin makes contact with the dispersive electrode, as well as a little moisturiser on the wart itself.

Constructors are advised not to use the circuit where current would flow across the head or the heart, and never during pregnancy, or where a person uses a pacemaker, or has any history of epilepsy. These are standard safety recommendations for TENS devices, which incidentally use some three times the peak power of the Wart Zapper.

If treating a wart e.g. on the lower or upper arm, hold a metal grip (the dispersive electrode) in the same hand. If it is not convenient to use a grip, rest the limb to be treated (e.g. a foot) on a metal plate instead, which is again connected as the dispersive electrode. The active electrode -- that is, the sharp(ish) metal point -- is rested directly and gently on the top of the wart. If treating a slightly larger wart (say more than 4mm at its widest point), it might be an idea to tackle one or the other side of it first, since the Wart Zapper is unlikely to kill it all at once.

Switch on, apply the Wart Zapper to a wart for up to five minutes (see above), then switch off. Potentiometer VR1 is used to turn up the power slowly to full after switching on -- however, for the brave, it may be turned up full immediately. Be prepared suddenly to experience perhaps half a minute of sharp pain. If you do not see this through until the pain subsides (which it will), the wart may not be destroyed.

Experience and Qualifications

Although most common warts were ultimately removed by the Wart Zapper, it was found that there were some differences in the effect that the device had.

In several cases, a wart was obliterated first time, never to return. These were usually small common warts about 2mm to 4mm at their widest point. However, with close constellations of warts (at first glance looking like a single wart), or with larger warts, the wart was sometimes destroyed in part, but needed follow-up treatments to destroy it all.

In most cases, little or no pain was experienced when the Wart Zapper was first applied, although one subject jumped when the device was first switched on, and another -- a dentist -- suggested a means of controlling the power at switch-on. This is taken care of in the present design with a potentiometer which the patient may slowly turn up once the so-called active electrode is resting on the wart. In most cases, however, this potentiometer would not be missed.

After a certain period of painlessness, which varied from about half a minute to three-and-a-half minutes, subjects suddenly felt a burning or even a "spine-chilling" pain, inside and under the wart. This pain only lasts about half a minute, then subsides. However, it is necessary for the removal of the wart, and needs to be "stuck out". When the pain has subsided (or after five minutes, whichever may come first), the probe is removed.

Be more careful with facial warts, since facial skin is delicate. Rather under-treat such a wart than over-treat it. You may always return to it again later.

Once a wart has been treated, it should immediately be apparent that it is "just not the same". In fact in many cases, the wart melted with a fizzle even before the treatment was over. The skin immediately surrounding the wart may be irritated for a few hours, and there may be a slight swelling close to the wart. Ultimately a scab may form. Don't ever remove a wart too soon, or break its surface, or even agitate it, since this could leave a deep wound, and there could be infection. If it is left alone, there should be no infection. If a treatment should have little or no effect, it would be sensible to consult a doctor.

While this circuit comes with no guarantees, it is no doubt a case nothing ventured, nothing gained! With the help of several willing "guinea-pigs", and further volunteers queuing up, I found that the Wart Zapper was entirely successful most of the time.

Theory and Practise

According to the original theory of Dr. John Crane, alien cells (such as viruses) begin to resonate when bombarded with a specific electrical frequency. Normal chemical processes at the cell boundary are thereby disrupted, or the cell ruptures, thus killing the cell. Healthy tissues are left almost entirely unscathed.

However, this is not the only theory in the running. By way of a process of elimination, I followed up further suggestions put to me by researcher Aubrey Scoon:

1. Electrolysis (a "flat" DC voltage). This also did significant damage to warts - however, it also did immediate, superficial damage to healthy tissues, and the experiment was not repeated. The conclusion is that electrolysis maycontribute to the destruction of warts, but it does not offer an adequate explanation for the Wart Remover's success.

2. Iontophoresis. This is the leaching of ions into a wart, which effectively kills the wart by poisoning. However, after experimenting with a variety of conductive electrodes, as well as graphite (all the electrodes were tried with success), this theory was safely ruled out.

3. The stimulation of immunomodulatory chemicals. The theory is that these chemicals, when stimulated by an electrical frequency, attack the wart and destroy it. However, this would be hard to explain in light of the spectacular destruction of some warts. In some cases, the Wart Eliminator appeared toexplode wart cells, and this could on occasion even be heard! Finally,

4. Frictional heating. Ionic agitation may raise the temperature within a wart, causing tissue coagulation. While I had no way of testing this theory, I thought it unlikely. Electrodesiccation typically raises the temperature within a wart above 47°C, and this requires a fewWatts of power. Since the Wart Remover pulses justone-thousandth as much power through a wart, this possibility would seem less probable.

Parts List

Qty    Part

1 Copper clad board 60mm x 44mm (2.5" x 1.8")

1 9V PP3 "matchbox" battery

1 Battery clip for battery - or suitable case with internal battery terminals

1 Panel mounting on-off switch

1 Suitable ABS plastic case approx. 100mm x 60mm x 22mm (4" x 2.5" x 1") external

1 1 metre (1 yard) plastic shielded wire for the electrodes

1 15 cm (6") long brass tube for the dispersive electrode

1 Needle sharp tip filed off - for the active electrode

1 8-pin dual-in-line (DIL) socket (not required for experienced constructors)

6 Solder pins

1 Etchant if a PCB needs to be etched

1 Solder

Semiconductors

1 6.8V Zener diode (¼-Watt is adequate)

1 Green LED (no other colour)

4 1N4148 signal diodes

1 IRF610 power "logic" MOSFET (alternatively IRF510, BUZ11, BUZ22)

1 7555 CMOS timer IC

Resistors

2 1k ¼-Watt carbon or metal film

1 47k ¼-Watt carbon or metal film

1 10k ¼-Watt crbon or metal film

1 470k or 500k potentiometer, carbon track or conductive plastic

1 Knob for potentiometer

Capacitors

1 680pF polyester or ceramic

2 100nF polyester or ceramic

2 220nF polyester or ceramic

1 100µF electrolytic 16V or higher

Wart Zapper

 

 

 

Schematic

Parts List

PCB

 

Download

Download this circuit in PDF

IR Remote Volume Controller

Description:

This circuit can be used to remotely monitor a loudspeaker, alarm, or audio source for presence of an audio waveform. It can also be directly connected across loudspeaker terminals used as a peak indicator.

Notes

If you need to monitor some audio signal at a location that is either soundproof are too far away to actually hear the signal then you can use a circuit similar to this one. A remote indication that the audio source is active is provided visually, here in the form of a LED. Referring to the above circuit, the 10k preset is a level control which should be adjusted so that when the source ( audio signal or alarm ) is producing the desired amount of noise, the LED lights. The input stage is an inverting x100 amplifier made with a 741 op-amp. The output of the 741 is capacitively coupled into a 10k load resistor, thus removing any dc component from the signal and the offset null voltage of the op-amp. As only the amplified audio signal appears across the 10k load resistor, this is half wave rectified by the 1N4148, filtered by the 10u capacitor, and this small dc voltage used to directly drive the 2N2222A transistor, lighting the LED. Any NPN transistor may be used in replacement of the 2N2222, and should work with any transistor having a current gain of over 100. Examples include, BC107/8/9, BC547/8, ZTX300, 2N3904 etc. The sensitivity of this circuit is high, and input signal level as low as 10mV RMS will light the LED. If the monitored audio signal level is much higher than this (i.e. 1000mV and higher) then the following simpler circuit may be used.

Operation is the same as the op-amp circuit. The input is now directly to a 10k preset resistor, the monitored audio signal being half-wave rectified and used to directly bias the transistor. As before any transistor with a forward dc current gain greater than 100 may be used.

Peak Monitor:
To use either circuit as a peak monitor adjust as follows. For example suppose the input signal is from an audio power amplifier. Audio voltages higher than 1 Volt may easily be generated at the loudspeaker output, so the second circuit is used and connected directly across the loudspeaker terminals. You adjust the volume of the audio power amplifier until the level is uncomfortable or to any desired level. You then adjust the preset so that the LED lights. You then return the volume control to a normal level. If this accidentally gets turned up the LED will light providing visual indication that the peak level has been reached.

Audio Indicator

Description

This is a simple Laser communication system. It can transmit and receive signal from any audio device.Communication distance is few meters. All components are not critical. Transistor 2N2222 may be on the coolrib. Laser diode is from laser pointer.

Croatian

Ovo je jednostavni komunikacijski sustav koji podatke prenosi laserskom zrakom. Na izlazu odasiljaca nalazi se laserska dioda koja signal prenosi do foto tranzistora u prijemniku. Lasersku diodu najbolje je uzeti iz laser pointera cija je cijena oko 20kn.

Tranzistori Q7 i Q6 cine pojacalo signala iz foto tranzistora. Na tranzistor Q6 potrebno je staviti mali hladnjak. Komponente nisu kriticne.

Laser Communication System

Description:

This circuit provides a delayed visual indication when a door bell switch is pressed. In addition, a DPDT switch can be moved from within the house which will light a lamp in the door bell switch. The lamp can illuminate the words "Please Wait" for anyone with walking difficulties.

Notes

The circuit uses standard 2 wire doorbell cable or loudspeaker wire. In parallel with the doorbell switch, S1, is a 1N4001 diode and a 12 volt 60mA bulb. The bulb is optional, it may be useful for anyone who is slow to answer the door, all you need to do is flick a switch inside the house, and the bulb will illuminate a label saying Please Wait inside the doorbell switch or close to it. The double pole double throw switch sends the doorbell supply to the lamp, the 22 ohm resistor is there to reduce current flow, should the doorbell switch, S1 be pressed while the lamp is on. The resistor needs to be rated 10 watts, the 0.5 Amp fuse protects against short circuits.

When S2 is in the up position (shown as brown contacts), this will illuminate the remote doorbell lamp. When down, (blue contacts) this is the normal position and will illuminate the lamp inside the house. Switch S1 will then charge the 47u capacitor and operate the transistor which lights the lamp. As a door bell switch is only pressed momentarily, then the charge on the capacitor decays slowly, resulting in the lamp being left on for several seconds. If a longer period is needed then the capacitor may be increased in value.

Doorbell for the Deaf

Description:

A very high quality intercom, which may also be used for room monitoring.

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Notes

This circuit consists of two identical intercom units. Each unit contains a power supply, microphone preamplifier, audio amplifier and a Push To Talk (PTT) relay circuit. Only 2 wires are required to connect the units together. Due to the low output impedance of the mic preamp, screened cable is not necessary and ordinary 2 core speaker cable, or bell wire may be used.

The schematic can be broken into 34 parts, power supply, mic preamp, audio amplifierand PTT circuit. The power supply is designed to be left on all the time, which is why no on / off switch is provided. A standard 12 V RMS secondary transformer of 12VA will power the unit. Fuses are provided at the primary input and also secondary, before the rectifier. The 1 A fuse needs to be a slow blow type as it has to handle the peak rectifier current as the power supply electrolytics charge from zero volts.

The microphone amplifier is a 2 transistor direct coupled amplifier. BC108B transistors will work equally well in place of the BC109C transistors. The microphone used is a 3 terminal electret condenser microphone insert. These are popular and require a small current to operate. The preamp is shown in my audio circuit section as well, but has a very high gain and low distortion. The last transistor is biased to around half the supply voltage; this provides the maximum overload margin for loud signals or loud voices. The gain may be adjusted with the 10k preset. Sensitivity is very high, and a ticking clock can easily be heard from the distantloudspeaker.

The amplifier is based on the popular National Semiconductor LM380. A 50 mV input is all that's required to deliver 2W RMS into an 8 ohm loudspeaker. The choice of loudspeaker determines overall sound quality. A small loudspeaker may not produce a lot of bass, I used an old 8 inch radio loudspeaker. The 4.7u capacitor at pin 1 of the LM380 helps filter out any mains hum on the power supply. This can be increased to a 10u capacitor for better power supply rejection ratio.

The push to talk (PTT) circuit is very simple. A SPDT relay is used to switch between mic preamplifier output or loudspeaker input. The normally closed contact is set so that each intercom unit is "listening". The non latching push button switch must be held to talk. The 100u capacitor across the relay has two functions. It prevents the relays back emf from destroying the semiconductors, and also delays the release of the relay. This delay is deliberate, and prevents any last word from being "chopped" off.

Setting Up and Testing:
This circuit does not include a "call" button. With this intercom pressing the Push to Talk button sends your voice to the opposite station, and vice versa. Setup is simple, set to volume to a comfortable level, and adjust the mic preset while speaking with "normal volume" from one meter away. You do not need to be in close contact with the microphone, it will pick up a conversation from anywhere in a room. If the units are a long way away, there is a tendency for the cable to pick up hum, or radio interference. There are various defenses against this. One way is to use a twisted pair cable, each successive turn cancels the interference from the turn before. Another method is to use a small capacitor of say 100n between the common terminal of each relay and ground. This shunts high frequency signals to earth. Another method is to use a low value resistor of about 1k. This will shunt interference and hum, but will shunt the speech signal as well. However as the output impedance of each mic preamp is low, and the speech signals are also low,this will have little effect on speech but reduce interference to an acceptable level.

IC Pinout:
The LM380 pinout viewed from above is shown below on the left. In the schematic, the LM380 has been represented as a triangle, the pins are shown on the right hand diagram. Pins marked "NC" have no connection and are not used.

   

PCB Layout:
Corey Rametta has kindly drafted a PCB layout for this project. First an oversized version to show component placement. Note the tracks on the bottom side, components on the top side.

Below is the actual size version shown track side.

