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700Hz Pulse Generator & Signal Tracer Circuit

Description

This simple circuit generates narrow pulses at approximately 700-800Hz frequency. The pulses, encompassing harmonics extending up to the MHz range, can be injected into audio or radio-frequency stages of amplifiers, receivers, and similar equipment for diagnostic assessments. A discernible high-pitched tone can be heard through the speaker of the device under test when the circuit is functioning correctly. The clip must be connected to ground by touching it with the probe to various stages of the circuit, commencing from the last stage and progressing towards the initial stages. When the tone ceases, the defective stage has been identified.

Connecting an earclip or headphones to J1 will automatically transition the circuit into a two-stage amplifier, and any audio signal emanating from the device under test and captured by the probe will be conveyed through the headphones. The testing of a circuit should be conducted in a reverse manner, commencing from the first stage and descending until the final stage. When no sound is detected, the malfunctioning stage has been located.

Circuit diagram:

Pulse Generator And Signal Tracer Circuit Diagram-Circuit diagram

Parts:

  • 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.)

Circuit operation:

Q1 & Q2 form a complementary astable multivibrator, and its operating frequency is primarily determined by the values of R3, C2, and C3. The output pulses are extracted from the collector of Q2 and applied to the probe via decoupling capacitor C1. D1 provides a symmetrical shape for the output waveform.

Notes:

  • If you intend to use the circuit to assess valve-operated devices, C1 must be a 630V type. When operating with low-voltage supply transistor devices, the voltage of C1 can be reduced to 63 or 100V.
  • Instead of a short probe, if connecting the circuit to the device under test using a wire exceeding a few centimeters, a small ceramic capacitor (470 to 1000pF) should be incorporated in parallel with D1 to prevent undesirable RF oscillations.
  • The current draw when in Pulse-Generator mode is approximately 60µA and 1.2mA when operating in Signal-Tracer mode. Therefore, SW1 can be omitted, provided the earclip or headphones are disconnected when the circuit is not in use.
  • J1 is a stereo switched jack socket wired to create a series connection of the two earpieces, forming a stereo headphone. This configuration increases the load impedance, thereby enhancing sensitivity.
  • Consequently, the higher the load impedance, the more sensitive the Signal-Tracer will become. Common 32 Ohm impedance mini-headphones suitable for walkman sets will function effectively.
  • A crystal (high impedance) earpiece represents a suitable solution, provided J1 is substituted with a mono switched jack socket.
  • The entire circuit can be readily housed within a pen-like enclosure, with the probe projecting like a nib.
circuit from http://www.extremecircuits.net/2010/04/pulse-generator-and-signal-tracer.html