Simple Smoggy Schematic

Even if your good old (Sony) Walkman  sees little use nowadays it would be a  shame to get rid of it altogether. The more  so when just removing the tape head  would allow the built-in audio amplifier  to become an outstanding electrosmog  detector for a variety of purposes. Looking at the schematic, readers with RF  experience will have no difficulty in recognising the diodes and coils of the two  detector-receivers, which serve to capture and demodulate RF signals. With its  coil of four turns (L2) one receiver covers the higher frequency range of the  electromagnetic waves, whilst the sec-ond detector takes care of the lower frequency range.

Simple Smoggy Circuit diagram:

For this reason a coil with a  greater number of turns is required: L1 is  an RF choke of about 250 µH. The precise  value is not critical and it could equally be  220 µH or 330 µH. The outputs of both detector-receivers  are connected to the cables disconnected  previously from the tape heads, feeding the  right and left channel inputs to the Walk-man’s audio amplifier. Please note here that  the screening of the tape head cable does not  have to be absolutely identical to the ground connection of the amplifier circuitry. As  we are dealing with a stereo amplifier,  we are listening into both channels and  thus both RF ranges at the same time.

One channel of the amplifier can also be  used to demodulate low-frequency magnetic alternating fields  via a capacitor  (C3) bypassing diode D1 and connecting either a third coil (L3, for instance;  a telephone recording adapter) as the  pickup device or else a long piece of wire  for acquiring low frequency AC electrical fields. Sources like this are discernible mainly by a distinct 50 Hz (or 60 Hz)  humming in the earphones. Predicting what you may hear down to  the very last detail is difficult, since every  locality has its own, individual interference sources. Nevertheless, with practice  users will succeed in identifying these  interference sources by their particular  audio characteristics.

To sum up, four different ‘sensors’ can be  connected to the inputs of this circuit:  ANT1 (approx. 50 cm long whip antenna),  ANT2 (3.5 cm short stub antenna), ANT3  (approx. 1 m long wire antenna for low frequency electrical fields) and a coil for magnetic fields. Finally, two more tips:

Use only ‘good old’ germanium diodes for  D1 and D2. Sensitivity will be much reduced if  silicon diodes are used, as these have a higher  threshold voltage.
Smoggy does not provide an absolute indi-cation of field strength and even more so can-not provide any guidance whether anything  it detects might be harmful. Its function is to detect electromagnetic signals and compare  their relative magnitude.

Author : Tony Ruepp  - Copyright : Elektor 

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Simple Oscillator-Pipe Locator

Sometimes the need arises to construct a really simple oscillator. This could hardly be simpler than the circuit shown here, which uses just three components, and offers five separate octaves, beginning around Middle C (Stage 14). Octave # 5 is missing, due to the famous (or infamous) missing Stage 11 of the 4060B IC. We might call this a Colpitts ‘L’ oscillator, without the ‘C’. Due to the reactance of the 100-µH inductor and the propagation delay of the internal oscillator, oscillation is set up around 5 MHz. When this is divided down, Stage 14 approaches the frequency of Middle C (Middle C = 261.626 Hz). Stages 13, 12, 10, and 9 provide higher octaves, with Stages 8 to 4 being in the region of ultrasound.

Simple Oscillator/Pipe Locator Circuit Diagram:

 
If the oscillator’s output is taken to the aerial of a Medium Wave Radio, L1 may serve as the search coil of a Pipe Locator, with a range of about 50 mm. This is tuned by finding a suitable hetero-dyne (beat note) on the medium wave band. In that case, piezo sounder Bz1 is omitted. The Simple Oscillator / Pipe Locator draws around 7 mA from a 9-12 V DC source.

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Automatic Headlight Reminder

Do you drive an older car without an automatic "lights-on" warning circuit? If so, you've probably accidentally left the lights on and flattened the battery on one or more occasions. This headlights reminder circuit will prevent that. It's more complicated than other circuits but it's also more versatile. As shown, the circuit uses two low-cost ICs. IC1 is a 555 timer which is wired to operate in astable mode. Its output clocks IC2, a 4017B decade counter. IC2 in turn drives a row of indicator LEDs and also resets IC1 (after about 10s) via transistor Q2.