High Quality Intercom

Circuit Notes

It's well known that many animals are particularly sensitive to high-frequency sounds that humans can't hear. Many commercial pest repellers based on this principle are available, most of them operating in the range of 30 to 50 kHz.
My aim was, however, to design a slightly different and somewhat more powerful audio frequency/ultrasonic sound generator that could be used to train dogs. Just imagine the possibilities - you could make your pet think twice before barking again in the middle of the night or even subdue hostile dogs (and I guess burglars would love that!).
From what I've read, dogs and other mammals of similar size behave much differently than insects. They tend to respond best to frequencies between 15 and 25 kHz and the older ones are less susceptible to higher tones. This means that an ordinary pest repeller won't work simply because dogs can't hear it. Therefore, I decided to construct a new circuit (based on the venerable 555, of course) with a variable pitch and a relatively loud 82 dB miniature piezo beeper.
The circuit is very simple and can be easily assembled in half an hour. Most of the components are not really critical, but you should keep in mind that other values will probably change the operating frequency. Potentiometer determines the pitch: higher resistance means lower frequency. Since different dogs react to different frequencies, you'll probably have to experiment a bit to get the most out of this tiny circuit. The circuit is shown below:

Despite the simplicity of the circuit, there is one little thing. The 10nF (.01) capacitor is critical as it, too, determines the frequency. Most ceramic caps are highly unstable and 20% tolerance is not unusual at all. Higher capacitance means lower frequency and vice-versa. For proper alignment and adjustment, an oscilloscope would be necessary. Since I don't have one, I used Winscope. Although it's limited to only 22 kHz, that's just enough to see how this circuit works.
There is no need to etch a PCB for this project, perf board will do. Test the circuit to see how it responds at different frequencies. A 4k7 potentiometer in conjunction with a 10nF (or slightly bigger) capacitor gives some 11 to 22kHz, which should do just fine. Install the circuit in a small plastic box and if you want to, you can add a LED pilot light. Power consumption is very small and a 9V battery should last a long time.
Possible further experimentation: I'm working on an amplified version of the whistle to get a louder beep. All attempts so far haven't been successful as high frequency performance tends to drop dramatically with the 555. Perhaps I could use a frequency doubler circuit - I just don't know and I've run out of ideas. One other slightly more advanced project could be a simple "anti-bark" device with a sound-triggered (clap) switch that sets off the ultrasonic buzzer as soon as your dog starts to bark.

A suitable piezo transducer for this project is available fromMaplin Electronics part code WF09K.

Ultrasonic Dog Whistle

Description

I've had a few requests for a quiz circuit, so here is a 4 input design which can easily be modified.

Notes

This design uses four IC's and has four input circuits and four independent outputs and a single master reset switch. The outputs here are LED's but may be modified to drive lamps or buzzers. Only one output LED can be lit at any time. The first person to press their input switch, A,B,C,D will light the corresponding output LED, disabling the other inputs.

The circuit uses all CMOS IC's part numbers shown on the diagram. The supply voltage may be anything between 3 and 15 volts. Alternatively, it may be built using equivalent TTL IC's and powered on 5 volts. The main component in this circuit is a bistable latch, here it is based on the dual 4013 D-type flip flop.

Circuit Operation

Pressing the reset switch will clear all flip flops and extinguish any lit LED's. Under this condition the Q outputs will all be low (logic 0) and NOT Q outputs will be high (logic 1). All four NOT Q outputs are fed to a 4 input AND gate, the 4082 whose output will also be high. The output of the 4082 is wired to one input of each 2 input AND gate (4081). Switch inputs A,B,C,D are all non latching push button switches, the first person to press their switch will cause the corresponding AND gate (4081) to go high and trigger the preset input of the 4013 D-type flip flop. This will latch and light the appropriate LED. Also the triggered flip flop will have its NOT Q output, set at low, this changes the 4082 output to low and prevents any further triggering of the other flip flops.Switch contact de-bouncing is not required as the first press will latch one of the bistables. Pressing the reset switch, restores the circuit to its former state. I would recommend using heavy duty push button switches, as in use they are likely to be under some stress. This circuit is available to download in the Circuit Maker section for anyone wishing to experiment further.

Quiz Circuit

Description:

A common problem with small torches is the short life-span both of the batteries and the bulb. The average incandescent torch, for instance, consumes around 2 Watts. The LED Torch in Fig. 1 consumes just 24 mW, giving it more than80 times longer service from 4 AA alkaline batteries (that is, up to one month's continuous service). Although the torchs light output is modest, it is nonetheless quite sufficient to illuminate a pathway for walking.
The LED Torch is based on a 7555 timer running in astable mode (do not use an ordinary 555). A white LED (Maplin order code NR73) produces 400 mcd light output, which, when focussed, can illuminate objects at 30 metres. Try Conrad Electronic for what appears to be a stronger white LED (order code 15 37 45-11).
A convex lens with short focal length is placed in front of the LED to focus the beam. If banding occurs at the beams perimeter, use another very short focal length lens directly in front of the LED to smooth the beam.
If a different supply voltage is preferred, the value of resistor R3 is modified as follows:
9V - 470 Ohm
12V - 560 Ohm

See my "Wind-up Torch" feature article in the October 2000 edition of Everyday Practical Electronics for a completely battery-free go-everywhere torch.

