Build Your Pulse Generator: 1.5V Circuit Explorer

Simple test-instrument

Very basic circuitry, 1.5V Battery-powered

Circuit diagram

Parts:

Device purpose:

This uncomplicated circuit generates narrow pulses at approximately 700-800Hz frequency. The pulses, including harmonics up to the MHz region, can be injected into audio or radio-frequency stages of amplifiers, receivers, and similar devices for diagnostic purposes. A high-pitched tone can be heard from the speaker of the device under test when operation is correct. The clip must be connected to the ground of the device under test, making contact with the probe at various stages of the circuit, starting from the final stage and proceeding toward the initial one. When the tone vanishes, the defective stage has been identified.

Connecting an earclip or headphones to J1, the connection from Q2 Collector and C2-C3 is interrupted by the switch incorporated into J1: in this instance, the circuit converts into a two-stage amplifier and any audio signal emitted from the device under test and captured by the probe is reproduced through the headphones. The testing of a circuit ought to be carried out in the reverse manner, namely, commencing from the first stage and proceeding towards the final one. If nothing is heard, the defective stage has been found.

Circuit operation:

Q1 & Q2 form a complementary astable multivibrator, whose operating frequency is primarily determined by R3, C2 & C3 values. Output pulses are taken at Q2 Collector and applied to the probe via decoupling capacitor C1. D1 provides a symmetrical form for the output waveform.

More about components

The BC547 and BC557 are NPN and PNP bipolar junction transistors, respectively. These transistors act as switches and amplifiers in the circuit. Bipolar junction transistors are three-terminal semiconductor devices. They consist of a base, a collector, and an emitter. They are used to amplify or switch electronic signals. The transistors’ operating voltages (45V) and currents (100mA) are chosen to ensure sufficient performance within the circuit’s operating conditions.

Notes: