This circuit is for detecting any audio signal with justable noise offset. It could be used to generate a radio mute signal eg. from a PDA.

The circuit is consisting of:

• Input impedance
• Signal amplifier
• Rectifier
• Justable comparator (Noise offset).

Schematic
Oszilogram

Audio Detector

Here's a simple lie detector that can be built in a few minutes, but can be incredibly useful when you want to know if someone is really telling you the truth. It is not as sophisticated as the ones the professionals use, but it works. It works by measuring skin resistance, which goes down when you lie.

Parts

Part	Total Qty.	Description
R1	1	33K 1/4W Resistor
R2	1	5K Pot
R3	1	1.5K 1/4W Resistor
C1	1	1uF 16V Electrolytic Capacitor
Q1	1	2N3565 NPN Transistor
M1	1	0-1 mA Analog Meter
MISC	1	Case, Wire, Electrodes (See Nots)

Notes

1. The electrodes can be alligator clips (although they can be painful), electrode pads (like the type they use in the hospital), or just wires and tape.
2. To use the circuit, attach the electrodes to the back of the subjects hand, about 1 inch apart. Then, adjust the meter for a reading of 0. Ask the questions. You know the subject is lying when the meter changes.

Simple Lie Detector

Detects 1.8 to 230 Volts DC or AC
Minimum parts counting.
Circuit diagram:

Parts:

D1 5 or 3mm. Red LED D2 5 or 3mm. Green or Yellow LED LP1 220V 6W Filament Lamp Bulb P1 Red Probe P2 Black Probe

Device purpose:
This circuit is not a novelty, but it proved so useful, simple and cheap that it is worth building.
When the positive (Red) probe is connected to a DC positive voltage and the Black probe to the negative, the Red LED will illuminate.
Reversing polarities the Green LED will illuminate.
Connecting the probes to an AC source both LEDs will go on.
The bulb limits the LEDs current to 40mA @ 220V AC and its filament starts illuminating from about 30V, shining more brightly as voltage increases.
Therefore, due to the bulb filament behavior, any voltage in the 1.8 to 230V range can be detected without changing component values.
Note:

• A two colors LED (Red and Green) can be used in place of D1 & D2.

Ultra-simple Voltage Probe

Description

A test circuit for BJT (Bipolar Junction Transistors). This circuit can measure both small signal h_fe and DC current gain h_FE of a low to medium power power transistor. In addition it can measure collector-base and collector-emitter leakage current. This circuit can also measure h_FE at different operating points. A multimeter can be used at multiple test sockets to make all measurements, or two DC ammeters can be used.

Notes

The circuit has two requirements: a variable DC power supply and a test and measuring circuit. Starting with the power supply, the input source is two 9 Volt batteries, series connected to create an 18 Volt supply. If the tester is designed to be portable then batteries can be used, if used in a workshop, then any variable DC power supply can be used. The power supply in this circuit is a standard L200 regulator circuit. R2 and VR1 allow the supply to be varied from 2.85 Volts to almost the full 18 Volt supply, current is limited by R1 to 450mA.
The test circuit is a collection of switches and passive components. The main tests are performed by S3 a 3 Gang 6 way switch, see this page in the practical section for help on switches. A transistor socket is used to connect the transistor under test, a multimeter switched to DC Milliamps can be connected to terminals M1 and M2 to measure collector current (a wire link should be used to short M3 and M4). The same meter can then be set to DC Microamps and measure the base current at M3 and M4 (terminals M1 and M2 short be shorted with a link). Alternatively analogue meters can be used for both meters, however as a digital meter offers better precision and resolution, a multimeter is the preferred choice.

Function of Switches

S1 is a DPDT switch wired to reverse polarity, as drawn it is used to test NPN transistors, in the opposite state it reverses the power supply and used to test PNP transistors. S2 is a normally open push to make switch. This is the general test switch and pressing this switch allows base current or collector current to be read on the multimeter.
S3. This is the selector switch and controls the different functions for the tester. S3 is a 3 gang, 6 way rotary switch. This means that one single shaft rotates arcs S3a, S3b and S3c simultaneously each turn.
S4. This switch reduces the base current and allows the small signal current gain h_fe to be measured.

Tests Using Rotary Switch S3

Position of S3	Function	Conditions	Result
1	ICO	VCB = VS	Read Multimeter Direct
2	hFE	IB = 20uA Set Vs to 6V	hFE = Meter reading / 19.5uA
3	hFE	IB = 100uA Set Vs to 6V	hFE = Meter reading / 92.6uA
4	hFE	IB variable Vs variable.	hFE = Meter reading M1,M2 / M3,M4
5	ICEO	VCEO = VS	Read meter M1,M2 direct.
6			No Function

General Usage

Using the tester is easy, starting with power off, insert a transistor into the test socket. Set S1 for NPN or PNP and rotate S3 to the required test position. Rotate VR1 so the desired collector emitter voltage. Pressing S2 now allows the measurement of h_FE to be made. Pressing S2 and S4 allows hfe to be measured. More detailed usage now follows.

Measuring Collector Base Leakage

With S3 in position 1, insert a transistor into the test socket and set S2 for NPN or PNP. M3 and M4 need to be shorted and a multimeter set to DC microamps between M1 and M2 now allows collector base leakage current to be measured. With silicon transistors, you may not see a reading at all, but germanium transistors have leakage current which can be measured.

Measuring DC Current Gain at 20uA

Set S1 for NPN or PNP and rotate S3 to position 2. Rotate VR1 so the power supply reads 6 Volt between terminal Vs and ground. Place a shorting link across M3 and M4 and a digital multimeter set to measure DC lamps across M1 and M2. Pressing S2 now allows the measurement of h_FE to be made. This will be the meter reading / 20 uA.

Measuring DC Current Gain at 100uA

Set S1 for NPN or PNP and rotate S3 to position 3. Rotate VR1 so the power supply reads 6 Volt between terminal Vs and ground. Place a shorting link across M3 and M4 and a digital multimeter set to measure DC milliamps across M1 and M2. Pressing S2 now allows the measurement of h_FE to be made. This will be the meter reading / 100 uA.

Measuring DC Current Gain at an Operating Point

Set S1 for NPN or PNP and rotate S3 to position 4. The parameter h_FE varies with different collector currents and temperatures. VR1 and VR2 allow you to set up different operating points. Suppose you have a circuit where a transistor is run from a 15 Vdc supply and base current is 15 uA. First set VR1 so the power supply reads 15 Volt between terminal Vs and ground. Place a shorting link across M1 and M2 and a digital multimeter set to measure DC microamps across M3 and M4. Press S2 and adjust VR2 until 15 uA is measured between M3 and M4. Now release S2, short terminals M3 and M4, remove the link across M1 and M2 and set the meter to read DC milliamps. Pressing S2 now allows the measurement of h_FE to be made. This will be the meter reading / 15 uA (or whatever base current you choose).

Measuring Collector Emitter Leakage Current

With S3 in position 5, insert a transistor into the test socket and set S2 for NPN or PNP. M3 and M4 need to be shorted and a multimeter set to DC microamps between M1 and M2 now allows collector emitter leakage current to be measured. With silicon transistors, you may not see a reading at all, but germanium transistors have leakage current which can be measured.

Measuring Small Signal AC Current Gain

The value of the small signal current gain h_fe can also be measured with this circuit, for base currents of approximately 20uA, 100uA or any particular operating point. Proceed as in the previous steps for measuring DC current gain and with S3 at position 2, 3, or 4. The voltage Vs should be set to 6 Volt, a short across meter terminals M3 and M4 is required then press S2 and read the current on the meter across terminals 1 and 2. This reading will be called IC1. Now keeping S2 pressed, also press S4, record the reading, this is measurement IC2.

h_fe is calculated as follows:


This is for S3 in postion 3 (20uA base current).

This is for S3 in postion 2 (100uA base current).

Measuring Small Signal AC Current Gain at a Particular Operating Point

You can also measure h_fe at any operating point within the voltage and current range of the power supply. The power supply can deliver 18 Volts at up to 500mA. Larger currents will drain the batteries so a bench power supply would be recommended.

To measure h_fe at a VCE of 12 Volts and collector current 1 mA. First adjust VR1 so that the supply Vs is 12 Volt. Short M3 and M4, press S2 and connect a multimeter to M1 and M2 and adjust VR2 to read 1 mA. Now release S2, short M1 and M2 and remove the short on M3 and M4 and set your meter to microamps and measure the current. Record both values of base and collector current. Now press S2 and S4 and measure both collector and base currents again. The value of h_fe is the difference in collector current divided by the difference in base current.

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How the Signal AC Current Gain is Calculated

The small signal AC current gain is achieved by changing the value of the base current. When S4 is pressed, the input voltage is reduced by the fraction R7 / (R4+R7) which reduces the input base current. h_fe is the change in collector current divided by the change in base current. The base current which is calculated as follows:



With S3 in position 2, the base current I_b is:



With S3 in position 2, and when S4 is pressed the base current I_b becomes:


The difference in base current is therefore 92.6uA - 72.8uA = 19.8uA This value is then used as the denominator for the larger change in collector current, as in the previous section.

h_FE versus h_fe

In practice, the difference between h_FE and h_fe is often so small that one value can be substituted for the other. Data sheets invariably quote the value for the dc current gain h_FE, the parameter h_fe is the ac quantity and decreases also at higher frequency. As this circuit measures the change in base current at dc the value of h_fe will only be approximate at low frequencies up to 1kHz. To measure h_fe at a particular frequency, then a signal generator would be required and the meter set to measure ac base and collector currents.

Testing on a Breadboard.

Although simple, the wiring of the switch can be troublesome, and if you already have a variable power supply, multimeter and a breadboard, then you can set up the circuit as shown below:

In my test transistor, a BC109C Base current was 94.6 uA. The power supply was set at 6V and a 56k and 1.2k resistor wired in series. Next the collector current was measured, see below:

My sample BC109C produced a collector current of 41.2 mA. The h_FE was therefore 41.2/0.0946 = 436.

