Category Archives: Testing

Game Show Indicator Lights (Who’s First)

This circuit 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.

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 resistors (1K and 50 ohm) 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.

Copyright 2007 [url=]Bill Bowden[/url]Parts:1x 4 Amp/400 Volt Bridge Rectifier
3x Silicon Controlled Rectifier (SCR)
3x 120 VAC/ 25 Watt incandescent lamp
1x 50-100 microfarad/ 200 volt capacitor
1x 1000 microfarad / 35 volt capacitor
2x 51 ohm resistor/ 5 or 10 Watt
3x Push Button Switch (normally open)
1x Push Button Switch (normally closed)
3x 2K resistor, 1/4 watt
4x 1N914 Diode
1x 51K resistor, 1 watt
1x 1K resistor, 1 watt
1x 2 Amp Fuse
1x Relay (SPDT) 9 Volt DC, 500 ohm coil

120 and 240Vac LED Voltage Indicator

This circuit, designed on request, has proven to be useful to indicate when the voltage in a power supply line is changing from 120V to 240Vac. It can be used in different circumstances and circuits, mainly when an increase in ac or dc supply voltage needs to be detected.
D3 illuminates when the line voltage is approaching 120V and will remain in the on state also at 240V supply. On the other hand, D6 will illuminate only when the line voltage is about 240V and will stay on because the latching action of Q1, Q2 and related components.
C1, D1 and D2 provide a low dc voltage in the 4.5V – 6V range in order to allow proper operation of latch circuit and LEDs.

* D4 value could require some adjustment in order to allow precise switching of the circuit at the chosen voltage. If the case, please try values in the 8.2V – 15V range.
* Warning! The circuit is connected to 240Vac mains, then some parts in the circuit board are subjected to lethal potential! Avoid touching the circuit when plugged and enclose it in a plastic box.

Source: [url=]RED Free Circuit Design[/url]

Copyright Flavio DellepianeParts:1x 470R 1/2W Resistor (R1)
1x 220K 1/4W Resistor (R2)
2x 470R 1/4W Resistors (R3,R7)
1x 1K 1/4W Resistor (R4)
1x 2K2 1/4W Resistor (R5)
1x 330R 1/4W Resistor (R6)
1x 330nF 630V Polyester Capacitor (C1)
1x 10µF 25V Electrolytic Capacitor (C2)
2x 1N4007 1000V 1A Diode (D1, D2)
2x LEDs (Color and shape at will) (D3, D4)
1x BZX79C10 10V 500mW Zener Diode (See Notes) (D4)
1x 1N4148 75V 150mA Diode (D5)
1x BC547 45V 100mA NPN Transistor (Q1)
1x BC557 45V 100mA PNP Transistor (Q2)

CCO Metal Detector

The metal detector shown here has, in concept, been widely recognised as a new genre. The general concept, of which I have developed three embodiments, is capable in principle of matching the performance of an Induction Balance (IB) metal detector. This is the first embodiment to be released on the Internet (the other two were published in Everyday Practical Electronics and Elektor magazines). When this circuit is correctly set up, an old Victorian penny (30mm diameter) should induce a shift in frequency of at least one tone on the Medium Wave band at 140mm (5½").

Apart from using two overlapping coils, the concept is fundamentally different to IB. Unlike IB, its Rx section is an integral part of the oscillator. Further, unlike IB, the design does not require the critical placement of the coils, which should have significant advantages for manufacture. A special characteristic is that sensitivity covers a wide area of the coils, thus making the design well suited to sweeping. It also provides discrimination. Further, while the design uses a beat frequency oscillator (in this case a MW radio), it differs fundamentally from a BFO metal detector. Its performance far outstrips that of BFO — and further, unlike BFO, it is dependent on the mutual inductance of two coils (BFO, of course, uses only one).

The circuit should be instantly recognisable as a transformer coupled oscillator (TCO) — a well known oscillator type. This essentially consists of an amplifier which, by means of a transformer, feeds the output back to the input, thus sustaining oscillation. In the circuit, the TCO transformer is replaced with two search coils, L1 and L2. These have the same action as the transformer in a TCO, L2 being the "transmitter", and L1 the "receiver". On the basis of its similarity with a TCO, I named this metal detector a Coil Coupled Operation (CCO) Metal Detector. The presence of metal induces changes both in the inductance and the coupling of the two coils, thereby inducing a shift in the oscillator frequency. A single stage common emitter amplifier provides 180 degrees phase shift, and the "transformer" provides a further 180 degrees. Base bias is provided by R1, and C1 provides decoupling. Depending on the placement of the coils, the oscillator frequency is around 200kHz. In the absence of a 2N3904 for TR1, a BF494 or BC109C may be pressed into service.

