Pulse generator and signal tracker

Circuit diagram:

part:

R1__________________1M 1/4W resistor

R2, R4 _______________ 2K7 1/4W resistors

R3________________150K 1/4W resistor

C1__________________2n2 630V ceramic or polyester capacitor (see note)

C2, C3_______________4n7 63V ceramic or polyester capacitors

D1_______________1N4148 75V 150mA Diode

Q1________________BC547 45V 100mA NPN transistor

Q2________________BC557 45V 100mA PNP transistor

SW1________________SPST Miniature slide switch (see note)

J1_________________ Stereo switch 3mm. Socket (see note)

Probe ______________ Metal probe, 3 to 5 cm long

Clips______________Mini alligator clips

B1_________________ 1.5V battery (AA or AAA battery, etc.)

Purpose of the equipment:

This simple circuit generates narrow pulses at a frequency of approximately 700-800 Hz. These pulses contain harmonics up to the MHz range and can be injected into the audio or RF stages of amplifiers, receivers, etc., for testing purposes. When everything is working correctly, a high-pitched sound can be heard from the speaker of the device under test (DUT). The clips must be connected to the ground wire of the DUT, and the probes are used to touch different stages of the circuit, starting from the last stage and working upwards to the first. When no more sound is heard, the faulty stage has been found.

When earbuds or headphones are connected to J1, the circuit automatically becomes a two-stage amplifier; any audio signal from the device under test (DUT) picked up by the probe will be heard through the headphones. The circuit should be tested in reverse order, starting from the first stage and working downwards to the last. The faulty stage is found when nothing can be heard.

Circuit operation:

Q1 and Q2 form a complementary astable multivibrator, whose operating frequency is mainly set by the values ​​of R3, C2, and C3. The output pulse is taken from the collector of Q2 and applied to the probe through the decoupling capacitor C1. D1 provides the symmetrical shape of the output waveform.

If an earphone or headphone plug is inserted into J1, the connection from the collector of Q2 to C1-C2 is disconnected by the built-in switch of J1: in this case, the circuit becomes a two-stage amplifier.

Note:

- If you intend to use this circuit to test tube-operated devices, C1 must be of the 630V type. When operating in low-voltage transistor devices, the voltage of C1 can be reduced to 63V or 100V.

- If you intend to use a wire several centimeters or more long to connect the circuit to the device under test instead of a short probe, a small ceramic capacitor (470 to 1000 pF) should be connected in parallel with D1 to prevent unwanted RF oscillations.

- Current consumption is approximately 60µA in pulse generator mode and 1.2mA in signal tracker mode. Therefore, SW1 can be omitted if the earbuds or headphones are unplugged when the circuit is not in use.

J1 is a stereo switch jack socket. Wiring allows for a series connection of two headphones to form a stereo headphone. This method results in a higher impedance circuit and improved sensitivity.

 Therefore, the higher the load impedance, the more sensitive the signal tracker. In any case, the common 32-ohm impedance mini headphones suitable for portable music players will work well.

- Crystal (high impedance) earbuds are a good solution, provided you replace the J1 with a mono switch jack socket.

- The entire circuit can be easily installed into a pen-shaped housing, with the probe protruding like a pen tip.