Transistor Tester

What can you use to test how effective your antennas are for 2.4 Ghz? Which antenna has the best gain or, how do you know that there is any 2.4Ghz RF transmitted? Here are the details on how to build a general purpose 2.4Ghz Radio Frequency Field Strength Meter. This one was built using the microwave rated diode from a MICROTEK solid state microwave leakage detector (purchased from Dick Smith Electronics for around $24) these diodes can be more expensive than that if purchased in single units from electronics suppliers. There may be other suitable diodes available. Electronics stores also sell Schottky Hot Carrier Diodes that will probably also be suitable for this application.

The antenna is a 2 element quad. I've orientated it in the diamond configuration so it should be effective for both horizontal and vertically polarised signals. You could build the antenna in the vertical or horizontal sense if you like. The antenna was constructed on a right angled BNC connector, however I'm sure you could come up with a different sort of plug setup that would still provide good results. Just keep the lead lengths to a minimum to reduce losses. I have used an attachment that allows the BNC connector to be inserted into my Voltmeter. I switch the Voltmeter to Millivolts, point it at the 2.4Ghz RF and read the result. The yellow plastic cylinder is used to keep the antenna separation at 10mm. I cut a channel into the plastic to allow the wire to sit tight, and pushed some liquid nails into the hole to hold it. The bottom of the reflector loop is held to the BNC connector with another dolop of glue.

The detail of the antenna plugged into my Voltmeter.

Above is the antenna plugged into the Volt meter. It works pretty well, pointing it at the SUN also gets a reading! Point it at the microwave oven and it will exceed the Millivolt scale! With a little work I'm sure you could build a radar detector... I tuned the capacitor with a plasitc screwdriver to get maximum reading from a 2.4Ghz RF source. You should use a Wireless LAN card as the source.

Here is the schematic detail (not to scale), you should make the elements of the anntenna as close to the correct size as possible. This will ensure maximum energy is absorbed at 2.4Ghz. The elements should be spaced around 10mm apart. The antenna will display some gain and uni-directionality, so point the smaller antenna loop (driven element) towards the RF source you wish to measure. I tried connecting the antenna directly to a microamp moving coil meter, however there was very little meter deflection from a Wireless LAN card. The electronic voltmeter is far superior.

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Field Strength Meter for 2.4 Ghz Wireless LAN

Description

An electromagnetic field probe designed to detect changing electric and magnetic fields. The probe has switchable gain, a frequency response up to 400kHz and independent audio and meter monitoring.

Circuit Notes

This EMF probe uses an inductor to locate stray electromagnetic (EM) fields. It will respond to both changing magnetic and electric fields as each will induce a voltage in the inductor. The circuit is built around a quad low noise FET input op-amp, type TL084.

Power supply is a single 9 Volt battery, the supply being divided by R5 and R6. C1 and C2 help smooth variations in battery voltage, S1 is the on off switch. The input stage U1, is direct coupled to the probe, a radial wound 1mH inductor, type Toko 8RB as shown in the probe construction. This part appears only available from Jabdog Electronics in the UK, part number 187LY-102J. If not available then the 1.2, 1.5 or 1.8mH inductor will work equally well. The reactance of the inductor changes with input frequency and stage gain is very high. As there is no offset null control in the TL084 then the output is capacitively coupled via C3 to the next Tl084 amplifier U2. This stage has switchable gain of approximately 1.5x and 4.7x controlled by S2.

The output gain of both U1 and U2 stages ( with switch S2 open ) is about 70dB at 1KHz. Gain is still about 30dB at 400KHz, although the signal meter will not be too accurate at such high frequency. The bode plot simulated in LTspice is shown below:

The output from U2 is split by C4 and C5 and drives an independent headphone amplifier built around U4. VR1 acts as a volume control the output being either a mono or stereo miniature jack plug as shown. The output stage of the TL084 is sufficiently low to drive 32 ohm headphones like Sennheiser or Ipod Shuffle, etc. U3 is the meter amplifier. All EMF fields are amplified across the load resistor R8. D1 now acts as a half wave rectifier and creates sufficient DC voltage to drive a small signal meter, shown below.

This signal meter is available from Maplin Electronics part number LB80B and has a FSD of 250uA and an internal resistance of 675 ohms. However any meter will work having a similar sensitivity. Meters of 100 or 50uA FSD can also be used providing a suitable series resistor is used. Because the circuit is responding to RF frequencies up to several hundred kHz a smoothing capacitor across the meter should not be used as this would appear as an effective short circuit reducing the average current through the meter to zero.

Probe Construction

The probe is made from an old pen tube, the end cap being removed. A 50cm length of audio screened cable is threaded through the pen tube and soldered to the radial inductor. The capacitance of 50cm audio cable is about 2pF, longer cable should not be used as high frequency performance will deteriorate.

