Pulse-Generator & Signal-Tracer

Dual-purpose test-instrument

Very simple circuitry, 1.5V Battery-operated

Circuit diagram:

Pulse-Generator & Signal Tracer


R1__________________1M   1/4W Resistor
R2,R4_______________2K7  1/4W Resistors
R3________________150K   1/4W Resistor

C1__________________2n2  630V Ceramic or Polyester Capacitor (See Notes)
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 Slider Switch (See Notes)

J1_________________Stereo switched 3mm. Jack socket (See Notes)

Probe______________Metal Probe 3 to 5 cm. long

Clip_______________Miniature Crocodile Clip

B1_________________1.5V Battery (AA or AAA cell etc.)

Device purpose:

This simple circuit generates narrow pulses at about 700-800Hz frequency. The pulses, containing harmonics up to the MHz region, can be injected into audio or radio-frequency stages of amplifiers, receivers and the like for testing purposes. A high-pitched tone can be heard from the speaker of the device under test when all is working properly. The clip must be connected to the ground of the device under test, touching with the probe the different stages of the circuit, starting from the last stage and going up towards the first. When the tone is no longer heard, the defective stage has been found.
Connecting an earclip or headphone to J1, the circuit will automatically change into a two-stage amplifier and any audio signal coming from the device under test and picked-up by the probe will be heard through the headphones. The testing of a circuit should be made in the reverse manner, i.e. starting from the first stage and going down until the last stage. When nothing is heard, the defective stage has been found.

Circuit operation:

Q1 & Q2 form a complementary astable multivibrator, whose operating frequency is set mainly by R3, C2 & C3 values. Output pulses are taken at Q2 Collector and applied to the probe by means of decoupling capacitor C1. D1 provides a symmetrical shape for the output waveform.
If an earclip or headphone jack is plugged into J1, the connection from Q2 Collector and C1 - C2 is broken by the switch incorporated into J1: in this case the circuit becomes a two-stage amplifier.