The two coils are each made of 50 turns 30swg (0.315mm, or 22awg) enamelled copper wire, wound on a 120mm (4¾") diameter former. Each coil has a Faraday shield, which is connected to 0V as shown. This is essentially a tin or aluminium foil screen, which does not quite make the full circuit of the coil — a gap of 10mm or so is left open. The coils are positioned side by side on the search head, with their beginning (B) wires to the left, and end (E) wires to the right. They are wired to the circuit as shown. The circuit will sustain oscillation with wide variations of coil overlap, and the best degree of overlap may be found through trial and error. The circuit is connected to a Medium Wave radio aerial by means of a screened cable as shown, and a suitable heterodyne is tuned in.

I present the circuit here merely as a bare bones or experimental idea, and look forward to seeing its further development in the future. To give an indication of what the concept is capable of, the Elektor design obtained nearly one-third better performance. I would welcome comments at my e-mail address However, while I would like to reply to all mail, I cannot guarantee that I shall be able. Happy hunting!

Copyright [url=]Rev. Thomas Scarborough[/url]

Super Probe, 16 functions with very few parts

The Superprobe project was designed to see how much could be done with a PIC chip and just a few parts. The image at the right shows the capacitance measuring mode. This device is designed around a PIC16F870, a 4 digit LED display module and very little else.


[i:eb8cceb438]Note: I have recieved a lot of inquiries on this project. To date, several have been duplicated world-wide. Many have been constructed in other types of cases. As long as the circuit is wired as shown, and the object code (below) is programmed into the PIC, the devices all worked perfectly.[/i:eb8cceb438]

Also, I get a lot of requests for translating the source file for MPASM. If anyone has done this, please send me a copy and I will post it on this page.

Also note: If you try to use the object file and your programmer ‘gags’ on it, try removing the second to last line which is simply a chip type designator.

The LTC4627 display is sold by [url=]Mouser[/url] as [url=]512-MSQC4911C [/url] for $2.50 in single quantity. The LM2931 is a low drop out regulator also sold by Mouser (24 cents).

The regulator allows the unit to operate on 5 volts or as much a 30 volts. It also provides 30 volts of reverse polarity protection.

As you can see in the schematic even the usual resistors associated with driving the display have been eliminated. Usually, separate resistors need to be used in series with each segment drive in order to evenly drive the display. The PIC chip, however, limits the current flow to about 25 ma per line. The software is written in such a way that only one segment is active at any one time. This eliminates the effect of multiple segments having to share the same current source at the same time and dimming some digits more than others.

Various testing modes use the resistors in different ways. Unused resistors for any particular mode are removed from the circuit by having their pic pins floated. R5, for instance, is used for the logic pulser funtion. R4 is used to charge a capacitor to measure its value.


The unit was built into a case from a scrapped Radio Shack logic probe (sold for $8 at a closeout price). Similar cases can be found if you look around a bit. All of the original electronics were removed and one side was cutout for a piece of transparant red plexiglass. The circuit was built up on pad-per-hole perf board.

Operation is via 2 pushbuttons. Holding down button #2 while pushing button #1 cycles through operating modes…..

Prob PULS FrEq Cnt VoLt diod CaP SIG ntSC 9600 Midi r/c [] Prn ir38 PWM
(Done on the 7 segment displays).

Here are how each of the modes work at present….

[b:eb8cceb438]Prob – Logic Probe[/b:eb8cceb438]
The logic probe shows ‘H’ for high (over 3.7 volts), ‘L’ for low (below 0.8 volts) and ‘-‘ for floating in the first display location. If a pulse is detected (0.5 usec minimum), the second location flashes a ‘P’.

[b:eb8cceb438]PULS – logic pulser[/b:eb8cceb438]
The logic pulser shows the pulse rate (5, 50, 500, 5.0) in the last 3 locations. The first location shows the sensed logic level as a dash in the bottom or top of the digit. When button #1 is held down, a series of 0.5 microsecond pulses are generated in the opposite direction and the center segment is lit. Pushing button #2 cycles thru the 4 pulse rates. The selected pulse rate is saved on power down.