The cable may be used with a 3.5mm mono plug and socket if desired. My completed probe is shown below. The diameter of the inductor fitted neatly against the body of the pen tube. A layer of insulating tape or glue may be used to secure the pen body to the inductor.

Simulation Model

To model this circuit in LTspice or any other simulator you have to take into account the input capacitance of the probe cable, and the impedance of the inductor itself. The cable capacitance was measured by a capacitance meter and came out at 1.9pF, so 2pF was added in parallel with L2 which is the probe inductor. The simulation schematic is shown below:

The Toko 8RB inductor has a series resistance of 7 ohms, at 100kHz the impedance is 628.3 ohms. The series resistance of L2 needs to be included, in LTspice the inductor L2 can be right clicked and a value for series resistance entered, or as shown above can be entered in the value of Rs. A transient response at 10kHz is shown below:

The simulation model has 3 nodes labeled Vgain, Vheadphone and Vmeter for clarity. These waveforms are shown above. The input has been simulated by a signal generator feeding another coil. The coupling coefficient of 0.9 is used and input voltage of 10mV pk-pk used.

Download Simulation Circuit

The simulation circuit for LTspice can be downloaded here. Please note that you will also have to download the model for the potentiometer and TL072 op amp from the LTspice yahoo group, more details in the simulation section. The TL072 simulation model is the same as the TL084 model.

Testing

If you have access to an audio or RF signal generator you can apply an input signal to the windings of a small transformer or another inductor. This will set up an electromagnetic field which will be easily detected by the probe. Without a signal generator, just place the probe near a power supply, mains wiring or other electrical device. There will be a deflection on the meter and sound in the headphones if the frequency is below 15KHz.

Parts List

IC1 TL084
D1 1N4148
L1 1mH radial inductor part 187LY-102J
R1 470k
R2,R5,R6,R7,R9 10k
R3 22k
R4 47k
R8 4k7
VR1 10k log
C1,C6,C8 220u
C2,C4,C5 10u
C3 1u
C7 2.2u
Signal meter 250uA FSD, Maplin LB80B or similar

In Use

Switch on, set VR1 to minimum and plug in headphones (optional). The circuit can be built on veroboard and is designed to be portable. Try moving the probe near a light switch or electric socket and a loud hum will be heard in the headphones and meter will deflect.

EMF Probe Version 2

Description

A low frequency test oscillator for testing tone controls and experimenting.

Circuit Notes

The circuit is a standard RC phase shift oscillator using a single bipolar transistor as the active element. When power is applied regenerative feedback is applied via C2 from collector to base of the transistor. The timing components, R1, R2, C1 and C3 dictate the oscillation frequency. In use preset RV1 is adjusted so that oscillation just begins. With values shown full amplitude oscillation takes about 4.8 seconds (see diagram below).

Frequency Calculation

This oscillator is designed and simulated on LTSpice IV. Once simulated click the "probe" cursor on the output wire "Vo", the above waveform is produced. To calculate the frequency, place your mouse on the graph where oscillations have reached full amplitude and draw a rectangle, ensuring maximum and minimum amplitude is enclosed within the rectangle. The diagram below shows such a zoomed portion of the output waveform, starting around 5.6 seconds into the simulation.

To add cursors left click on the name of the output waveform, this is called "V(vo)". A single cursor is added to the graph which can be moved with the mouse or keyboard arrows. Now right click the mouse on the Vvo waveform. In the window that appears click on the attached cursor menu and change to "1st & 2nd". Now two cursors will be visible and controllable. Make sure both cursors pass through the zero volt horizontal meridian and measure one output cycle. A sub window allows you to read the value as shown above, for this oscillator the time for one cycle is 127.2ms and frequency a little under 8Hz.

Downloadable Circuit

The LF oscillator may be downloaded here. Please note that you will also have to include a modified list of components to simulate the BC549B, see the LTspice Section for more details.

Low Frequency Oscillator

Description

A Coil Coupled Operation Metal Detector made from readily obtainable components and using an ordinary medium receiver as a detector.

Notes

The metal detector shown here may well represent a new genre. At any rate, after some exposure, it is regarded as such by those who have seen it. It is based on a standard transformer coupled oscillator (TCO) - hence the name Coil Coupled Operation (CCO) Metal Detector. Although requiring a BFO (in this case provided by a Medium Wave radio), it differs from a typical BFO detector in that its performance far outstrips that of BFO. Also, unlike BFO, it is dependent on the balance of two coils to boost sensitivity. It also differs from IB, in that its Rx section is an active, rather than passive, component of the oscillator. Further, unlike IB, the design does not require critical placement of the coils. As with both BFO and IB, the design provides discrimination. Experiments with different embodiments of the idea have shown that it has the potential to match the best of IB. Happy hunting!

Coil Coupled Operation Metal Detector

Description

A Beat Balance Metal Detector made from discrete components.