Automatic Headlight Reminder Circuit Diagram:

The circuit works like this:
When the ignition is on, transistor Q1 is also on and this pulls pin 4 of IC1 low. As a result, IC1 is held reset and no clock pulses are fed to IC2. Conversely, if the ignition is turned off, Q1 will turn off and so IC1 will start oscillating and sound the piezo siren. At the same time, IC1 will clock IC2 and so LEDs 1-10 will light in sequence and stop (after about 10s) with the last LED (LED10) remaining on. That's because, when IC2's O9 output (ie, pin 11) goes high, Q2 also turns on and this pulls pin 4 of IC1 low, thus stopping the oscillator (and the siren).


Note:
That different colored LEDs are used to make the display look eye-catching but you make all LEDs the same color if you wish. Installing optional diode D1 will alter IC1's frequency and this will alter the display rate. Finally, if the lights are turned off and then back on again, the alarm will automatically retrigger. LED1 is always on if the lights are turned on. If you don't want the LED display, just leave the LEDs out.

Author: L. Marshall - Copyright: Silicon Chip

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Game Show Indicator Lights (Who's First)

The circuit below turns on a light corresponding to the first of several buttons pressed in a "Who's First" game. Three stages are shown but the circuit can be extended to include any number of buttons and lamps.

Game Show Indicator Lights Circuit Diagram:

Three SCRs (silicon controlled rectifiers) are connected with a common cathode resistor (50 ohm) so that when any SCR conducts, the voltage on the cathodes will rise about 7 volts above the voltage at the junction of the 51K and 1K ohm resistors and prevent triggering of a second SCR. When all lamps are off, and a button is pressed, the corresponding SCR is triggered due to the voltage at the divider junction being higher than the cathode. Once triggered, the SCR will remain conducting until current is interrupted by the reset switch. Or, you can just turn the power off and back on.

A 50 ohm, 5 watt resistor was selected to produce a 10 volt drop at 200 mA when a single 25 watt lamp comes on. Higher wattage lamps would require a lower value resistor, and visa versa. For example to use 60 watt lamps and maintain the 10 volt drop, the peak current would be 60/160 = 375 mA and the resistance would be E/I = 10/.375 or about 27 ohms at 3.75 watts. The SCRs are "Sensitive Gate' types which trigger on about 200 uA and the gate current is around 1.5 mA when the first button is pressed. The 1N914 diodes in series with the buttons gates are used to prevent a reverse voltage on the gate when a button is pressed after an SCR is conducting. The two 51 ohm resistors will be fairly large in physical size (compared to a 1/4 watt size) and should be rated for 5 watts of power or more. Use caution and do not touch any components while the circuit is connected to the AC line.

Adding a Buzzer:

The relay shown in parallel with the 50 ohm cathode resistor can be used to momentarily power a buzzer with an external circuit through the contacts. The 1000 uF capacitor causes the relay to energize for about one second, longer times can be obtained with a larger capacitor.

Parts List:

Quantity       Description                   Radio Shack Part Number

1         4 Amp/400 Volt Bridge Rectifier            276-1173
3         Silicon Controlled Rectifier (SCR)         NTE5457
3         120 VAC/ 25 Watt incandescent lamp
1         50-100 microfarad/ 200 volt capacitor
1         1000 microfarad / 35 volt capacitor        272-1032
1         50 ohm resistor/ 5 or 10 Watt              271-133
3         Push Button Switch (normally open)
1         Push Button Switch (normally closed)
3         2K resistor, 1/4 watt                      271-1325
4         1N914 Diode
1         51K resistor, 1 watt
1         2 Amp Fuse                                 270-1064
1         Relay (SPDT) 9 Volt DC, 500 ohm coil       275-005

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Battery Equal Charge Indicator

This is a Simple Electronic Circuit Project of battery equal charge indicator. The circuit below illuminates an LED to indicate unequal charges between two 12 volt lead batteries. It can be used to verify that two batteries are connected in parallel or isolated since the LED will be off when the voltages are equal within a tollerance, or on if the voltage difference is greater than 100 millivolts.