[b:eb8cceb438]FrEq – Frequency Counter[/b:eb8cceb438]
In the frequency counter mode, hitting button #1 switches the display to the next 4 digits of the count. For instance, the display shows ‘12.57’ for a frequency of 12,576 hz. Holding down button #1 shows ‘2576’ – the lowest 4 digits. If a decimal point shows, the value is in Khz, if the decimal is flashing, the value is in Mhz. Hence, a frequency of 42,345,678 hz is displayed as 42.34 with a flashing decimal. Holding down button #1 in this case will display 5678.

[b:eb8cceb438]Cnt – Event Counter[/b:eb8cceb438]
In the event count mode, the display shows the lowest 4 digits. Button #1 switches to the next higher 4 digits while held down. Button #2 resets the count.

[b:eb8cceb438]VoLt – Voltmeter[/b:eb8cceb438]
The voltmeter uses the power going into the probe as a voltage reference. The current implementation shows only an approximate voltage – about 2% high. This can still be very usefull for most measurements. Do not connect the probe to voltages that exceed 5 volts under any conditions.

[b:eb8cceb438]diod – Diode Junction Voltage[/b:eb8cceb438]
This is just the voltmeter function with 10k resitor feeding current to the probe tip. When a diode or transistor junction is connected from the tip to the ground lead, the drop voltage is displayed.

[b:eb8cceb438]Cap – Capacitance Measurement[/b:eb8cceb438]
When a capacitor is connected from the tip to the ground lead, and button #1 is pushed, its value is displayed. Values from .001 uf to about 500 uf are displayed. The larger the capacitor, the longer it takes to measure. A value of 100uf takes a couple of seconds.

[b:eb8cceb438]SIG – Signal Generator[/b:eb8cceb438]
This mode generates a 500hz squarewave at about 0.5 volts. The signal is only generated while button #1 is held down.

[b:eb8cceb438]ntSC – Video Patern[/b:eb8cceb438]
Generates an NTSC video frame with a white dot pattern when button #1 is held down.

[b:eb8cceb438]9600 – Serial Ascii[/b:eb8cceb438]
Each time button 1 is pushed, the letters A-Z followed by cr/lf is generated. Auto polarity sensing. If the signal injection point is orignally high, then normal (zero start bit) ascii is generated. Otherwise, the other polarity is done. New feature: Button #2 cycles thru 1200, 2400, 4800, 9600 baud.

[b:eb8cceb438]Midi – Midi Note[/b:eb8cceb438]
Sends note number 60 (middle C) on any of the 16 midi channels. Holding button 1 sends ‘note on’. Release of button 1 sends ‘note off’. Buttton 2 cycles thru the 16 channels. The midi channel number is stored.

[b:eb8cceb438]R/C Servo[/b:eb8cceb438]
Generates 1ms to 2ms pulse for r/c servos. Button 1 increases pulse, Button 2 decreases pulse. Defaults to 1.5 ms each time mode is entered.

[b:eb8cceb438][ ] Sqaure Wave[/b:eb8cceb438]
Generates 1 – 9999 hz squarewave. Button 1 decreases frequency, Button 2 increases frequency.

[b:eb8cceb438]Prn – Pseudo Random Number[/b:eb8cceb438]
Generates 10khz digital PRN series.

Generates 1 millisecond on and 2.5 millisecond off of 38khz square wave. When connected to IR LED, used for testing IR reciever modules.

Generates variable pulse width 3-97 percent of a 6khz (approx) digital signal. Button 1 decreases pulse width, Button 2 increases pulse width.

In any mode, holding down both buttons exits to the menu. Once there, releasing and pressing button #1 cycles thru its modes forward. New Feature: Button #2 cycles thru modes backwards.

The mode is saved on power down. Since this device takes its power from the circuit being tested, powering down and back up will restore the same operating mode.

For anyone interested, the Source Code and Object Code are available along with my own PIC assembler, see below.

Any other suggestions for this project? See email address on [url=]home page[/url].

Copyright 2006 [url=]Luhan Monat[/url]Parts:a PIC16F870
a 4 digit LED display module
very little else

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.

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!

Copyright Rev. Thomas Scarborough
[Contact the author of this article at]

Beat Balance Metal Detector

A Beat Balance Metal Detector made from discrete components.

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!

Copyright []Rev. Thomas Scarborough[/url]
[Contact the author of this article at ]