LED Torch

Notes

Repell those repugnant insects from your Garden this Summer with this insect repellent circuit. Designed by Graham Maynard the circuitry consists of a phase locked loop (CMOS 4047) wired as a 22KHz oscillator. The output is amplified by a pair of complimentary output transistors and drives a Motorola 3.25 inch Piezo. Current drain is around 120mA so an external power supply is recommended.

The piezo used was a standard 85mm square Motorola Horn, Maplin part number WF09K or WF55K. These are rated +/-3dB to 28kHz.

Insect Repellent Circuit

Description

Picured in Figure 1 is a miniature magnetic gun. When optimally tuned, it will propel a small slug about 1.5 metres high, or 2.5 metres horizontally.

Notes

IC1 is a 555 timer in astable mode, sending approx. 10 ms pulses to decade counter IC2. IC2 is continually reset through R3, until pin 15 is taken low through the "Fire" button. IC2 then sequences through outputs Q1 to Q7, to feed power transistors TR1 to TR4, which fire electromagnets L1 to L4 in rapid sequence.
Transformer T1 secondary is 18 volts 1 amp A.C. When rectified and smoothed, this provides 25.2 V D.C for electromagnets L1 to L4. Resistor R4 drops 12 V to obtain a supply voltage low enough for IC1 and IC2.
The electromagnets are wound on a 25 cm long, 3 mm dia. copper tube (available at hobby shops). Two "stops" may be cut from tin for each electromagnet, and 500 turns of approx. 30 swg. enamelled copper wire wound between them. The electromagnets should be wound on a base of reversed sellotape, so that one may slide them on the copper tube. The slug (or "bullet") is a 3 cm long piece of 2 mm dia. galvanized wire, which should slide loosely inside the copper tube.
Most crucial to the effectiveness of the gun are the setting of VR1 and the positions of electromagnets L1 to L4 on the copper tube (the values and measurements shown are merely a guide). Firstly, with L2 to L4 disconnected, VR1 should be tuned and L1 positioned for optimum effectiveness (place a wire inside the tube to feel how far the slug jumps with L1). Then L2 (now connected) should be positioned for optimum effectiveness (the slug will now exit the tube). Repeat with L3 and L4.
Electromagnets L2 to L4 were each found to substantially increase the range of the gun. In a forthcoming edition of EPE, the author will describe how readers may land a small projectile on Mars.

Magnetic Gun

Description:

This circuit takes an ordinary loudspeaker and allows it to be used in reverse, as a microphone.

Notes

This circuits allows you to use a cheap loudspeaker as a microphone. Sound waves reaching the speaker cone cause fluctuations in the voice coil. The voice coil moving in the speakers magnetic field will produce a small electrical signal . The circuit is designed to be used with an operating voltage between 6 and 12 volts dc. The first transistor operates in common base mode. This has the advantage of matching the low input impedance of the speaker to the common base stage, and secondly has a high voltage gain. The second stage is direct coupled and operates in emitter follower. Voltage gain is slightly less than unity, but output impedance is low, and will drive long cables.

Frequency Response

As speakers do not give respond well (as a microphone) to high frequency gain is rolled off by C1, the frequency plot against gain (more commonly known as Bode Plot is shown above). This roll-off also helps with RF noise immunity when driving long cables.

Harmonic Distortion

As the amplifier is biased to almost half supply allowing for maximum amplitude swings, its distortion is low, a fourier series to the 16th harmonic (fundamental 1kHz) calculated THD at just over 1.5%, see above. Speech quality is not as good compared to an ordinary or ECM microphone, but quite acceptable results can be obtained. Speaker cones with diameters of 1 inch to 3 inches may be used. Speaker impedance may be 4 ohm to 64 ohm. The 8.2 ohm resistor value may be changed to match the actual speakers own impedance.

Speaker Mic Circuit

Description

The simple conjuring trick in Figure 1 is intended to provide some enjoyment for the beginner in electronics or conjuring, and should take only an hour or two to build.

The trick works as follows: a wand (with a magnet mounted in one end) must pass in a 1-2-3 sequence over reed switches S4 to S6 before the bulb LP1 will light. If the wand passes over reed switches S1, S2, or S3, the 1-2-3 sequence will be reset (that is, cancelled). Or, if the bulb is already burning, the activation of reed switches S1, S2, or S3 will extinguish it.
All the reed switches - S1 to S6 - are glued just beneath the surface of a 10 cm² box (Figure 2). A general purpose adhesive is suggested, so that the reed switches may later be moved if necessary. The bulb, LP1, is mounted in the centre of the box. A small PP3 9V battery may be used. The prototype box was built using balsa wood.
The wand may be waved back and forth in various motions over the box, on condition that it finally passes in the correct 1-2-3 sequence over S4 to S6 (at which point LP1 will light). This should thoroughly confuse any onlooker and make it virtually impossible for another person to repeat the correct motions with the same wand. The wand may also be lifted just high enough over reed switches S1 to S3 so as not to trigger them.
A 7.2V filament bulb, LP1, was used - instead of, say, a LED - so as not to give the trick an "electronic" appearance. The operation of the circuit is fairly simple. Three AND logic gates of a 4081 CMOS IC are employed, with gates IC1a to IC1c beingconfigured as a standard cascaded latch circuit. S1 to S3 serve as reset switches. The output at pin 10 will only switch to logic high when reed switches S4 to S6 are closed in sequence. Power transistor TR1 amplifies the output current to light bulb LP1.
Instead of a wand, a small neobdymium (super-strength) magnet may be stuck to one finger, and one's finger used in place of the wand.
In "stand-by" mode (with the bulb extinguished) the circuit will use very little current. Therefore a switch is not included in the circuit (of course, one may be added). The box may be opened and the battery simply clipped on or off.

