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