Battery Equal Charge Indicator Circuit Diagram:

Three comparators and three voltage dividers are used to determine battery conditions. The upper left comparator (+) input at pin 5 is set to about 10 volts with battery #1 at 12 volts. The negative input (pin 4) is set to a slightly lower voltage by adding an additional 240 ohms to the voltage divider so that the output of the comparator will be positive when both battery voltages are equal and negative if battery 2 rises above battery 1 by 100 millivolts or more.

The voltage at pin 5 is used as a reference for the lower comparator and the negative input of the lower comparator is set to a lower voltage with the addition of 510 ohms, so that the output will also be positive when the battery voltages are equal and negative when battery #1 is greater than #2 by 100 millivolts or more. The two comparator outputs are both connected to the positive input of the third comparator at pin 9 so that the LED will illuminate when either condition exists,
(Battery #1 > Battery #2) OR (Battery #2 > Battery #1).

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Simple and Small Bench Amplifier

This is very simple and small A small bench amplifier circuit. 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.

Bench Amplifier Circuit Diagram:

Bench Amplifier Circuit Image:

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.

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Alternating Square Pulse Generator

The generator circuit was designed to produce alternating square pulses with vibrations in different parts of the circuit.

4027 – a dual JK flip flop that has independent clock, set, and reset inputs for each flip flop used in toggle, register, and control functions due to its features such as capability of driving two low power TTL loads, logic edge clocked flip flop design, logic swing independent of fanout, toggle rate of 3 MHz at 5 Vdc, supply voltage range of 3 V to 6 V, protection of diodes on all inputs, noise immunity, and quiescent current of 2 nA at 5 Vdc. 4001 – a quad 2-input NOR gate integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption.

Alternating Square Pulse Generator Circuit Diagram

BC550 – an NPN general purpose transistor with low current and low voltage used for low noise stages in audio frequency equipment

The generator functions without distortion when it reaches 100 KHz. The square pulses are produced when the selector switch S1 is turned ON, which matches the output of Q3 at pin 7. The 5V pulse line is applied to J1 input, the signal is fed to T-flip flop IC1A through S2 which creates pulses indicated at half of the duration time where pin 2 handles the division of pulse frequency. The pulses are applied to IC2 pin 14 and IC3A input. IC2 acts as decimal counter with decode outputs where each entry produces HIGH on one of the outputs. From the initial pulse entry, the output of Q1 becomes HIGH while others are LOW. The second pulse entry causes HIGH output on Q2, and third entry applies on Q3. The same operation occurs for the succeeding pulse line entry, since the counter checks the number of pulses that passes the generator output.

The generator output stage is considered on levels where the driving stage of Q2 creates positive output voltage and the saturation of Q 3 in the cutoff region. Through the potentiometer, the signal is applied in the output of J2. Using the gates of IC3B-C-D, the sine wave or triangular wave can be changed to square pulses in the circuit input. The conversion is directly made from the command of S2. The switching can also be done by DIP switch S2. A suitable power supply or two NiCd batteries can maintain the circuit with the stabilization of voltage achieved by two Zener diodes.

R1-10= 10Kohm
R2= 47Kohm
R3= 22Kohm
R4-5= 18Kohm
R6-7= 4.7Kohm
R8= 1.2Kohm
R10= 100Kohm
R11-13= 470 ohm
R12= 1.5Kohm RV1= 1.2Kohm linear pot.
C1= 15pF ceramic
C2-3= 10nF 63V MKT
C4-5= 470uF 16V
C6= 1uF 63V MKT
D1-2= 5.1V 0.5W zener
D3-4= 1N4148
Q1-2= BC560C
Q3= BC550C IC1= 4027
IC2= 4027
IC3= 4001B
S1= switch DIL 10S
S2-3= 2X2 mini switch
LD1= LED
BATT= 9V Battery NiCd

The generation of square pulses can bring about plenty of usages by adjusting the input in digital circuits and controlling the frequencies of amplifiers, loudspeakers, rooms of hearings, and others. One popular application is in the camera flash temporal profile where square pulses act as a heating source in the photoflash technique. The amount of light emitted by an electronic flash is controlled by the shunted output of a pulse generator known as tailbiters where longer pulses are made with the biting off the tail of the impulse.