Notes

Various embodiments of the BB metal detector have been published, and it has been widely described in the press as a new genre. Instead of using a search and a reference oscillator as with BFO, or Tx and Rx coils as with IB, it uses two transmitters or search oscillators with IB-style coil overlap. The frequencies of the two oscillators are then mixed in similar fashion to BFO, to produce an audible heterodyne. On the surface of it, this design would seem to represent little more than a twinned BFO metal detector. However, what makes it different above all else, and significantly increases its range, is that each coil modifies the frequency of the adjacent oscillator through mutual coupling. This introduces the "balance" that is present in an IB metal detector, and boosts sensitivity well beyond that of BFO. Since the concept borrows from both BFO and IB, I have given a nod to each of these by naming it a Beat Balance Metal Detector, or BB for short. Happy hunting!

Beat Balance Metal Detector

Description

A simple test circuit to fault find audio and radio equipment. Can be used to inject a square wave signal, rich in harmonics, or used with headphones as an audio tracer.

Notes

A single pole double throw sitch is used to switch between inject and trace modes. The diagram is drawn in trace mode, the earpiece being connected to the collector of the last transistor. Both transistors are wired as emitter followers, providing high gain. DC blocking is provided by the 1n capacitor at the probe end, and the two stages are capacitively coupled.

when the switch is thrown the opposite way (to the blue dot) both transistors are wired as an astable square wave generator. This provides enough harmonics from audio up to several hundred kilohertz and is useful for testing AM radio Receivers.

The Circuit on Veroboard

Below is the actual signal generator built on Veroboard by one of Circuit Exchange International viewers from Holland. Special thanks to Henry for his pictures.

Output Injector Waveform

Henry has also kindly provided on oscillogram on the injector in action, this is shown below.

The fast switch on time of the transistors produces the switching spike which is rich in harmonics.

Signal Tracer and Injector

Description:

A small 325mW amplifier with a voltage gain of 200 that can be used as a bench amplifier, signal tracer or used to amplify the output from personal radios, etc.

Notes

The circuit is based on the National Semiconductor LM386 amplifier. In the diagram above, the LM386 forms a complete non-inverting amplifier with voltage gain of x200.

A datasheet in PDF format can be downloaded from the National Semiconductor website. The IC is available in an 8 pin DIL package and several versions are available; the LM386N-1 which has 325mW output into an 8 ohm load, the Lm386N-3 which has 700mW output and the LM386N-4 which offers 1000mW output. all versions work in this circuit.

The gain of the Lm386 can be controlled by the capacitor across pins 1 and 8. With the 10u cap shown above, voltage gain is 200, omitting this capacitor and the gain of the amplifier is 20.

The IC works from 4 to 12Volts DC, 12Volt being the maximum recommended value. The internal input impedance of the amplifier is 50K, this is shunted with a 22k log potentiometer so input impedance in this circuit will be lower at about 15k. The input is DC coupled so care must be taken not to amplify any DC from the preceeding circuit, otherwise the loudspeaker may be damaged. A coupling capacitor may included in series with the 22k control to prevent this from happening.

The finished circuit.

Step by step instructions on how to produce this amplifier now appear in my practical section

Bench Amplifier

Description:

Two simple test circuits to check operation of quartz crystals.

Notes

In the first circuit, above the BC548 is wired as a colpitts oscillator, the frequency tuned by insertion of a crystal. A good crystal will create high frequency oscillations, the output at the collector is rectified by the germanium OA91 diode and a deflection will appear on the meter. Thw more active the crystal, the higher the output deflection which may be adjusted with the preset.

Notes

The next circuit uses a working crystal again used to control the frequency of a colpitts oscillator. This time the output from the oscillator is taken from the emitter and is full wave rectified, the small dc bias will then directly cause the second BC548 to light the LED.

Two Simple Crystal Test Circuits

Description

A low resistance ( 0.25 - 4 ohm) continuity tester for checking soldered joints and connections.

Notes

This simple circuit uses a 741 op-amp in differential mode as a continuity tester. The voltage difference between the non-inverting and inverting inputs is amplified by the full open loop gain of the op-amp. Ignore the 470k and the 10k control for the moment, and look at the input of the op-amp. If the resistors were perfectly matched, then the voltage difference would be zero and output zero. However the use of the 470k and 10k control allows a small potential difference to be applied across the op-amp inputs and upset the balance of the circuit. A small resistance across the input probes (circuit under test) causes the balance to be amplified and op-amp output swinga to full supply voltage and light the LED's. Although two LED's are shown this is cosmetic and single or multiple LED's could be used as the display.
Input current to the circuit under test is minimal and is approximately 9/44k or 205uA. It is important that the circuit under test contains no voltage and all capacitors (if any) are discharged. However you may find a primary use for this circuit in testing soldered connections down to 0.25 Ω

Setting Up and Testing

The probes should first be connected to a resistor of value between 0.22 ohm and 4ohm. The control is adjusted until the LED's just light with the resistance across the probes. The resistor should then be removed and probes short circuited, the LED's should go out. As the low resistance value is extremely low, it is important that the probes, (whether crocodile clips or needles etc) be kept clean, otherwise dirt can increase contact resistance and cause the circuit to mis-operate. The circuit should also work with a MOSFET type op-amp such as CA3130, CA3140, and JFET types, e.g. LF351. If the lED's will not extinguish then a 10k preset should be wired across the offset null terminals, pins 1 and 5, the wiper of the control being connected to the negative battery terminal. A pin out for the 741 can also be found on my practical section.