Magic Wand Conjuring Trick

Description

A simple device to indicate various levels of hot water in a tank.

Notes

Save fuel bills and the economy of the planet with this circuit. SW1 is a normally open press button switch which allows you to view the level of hot water in a hot water tank. When pressed the voltage difference at the junction of the thermistor and preset is compared to the fixed voltage on the op-amps non-inverting input. Depending on the heat of the water in the tank, the thermistors resistance will toggle the op-amp output to swing to almost full voltage supply and light the appropriate LED.

Construction

Masking tape was used to stick the bead thermistors to the tank. Wires were soldered and insulated at the thermistors ends. A plastic box was used to house the circuit. Battery life will probably be 4 to 5 years depending on how often you use the push switch, SW1.

Sensor Placement

Thermistors NTC1-4 should be spread evenly over the height of the tank. I placed NTC1 roughly 4 inches from the top of my tank and the others were spaced evenly across the height of the hot water tank. As hot water rises the lowest sensor indicates the fullest height of hot water and should be about 8 to 10 inches from the bottom of the tank.

Calibration

With a full tank of hot water adjust P1-4 so that all LED's are lit. As hot water rises, the sensor at the bottom of the tank will be the maximum level of hot water. "Hot" can be translated as 50C to 80C the presets P1-4 allow adjustment of this range.

Parts

I have used a quad version of the LM324 but any quad opamp can be used or even four single op-amps.
R2-R5 I used 330ohm resistors, but value is not critical. Lower values give brighter LED output.
NTC1-4 The thermistors maximum resistance must roughly equal the resistance of the fixed resistor and preset. As negative temparature coefficient (NTC) thermistors are used, then their resistance decreases for increases in temperature. I used a thermistor from theMaplin Catalogue. Cold resistance was around 300K, hot resistance 15k. Alternative thermistors may be used with different resistance ranges, but the presets P1 to P4 must also be changed as well.
R7-10 series resistance, only required if your thermistors resistance is several ohms at the hottest temperature.
P1 - P4 Chosen to match the resistance of the thermistor when cold.
R1 & R6. These resistors are equal and bias the op-amp inverting input to half the supply voltage. I used 100k.

Hot Water Level Indicator

Description:

A simple op-amp circuit that will trigger a relay when a preset temperature is reached. Please note that there is no hysteresis in this circuit, so that if the temperature changes rapidly, then the relay may switch rapidly.

Description

This circuit uses an ordinary NTC thermistor with a resistance of 47k at room temperature. A suitable part fromMaplin Electronics is FX42V. The circuit is set in balance by adjusting the the 47k potentiometer. Any change in temperature will alter the balance of the circuit, the output of the op-amp will change and energize the relay. Swapping the position of the thermistor and 47k resistor makes a cold or frost alarm.

Calibration

At room temperature (25 degrees Celsius) a 47k NTC thermistor resistance is approximately 47k. The non-inverting op-amp input will then be roughly half the supply voltage, adjusting the 47k pot should allow the relay to close or remain open. To calibrate the device, the thermistor ideally needs to be at the required operating temperature. If this is for example, a hot water tank, then the resistance will decrease, one way to do this is use a multimeter on the resistance scale, read the thermistors resistance and then set the preset so that the circuit triggers at this temperature.

Please note that if the temperature then falls, the relay will de-energize. If the environment temperatures changes rapidly, then the relay may chatter, as there is no hysteresis in this circuit.

Hysteresis, allows a small amount of "backlash" to be tolerated. With a circuit employing hysteresis, there will be no relay chatter and the circuit will trigger at a defined temperature and require a different temperature to return to the normal state. Hysteresis can be applied to the circuit using feedback, try a 1Meg resistor between op-amp output, pin 6 and the non-inverting input pin 2 to give the circuit hysteresis.