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Simple But Reliable Car Battery Tester

This circuit uses the popular and easy to find LM3914 IC. This IC is very simple to drive, needs no voltage regulators (it has a built in voltage regulator) and can be powered from almost every source. This circuit is very easy to explain: When the test button is pressed, the Car battery voltage is feed into a high impedance voltage divider. His purpose is to divide 12V to 1,25V (or lower values to lower values).

This solution is better than letting the internal voltage regulator set the 12V sample voltage to be feed into the internal voltage divider simply because it cannot regulate 12V when the voltage drops lower (linear regulators only step down). Simply wiring with no adjust, the regulator provides stable 1,25V which is fed into the precision internal resistor cascade to generate sample voltages for the internal comparators. Anyway the default setting let you to measure voltages between 8 and 12V but you can measure even from 0V to 12V setting the offset trimmer to 0 (but i think that under 9 volt your car would not start).

 Car Battery Tester  Circuit diagram:

There is a smoothing capacitor (4700uF 16V) it is used to adsorb EMF noise produced from the ignition coil if you are measuring the battery during the engine working. Diesel engines would not need it, but I'm not sure. If you like more a point graph rather than a bar graph simply disconnect pin 9 on the IC (MODE) from power. The calculations are simple (default)
For the first comparator the voltage is : 0,833 V corresponding to 8 V
* * * * * voltage is : 0,875 V corresponding to 8,4 V
for the last comparator the voltage is : 1,25 V corresponding to 12 V
Have fun, learn and don't let you car battery discharge... ;-)

author: Jonathan Filippi
e-mail: jonathan.filippi@virgilio.it

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Tri-Waveform Generator

The Tri-Waveform Generator can be used for a number of different uses. The one that I use it for is a signal generator to test circuits. The frequency range is 20 to 20khz. and can be adjusted by R1. The duty cycle or the time that the waveform is high and the time that the waveform is low can be adjusted by R4. The purpose of R2 and R3 are to clean up any distortion on the sine wave output. To do this you must hook up the sine wave output to and oscilloscope and adjust R2 & R3 to make the sine wave as accurate as possible.

Tri-Waveform Generator Circuit Diagram

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Simple Circuit Transistor Checker

This simple circuit has helped me out on many occasions. It is able to check transistors, in the circuit, down to 40 ohms across the collector-base or base-emitter junctions. It can also check the output power transistors on amplifier circuits.

Simple Circuit Transistor Checker Schematic

Circuit operation is as follows. The 555 timer ( IC1 ) is set up as a 12hz multi vibrator. The output on pin 3 drives the 4027 flip-flop ( IC2). This flip-flop divides the input frequency by two and delivers complementary voltage outputs to pin 15 and 14. The outputs are connected to LED1 and LED2 through the current limiting resistor R3. The LED's are arranged so that when the polarity across the circuit is one way only one LED will light and when the polarity reverses the other LED will light, therefore when no transistor is connected to the tester the LED's will alternately flash.

The IC2 outputs are also connected to resistors R4 and R5 with the junction of these two resistors connected to the base of the transistor being tested. With a good transistor connected to the tester, the transistor will turn on and produce a short across the LED pair. If a good NPN transistor is connected then LED1 will flash by itself and if a good PNP transistor is connected then LED2 will flash by itself. If the transistor is open both LED's will flash and if the transistor is shorted then neither LED will flash.

