We found this circuit in a 1971 documentation. The circuit can be assembled with more modern transistors such as BC548 for Q1, BC558 for Q2 and BD135 for Q3.
Signals ranging from a few millivolts to a few volts can be used to excite a relay with this...
The frequency of the signal produced by this circuit depends on the temperature of the diode used...
This simple digital circuit tester can work with TTL technology when powered by 5 V and with CMOS...