Without offset null adjustment, the output of the 741 IC will be around 2 Volts (quiescent) swinging to nearly full supply when triggered. The 4.7k and 1k resistor form a potential divder so that under quiescent conditions the transistor will be off. Quiescent or steady state means no signal, or in this case (when the temperature does not cause the output to swing to full voltage).

Temperature Monitor

The basic receiver is shown below. The antenna is a telescopic antenna that extends to two or three feet, the length is not critical. A high-value resistor (270k) is connected from the antenna to ground to control the Q and this value may be lowered if the circuit seems unstable but too low a value will destroy the sensitivity. The 10 mH and 1 mH ( not uH) chokes are molded types but most moderately high-Q inductors will work fine and the rest of the parts are run-of-the-mill and not particularly critical. The transistors are all general-purpose types. 

Note: This circuit is intended to be used with one of the lamp options and any or all of the other options. If no lamp is desired, add a 1 k resistor from the "pulses" output to 5 VDC.

Theory of operation:

The antenna, 10 pF capacitor, and the two inductors form a resonant tank at about 300 kHz, a good frequency for receiving energy from lightning. The two series inductors act as a matching network, feeding Q1 with a lower impedance version of the signal received by the antenna. The 270k resistor lowers the Q of the resonant tank to prevent oscillation. Q1 amplifies the 300 kHz bursts and applies the larger signal to the base of a PNP transistor that forms a monostable "flasher" circuit with the last NPN transistor. When the RF signal pulls the PNP base voltage below the voltage on the 10 uF capacitor (plus about 0.6 volts) the PNP turns on, turning on the NPN. Since the NPN is connected to the base of the PNP through the 82 k resistor, the PNP turns on even harder. This regenerative action causes the circuit to turn on quickly and fully, pulling the "pulses" line to nearly zero volts. The circuit stays on until the 10 uF capacitor discharges at which point a similar reverse regenerative action causes the circuit to quickly switch off. The capacitor then quickly charges through the 1k resistor (in one of the lamp circuit options) and the diode and is ready for another pulse.

The prototype is built into a phenolic box using point-to-point wiring. The power switch is a single pole, double throw type with a center-off position. The power supply is connected to the center terminal and the speaker is connected to one of the outer terminals. Both of the outer terminals are also connected to the other circuitry through a couple of silicon diodes, one from each terminal. One diode keeps the speaker from getting power in the 'speaker off' position and the other diode is simply there so that the circuitry sees the same voltage in both 'on' positions.  Alternately, a switch could be added in series with the speaker to turn it off. After one storm, you will add the switch if you don't include it at first!

The diodes can be seen in the close-up of the power switch. Alternately, a  ordinary single pole, single throw switch could be used with another switch in series with the brown wire to disconnect the speaker when the constant crackle becomes too annoying!

The basic circuit above is combined with any of the following circuits to complete the lightning detector. The prototype used the 5 volt lamp driver, meter circuit, and speaker circuit.

Lamp Circuits

The second circuit will drive higher current lamps, up to 500mA. Flashlight bulbs make a bright flash.

Meter Circuit

The meter sensitivity may be altered by changing the 5.1k resistor.

Speaker Circuit

No volume control is included but the sound level is not particularly loud.

Choose any or all of the above circuits and connect them across the indicated terminals on the basic receiver. An alarm, buzzer, or other load may be activated when the lightning activity exceeds a preset level using the following circuit:

A different op-amp or comparator may be used, as long as it is "ground sensing". Look for single-supply types for substitutes. The '339 requires a pull-up resistor on the output but op-amps will not require this part (the 1k resistor). The power supply may be the same 5 volt supply or a different voltage as long as the voltage is within the operating range of the op-amp or comparator. The VMOS transistor may be any type sufficient for driving the load. A VN10KM is a typical part for lower current loads. Add a power switch in series with the load, if desired.

Older Circuits

The following two circuits maybe improved by modifying the antenna/RF section to look like the new circuit above ( the 10mHy and 1 mHy chokes, the 10pF capacitor, and the 270k resistor).  This modification removes a resonance in the lower part of the broadcast band that might make the detector succeptible to interference. The resulting circuits will work great and may be adjusted for extreme sensitivity.

Egor! Come quick! A storm approaches!

Here is a LF receiver tuned to 300 kHz designed to detect the crackle of approaching lightning. A bright lamp flashes in synchrony with the lightning bolts indicating the proximity and intensity of the storm. Figure 1 shows the simple receiver which consists of a tuned amplifier driving a modified flasher circuit. The flasher is biased to not flash until a burst of RF energy, amplified by the first 2N4401, is applied to the base of the 2N4403. The receiver standby current is about 350 microamps which is nothing at all to a couple of D cells, hardly denting the shelf life. Of course, the stormier it gets, the shorter the battery life.

For best effect, mount the lamp in an old-fashioned holder with an extra-large colored glass lens. Or construct your own fixture with a plate of textured colored glass behind a panel painted with black-crackle paint. Watch a few old science fiction movies for other ideas.