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Signal Tracer using LM386 Amplifier Chip

The main part of this circuit is the LM386 amplifier chip. It also uses a transistor input to buffer the input signal and provide extra gain for the LM386. The little unit has helped me out on numerous occasions when trouble shooting any amplifier circuit like a stereo receiver, tv / vcr audio section, radios, cd players and car stereos.

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Glitch Detector Schematic

In the circuit, two op amps (half of an LM324 quad op amp) and an SCR are direct coupled in a de-voltage monitoring circuit. Op-amp U1-a is configured as a voltage follower, which feeds the bridged inputs of the second op amp, Ul-b. A resistor/capacitor combination (R2/C1) connected to the negative input of U1-b forms an RC time-delay circuit.

Glitch Detector Circuit Schematic

As long as there is no change in the de-voltage level at either of U1-b's inputs, its output is near zero. If a voltage glitch occurs, the RC timing circuit will delay the voltage change at the op amp's inverting input, causing its output to go high, triggering SCR1 and causing LED1 to light. The circuit's sensitivity allows it to detect voltage changes in the millivolt range. Pressing S1 diverts the SCR's holding current to ground, causing it to turn off and reset the circuit.

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Digital Radar Speedometer Schematic

This circuit is a Digital Radar Speedometer. It allows us to evaluate the speed of any object moving, especially cars and other vehicles. The speed is calculated in kilometers per hour (KPH). Its display has three digits. This radar works with the laser reflexion. It sends laser radiation to the object and this object reflects the laser radiation to the radar. To evaluate the speed of a vehicle, we must be in front of it. In other words, the vehicle must come in our direction. The front of the radar must point the front of the vehicle. The radar has the shape of a pistol. In this radar, it has a laser LED and a laser diode. Both have a lens.

Digital Radar Speedometer Circuit Diagram

The laser LED can send a spot of light to a distance of 90 m (295 ft). It's very important that the distance range of the laser LED is 90 m, if not, the speed will not be calculated properly. The laser diode, which receives the light signal by the laser LED, must be able to detect the light which is same color as that emitted by the laser LED. The laser diode and the laser LED must be placed one beside the other. They are protected by a tinted pane. They must be placed at the front of the radar and point the outside. The radar is powered by a 9V battery and it has a SPST switch to control its power state.

The display, or the speed indicator, is placed at the rear of the radar, just on the right of the overload LED indicator. All the logic components of the circuit must be of the 74AS series and TTL type. Because they have short time of response (less than 1.7 ns) and have high frequency supports (more than 200 MHz). The radar can evaluate the speed of an object moving between 0 to 999 km/h. After this speed, the overload LED indicator will turn on and the "999" will still displayed. The radar displays the speed during 3 seconds, after this time, it displays "zero" (0).

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Signal Tracer and Injector

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.

Signal Tracer and Injector Circuit diagram

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.

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Bench Amplifier Based on LM386

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.

 Circuit Diagram

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. 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.

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Ultra Low Power LCD Indicator

This circuit serves as an ultra-low power replacement for multiple LED on-off indicators. It also has the advantage of being easy to read in full daylight. With the parts shown, it is possible to display four bits of information.

The display that I used has three digits and 2 decimal points for a total of 23 segments. Different groupings of segments can be used for the four indicators. I chose to use three squares (shown) and the three lower segments together (not shown) for the four indicators. Many other combinations could be used, one possibility would be to hard-wire numbers or letters out of each of the digits. Other LCD displays could also be used for different effects.

Circuit Diagram

A part that doesn't exist as far as I know, but should, is a single pixel LCD indicator (2 wire). An LCD manufacturer could probably make a lot of money with such a part. If such a thing exists, I'd love to hear about it.

Specifications:
Operating Voltage: 3-15V (5V Nominal) DC
Operating Current: 250 microamps to 1 milliamp (400 microamps at 5V)
Operating Frequency: approximately 60 Hz

Theory:
The 7555 IC (CMOS 555 timer) generates a square wave clock signal at approximately 60 hz. This signal is sent to the LCD backplane and the inputs of the four CMOS 4070 XOR gates. If the other input (ind*) of an XOR gate is low, the gate's output is a square wave that is in phase with the clock signal. If the ind* input is high, the gate's output is out of phase with the clock.