Connection Tester

Description:

A multi wire cable tester with a separate LED for each wire. Will show open circuits, short circuits, reversals, earth faults, continuity and all with four IC's. Designed initially for my intercom, but can be used with alarm wiring, CAT 5 cables and more.

Full circuit can be viewed with resolution of 1024x768
IC Pinouts of the 4011 and 4017 can also be viewed here

Notes

Please note that for clarity this circuit has been drawn without showing power supplies to the CMOS 4011 and CMOS 4050 IC's. The positive battery terminal connects to Pin 14 of each IC and negative to Pin 7. The CMOS 4017 uses Pin 16 and Pin 8 respectively. Note also that as the CMOS 4050 is only a hex buffer, you need 8 gates so two 4050's are required, the unused inputs are connected to ground (battery negative terminal).

Circuit Description
The circuit comprises transmitter and receiver, the cable under test linking the two. The transmitter is nothing more than a "LED chaser" the 4011 IC is wired as astable and clocks a 4017 decade counter divider. The 4017 is arranged so that on the 9th pulse,the count is reset. Each LED will light sequentially from LED 1 to LED 8 then back to LED 1 etc. As the 4017 has limited driving capabilities, then each output is buffered by a 4050. This provides sufficient current boost for long cables and the transmitter and receiver LED's. The receiver is simply 8 LED's with a common wire...read on.

Wiring the CMOS 4017
The pinout for the CMOS 4017B is shown below. Please note that in the main schematic above, alternate naming of the pins has been used. The pin equivalence is as follows:-

CP0 (clock pulse zero) is the Clock input, Pin 14 on the diagram above.
CP1 (clock pulse one) is the clock inhibit or Pin 13 on the pinout above.
MR (master reset) is the reset pin 15 in the diagram above.
Q0-9 represent the decoded decimal outputs. Hence Q0 is Pin 3 on the pinout and Q8 is Pin9.

7 Led's 8 Wires
Not a mistype. The problem with testing each wire individually is that if you had 7 individually addressable LED's, then you would need an eighth return or common wire. In the case of testing 8 wires you would need a ninth wire. You could use a domestic earth but its not really practical, and also if the cable was shorting to earth anyway it would be no good anyway. The solution had me thinking for a while, but since this is a logic circuit, there are only two conditions, logic high or zero. As the 4017 outputs are either high or low, any output can provide a common return path for a LED. So LED's 1 - 3 use the 4th output of the 4017, which will be zero, and the 4th LED is wired with reverse polarity. On the 4th pulse, output 4 is high, output 3 is low and so the LED will light. If the common return wire is open circuit then LED's 1-4 will not light. A similar situation occurs with outputs 5 to 8. The common wire in can be taken from any output terminal from the 4017, but the same rule would still apply. The ability to test all wires quickly outweighs this small disadvantage. If a cable of just 4 or 6 wires is tested then it must use the wires with LED's numbered 1 to 4 or 1 to 6, which is why the LED's are numbered that way.

Testing
With a good cable and all wires connected then LED 1 will light at both cable ends, followed in sequence by LED 2 ,3, 4 etc to LED 8, the sequence then repeating. If a 4 wire cable is used, it must be connected to use the common return wire as described in the preceding paragraph. The sequence would be LED 1,2,3,4 repeating with a delay as the 4 unused outputs are stepped through.
To check for earth contact faults, the probe labeled "to earth connection" would be physically connected to a local earth. A wire that is earthing will dim or extinguish the LED's at both ends of the cable. An LED not lighting at the receiver, indicates a broken or open circuit. If two wires are short circuit, example 3 and 4 then at the receiver the sequence would be 1, 2, 34, 43, 5, 6, 7, 8. A reversal would be indicated by an out of pattern sequence of LED's. Here's an example, the probe is connected to an earth at the transmitter, the cable is very faulty, wire 1 is OK, 2 is earthing, 3 and 5 are reversed 4 is OK, 6 is open circuit and 7 and 8 are short circuit. See below.

Test Result for Above Faulty Cable:

The transmitter pattern:                         The receiver pattern would be:
            1 ON                                             1 ON
            2 OFF or Faint                                   2 OFF or faint
            3 ON                                             3 (would show LED 5)
            4 ON                                             4 ON
            5 ON                                             5 (would show LED 3)
            6 ON                                             6 OFF
            7 ON                                             7 (would show 7 & 8)
            8 ON                                             8 (would show 7 & 8)
     

The LED sequence of course is stepped through, as you know the transmitter "pattern" it is easy to tell the state of the cable by viewing the receiver pattern. The earth condition will only show up if the contact to earth is less than 1000 ohms, a better but more time consuming method for earth faults is to use a meter on the Megaohms range.