A totally different approach is to mount the circuit in an empty glass jar with the antenna and bulb protruding through the top. (A malted-milk jar has a nice, red plastic lid which is easy to work and looks good.) Use a pin jack for the antenna. The gadget looks quite home-made but fascinating.

Boat owners may wish to replace the lamp with a 3-volt beeper to provide an early warning of approaching bad weather. Choose one of those unbreakable clear plastic jars like the large jars of coffee creamer. A little silicone rubber will seal the antenna hole in the lid of the jar. Use a longer antenna for increased sensitivity since there are few electrical noise sources on the lake.

Tune-up is simple: adjust the potentiometer until the regular flashing just stops. (Use a multi-turn trimmer.) When properly adjusted, the lamp will occasionally flash when large motors or appliances switch on and off and an approaching storm will give quite a show. Obviously, tune-up is a bit more difficult during stormy weather. Adjust the pot with no antenna if lightning is nearby. Tune an AM radio to the bottom of the dial to monitor the pulses that the lightning detector is receiving.

This lightning detector is not so sensitive that it will flash with every crackle heard on the radio but will only flash when storms are nearby. Increased sensitivity may be achieved by increasing the antenna length. The experienced experimenter may wish to add another gain stage after the first by duplicating the RF amplifier circuitry including capacitor coupling with the addition of a 47 ohm emitter resistor to reduce the gain somewhat. This additional gain can cause stability problems if the layout is poor so novices are advised to use a longer antenna or adjust the sensitivity potentiometer more delicately instead! (When operating properly, the additional gain makes the pot adjustment much less critical.)

Theory of Operation:

Lightning flashes generate a broad spectrum of radio frequencies with especially intense emissions in the VLF band. This receiver is designed to pick up a band near 300kHz which is fairly empty except for lightning static. These radio "crackles" are picked up by the antenna with the help of the 10 millihenry choke. Short antennas (short compared to the wavelength, that is) behave as though a very tiny capacitor is connected in series and this choke resonates with this capacitor allowing current to flow into the receiver. The 330 uH and 680 pF form a tuned circuit at 300 kHz and the 0.01 uF couples this tank into the base of the first transistor amplifier. The amplified radio signal on the collector is coupled into the base of the second transistor which is part of a lamp flasher circuit. The flasher is biased such that it doesn't flash (by careful adjustment of the pot) until a radio burst pulls the base of the 2N4403 down. Positive feedback causes the flasher to quickly turn full on until the 100 uF capacitor discharges giving a good lamp flash. The circuit quickly resets by charging the 100 uF capacitor through the 1N914.

Transistor substitutions are fine. Most modern small-signal transistors will work well in the circuit including 2N3904 (NPN) and 2N3906 (PNP). Avoid high frequency "RF" transistors since unwanted oscillations may result.

Note: A reader, Bob Radmore (N2PWP) has written an article featuring this lightning detector for the April, 2002 issue of QST, ARRL's monthly membership journal.

Inspired by this season's first thunderstorm here is another experimental version of the lightning detector that features lower battery drain and additional functions:

Basic Low-Power Receiver

The basic receiver is similar to the first version except that the RF amplifier is a bit starved for current which saves power and also provides demodulation for listening to the lightning crackles. The flasher portion uses much less current but only provides a low current positive pulse which needs further conditioning for most purposes. When idle this new circuit draws only about 100 uA so applications using smaller batteries are practical. One or more of the following options are connected to the receiver to complete the detector:

LED Driver

This LED driver consumes much less current than the previous design but the light output is less.

Audio Amplifier

The audio amplifier connects to the collector of Q1 and allows the user to listen to the received signal. The volume is not great but an optional volume control may be added by replacing Q1's 10k collector resistor with a 10k potentiometer with the wiper connecting to the 10k on the input of the amplifier. A power switch is included since this amplifier will draw several milliamps. Adjust the resistor from the cathode to reference for 5 to 20 mA DC power consumption.

Averaging Meter

The averaging meter shows a fairly steady reading that is proportional to the lightning activity. A DC output is provided for driving a comparator for alarms, automatic controls, etc.:

Alarm Comparator

The alarm comparator is used to drive a buzzer or other type alarm, a motor, relay, or other heavy electrical load. A separate power supply provides power to the comparator and load. The 10k input resistor connects to the output of the averaging meter circuit  and the 10 megohm provides some hysterisis for quick switching. The 50k pot is adjusted for the desired trip point. Any n-channel VMOS power transistor will work as long as it is rated for the load current. An LM358 op-amp or other ground-sensing op-amp may be substituted for the comparator.

All of the options may be included in one unit. The photo below is my detector in my living room. It is built into an abandoned control panel for an electronic air filter and includes the incandescent lamp, meter and speaker options.

Lightning Detectors