Sending a signal to an LCD segment that is in phase with the backplane signal causes the display to stay blank. Sending an out of phase signal to the LCD segment causes an AC waveform to be applied to the segment which turns it black. Multiple segments are wired in parallel to generate the desired display patterns. The LCD segments require a tiny amount of current to operate, the CMOS gates also take very little power, hence the efficient nature of the circuit. It is necessary to tie the unused segments to the LCD backplane, otherwise they may partially turn on.

If more dislay bits are needed, additional XOR gates can be connected in the same manner. Up to 23 XOR gates could be used to drive the entire display, but a microprocessor and driver software would probably be easier to put together. By generating all of the signals with a microprocessor, all of the driving circuitry can be eliminated.

Other logic families could be used to make this circuit, it should work with a standard 555 timer chip and a 74LS86 XOR gate (different pinout), for example.

Some LCDs may not operate at very cold temperatures, an engineer at Lumex said that their components will work from -30C to +75C.

Construction:
The circuit was built on a standard prototyping plug board. All of the parts can be purchased for under ten dollars.

Use:
The four inputs of the CMOS 4070 IC can connect to outputs on a microprocessor, or any other logic output that needs monitoring. The supply voltage of this circuit should be the same as the driving logic's supply voltage.

Parts:
1X Lumex LCD-S301C31TR 3 digit LCD display (from Digi-Key), or equivalent
1X CMOS 4070 quad XOR Gate, a CMOS 4030 should also work.
1X 7555 CMOS 555 timer chip
2X 100nF capacitor
1X 100uF 25V electrolytic capacitor
1X 10K 1/4W resistor
1X 100K 1/4W resistor

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Beat Balance Metal Detector

A Beat Balance Metal Detector made from discrete components.

Beat Balance Metal Detector Schematic

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!

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Coil Coupled Operation Metal Detector

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

Coil Coupled Operation Metal Detector Circuit Diagram

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!

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Solar Panel Current Meter

This circuit is used to measure the current from a solar panel. It has very low power loss for currents in the 0-10A range. It also works as a general purpose DC current meter. The circuit can be used on either the positive or negative side of a DC circuit.

Specifications

Measured Current: 0-10 Amps DC
Circuit Voltage: Will work with DC circuits at any practical voltage.
Accuracy: approximately 2%, depending on the meter movement.

Theory

The current to be measured flows through the 0.01 ohm resistor which causes a small voltage drop across the resistor. The 100 microamp meter is set up with the series 50 ohm and 500 ohm variable resistor in a voltage measurement configuration to measure this voltage drop. The 500 ohm variable resistor is used to adjust the meter's full scale reading. The 50 ohm resistor limits the maximum current to the meter no matter what setting is on the 500 ohm resistor, this protects the meter from passing too much current and burning up. The series resistance of the meter, 500 ohm (or less) variable resistor and 50 ohm resistor should total 1000 ohms. Different meters may require a different variable resistor to achieve the 1000 ohm value.

Construction

Build the meter into a metal box with the meter and two connectors mounted on the outside of the box.

Alignment

Put the meter circuit in series with a known current meter such as a digital VOM meter set to measure current. Run a known current through both meters. Adjust the 500 ohm resistor until both meters read the same current. A good way to get a known current is to put a 12V lead acid battery in series with a 2 ohm 100 watt current limiting resistor. This will produce approximately 6 Amps of current. Put the two meters in series with this loop and adjust for the same reading. Beware, the resistor will get fairly hot in a short time.

Use

Connect this circuit in series with a nominal 12V or 24V solar panel array. The meter can go in either the positive or negative side of the solar panel circuit. The current flowing through the solar panel to the load will be shown on the meter.

Parts

1x 100 microamp DC meter
1x 0.01 ohm 5 W resistor
1x 50 ohm 1/4 W resistor
1x 500 ohm 10 turn variable resistor
2x banana plugs or a 2 pin screw type terminal block.
1x metal box

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