This is a simple circuit that can used to generate 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. This circuit is built by two transistor. This is the figure of the circuit.
Operation of the circuit is Q1 & Q2 form a complementary unstable multi vibrator, 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. If you intend to use the circuit to test valve operated devices C1 must be a 630V type. Working with low voltage supply transistor devices the voltage of C1 can be lowered to 63 or 100V. A crystal (high impedance) earpiece is a good solution, provided you substitute J1 with a mono switched jack socket.
In below is the listing component that can be used to built up the circuit.
R1 1M 1/4W Resistor
R2, R4 2K7 1/4W Resistors
R3 150K 1/4W Resistor
C1 2n2/630V
C2, C3 4n7/ 63V
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).
Operation of the circuit is Q1 & Q2 form a complementary unstable multi vibrator, 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. If you intend to use the circuit to test valve operated devices C1 must be a 630V type. Working with low voltage supply transistor devices the voltage of C1 can be lowered to 63 or 100V. A crystal (high impedance) earpiece is a good solution, provided you substitute J1 with a mono switched jack socket.
In below is the listing component that can be used to built up the circuit.
R1 1M 1/4W Resistor
R2, R4 2K7 1/4W Resistors
R3 150K 1/4W Resistor
C1 2n2/630V
C2, C3 4n7/ 63V
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).