Multi Wire Cable Tester

Description

The circuit of Fig.1 provides an easy yet reliable way to detect the intensity of a.c. (or e.l.f.) fields around the home or workplace. It is doubly effective because it does not merely detect the electromagnetic radiation emitted by electrical appliances, but the electromagnetic energy actually absorbed by the body.

Fig. 1

The circuit in Fig.1 is a standard charge pump which is charged by the alternating eddy currents induced in the human body by a.c. fields. C1 charges virtually instantly, and is read by a digital (or high impedance) voltmeter.

To obtain a very rough translation from millivolts to milligauss (the unit of magnetic field strength), divide the millivolts reading by four. For example, 1000mV will yield 250 milligauss. A rough guide to the readings follows:

Up to 3 milligauss - Low electromagnetic radiation

25 milligauss - Significant electromagnetic radiation

100 milligauss - High electromagnetic radiation

250 milligauss - Maximum risk exposure

Detrimental effects have been reported at doses as low as 3 milligauss, and a series of studies since the 1970's has shown that sustained exposure to high e.l.f. doses heightens the risk of certain cancers and miscarriage.
Readings are taken while holding the probe in one hand. The closest proximity to the electromagnetic source does not necessarily give the highest reading, probably because the induced currents in the body remain localised at close proximity.

The Sensor

Is any piece of metal (e.g. a short stub of copper piping, even a short piece of fencing wire) that makes good contact with the hand.

Milligaus Meter

Description:

A function generator using the ICL8038 integrated circuit. Is has four ranges and capable of sine, square and triangle outputs.

Notes

Built around a single 8038 waveform generator IC, this circuit produces sine, square or triangle waves from 20Hz to 200kHz in four switched ranges. There are both high and low level outputs which may be adjusted with the level control. This project makesa useful addition to any hobbyists workbench as well. Allof the waveform generation is produced by IC1. This versatile IC even has a sweep input, but is not used in this circuit. The IC contains an internal squarewave oscillator, the frequency of which is controlled by timing capacitors C1 - C4 and the 10k potentiometer. The tolerance of the capacitors should be 10% or better for stability. The squarewave is differentiated to produce a triangular wave, which in turn is shaped to produce a sine wave. All this is done internally, with a minimum of external components. The purity of the sine wave is adjusted by the two 100k preset resistors.
The wave shape switch is a single pole 3 way rotary switch, the wiper arm selects the wave shape and is connected to a 10k potentiometer which controls the amplitude of all waveforms. IC2 is an LF351 op-amp wired as a standard direct coupled non-inverting buffer, providing isolation between the waveform generator, and also increasing output current. The 2.2k and 47 ohm resistors form the output attenuator. At the high output, the maximum amplitude is about 8V pk-pk with the square wave. The maximum for the triangle and sine waves is around 6V and 4V respectively. The low amplitude controls is useful for testing amplifiers, as amplitudes of 20mV and 50mV are easily achievable.
Setting Up: The two 100k preset resistors adjust the purity of the sine wave. If adjusted correctly, then the distortion amounts to less than 1%. The output waveform ideally needs to be monitored with an oscilloscope, but most people reading this will not have access to one. There is however, an easy alternative:- Winscope. This piece of software uses your soundcard and turns your computer into an oscilloscope. It even has storage facility and a spectrum analyser, however it will only work up to around 20KHz or so. Needless to say, this is more than adequate for this circuit, as alignment on any range automatically aligns other ranges as well. Winscope is available at my download page click here. Winscope is freeware and designed by Konstantin Zeldovich. After downloading, read the manual supplied with winscope and make up a lead to your soundcard. My soundcard is a soundblaster with a stereo line input, i made up a lead with both left and right inputs connected together. Connect the lead to the high output of the function genereator, set the output level to high, shape to sine, and use the 1k to 10k range, (22nF capacitor). A waveform should be displayed, see the Figure 1 below:-

Figure 1.

Here an undistorted sine wave is being displayed. The display on winscope may flicker, this is normal as it uses your soundcard to take samples of the input waveform. The "hold" button on winscope will display a steady waveform.
Alignment:
First adjust the 100k preset connected to Pin 1 of the 8038. An incorrect setting will look similar to the waveform below:-

Adjust the preset so that the top of the sine wave has a nicely rounded peak. Then adjust the other preset, again an incorrectly adjusted waveformis shown below:

The two presets work together, so adjusting one affects the other. A little is all that's needed. When your waveform is asjusted and looks similar to Figure 1 press the FFT button on winscope. This will preform a fast fourier transform and the displayed output will be a spectrogram of the input.For a pure sine wave, only one signal is present, the fundamental frequency, no harmonics will be present and so a spectrogram for a pure sine should contain a single spike, see Figure 2 below:-

Figure 2.

A distorted sine wave will contain odd and even harmonics, and although the shape of the sine may look good, the spectrogram will reveal spikes at the hormonics, see below:-

Once alignment of the sine wave is complete, the other wave shapes will also be set up correctly. Below is a picture of the triangle waveform generated from my circuit:-

Finally the ICL8038PCD is available from Maplin Electronics order code YH38R.

Function Generator

Description:

This logic probe uses a single CMOS 4001 IC and can display high, low and pulsing outputs.

Notes

This logic probe is based on a single CMOS 4001 IC. This IC contains four 2-input NOR gates, all of the gates are used in this circuit. Power for the logic probe is taken from the circuit under test and because CMOS technology is used, the circuit will work with voltages from 3 to 15 Volts DC. The IC pinout is hown below:

CMOS IC's use MOSFET's, so the power terminals are commonly called V_DD for Drain voltage (positive) and V_SS for Source Voltage (ground terminal). In transistor circuits and logic circuits using TTL IC's the positive supply is often termed V_CC which is the collector volatge. As long as Pin 14 is wired to the circuit under test positive supply and pin 7 to ground then the IC will be powered up correctly. The gates in the circuit can be used in any order e.g. Pins 1,2,3 for gate IC1a, pins 4,5,6 for gate IC1b, etc.

Output States

This logic probe can dispay four output states, High, Low, Pulsing and tri-state (or high impedance). The tri-state output is a high impedance state in which the output pin has no value (it is not at logic 0 and not at logic high, you can think of this state as infinite impedance).

IC1a (Pinout pins 1,2,3) is wired as an inverter with a 2M2 feedback resistor. With no input, i.e. probe not connected to a logic circuit, then the output of gate 1a (pin3) is fed back to the input (pins 1 and 2) via the 2M2 resistor. This gate will oscillate at a very high rate and resultant output voltage at pin 3 will be approximately half the supply voltage. The Hi and Lo logic indicator LED's are also connected to a potential divider consisting of the two 1k resistors. The voltage at the resistor junctions is also half supply voltage hence with no input, no output LED's light representing the tri-state.

A Hi or Lo logic condition at the probe input, will cause IC1a to rest in a permanent state indicated by either the Hi or Lo LED illuminating.

With a fast oscillator or clock signal input both Hi and Lo LED's will light but appear quite dim. To increase brightness, the input signal is slowed down. This is achieved by gates IC1b and IC1c which are wired as a monostable. The time constant for the monostable is determined by the 100n capacitor and 4M7 resistor. A fast input pulse now continually triggers and re-triggers, the monostable, effectively slowing the input signal. The output of the monostable is inverted by IC1d, wired as an inverter, and increasing output current to the pulsing LED.

Logic Probe

Description:

A very basic square wave generator using a CMOS 4011 NAND gate.

Notes

Using two gates from a CMOS 4011 NAND chip, a simple squarewave oscillator can be made. Pinouts for the CMOS 4011 can be found in the practical section click here. All unused inputs should be tied to ground. Alternatively a CMOS 4001 chip can also be used, or a TTL equivalent. In this circuit the mark space ratio can also be independantly controlled by varying the value of the resistors. The rise and fall times of the output pulses depend on the operating voltage of the IC and type of IC, but will be typically in the order on tens of nanoseconds.

Square Wave Oscillator

Description

An electromagnetic field probe designed to detect changing electric and magnetic fields. The probe has a meter output and headphone socket as well.

Circuit Notes

This tester is designed to locate stray electromagnetic (EM) fields. It will easily detect both audio and RF signals up to frequencies of around 100kHz.Note, however that this circuit is NOT a metal detector, but will detect metal wiring if it conducting ac current. Frequency response is from 50Hz to about 10kHz gain being rolled off by the 150p capacitor, the gain of the op-amp and input capacitance of the probe cable. Stereo headphones may be used to monitor audio frequencies at the socket, SK1.

Probe Construction

I used a radial type inductor with 50cm of screened cable threaded through a pen tube. The cable may be used with a plug and socket if desired. My probe is shown below:

A layer of insulating tape or glue is used to secure the pen body to the inductor.

Meter Circuit

The output signal from the op-amp is an ac voltage at the frequency of the electro-magnetic field. This voltage is further amplified by the BC109C transistor, before being full wave rectified and fed to the meter circuit. The meter is a small dc panel meter with a FSD of 250uA. Rectification takes place via the diodes, meter and capacitor.

Testing

If you have access to an audio signal generator you can apply an audio signal to the windings of a small transformer. This will set up an electromagnetic field which will be easily detected by the probe. Without a signal generator, just place the probe near a power supply, mains wiring or other electrical device. There will be a deflection on the meter and sound in the headphones if the frequency is below 15KHz.

In Use

Switch on, plug in headphones (optional) and move the probe around. Any electrical equipment should produce a hum and indicate on the meter.I remember once building a high gain preamp (for audio use). I made a power supply in the same enclosure. The preamp worked, but suffered from an awful mains hum. This was not directly from ripple on the power supply as it was regulated and well smoothed.What I had done was built the audio circuit on a small piece of veroboard, and placed it within a distance that was less than the diameter of the transformer. The transformers own electromagnetic field was responsible for the induced noise and hum. I should however note, that this was when I was new to electronics with very little practical experience. You can now buy toroidal transformers which have a much reduced hum field.

EMF Probe with Meter