We found this circuit in the November 1984 Radio Electronics magazine. The base frequency is given by C1. The power supply can be between 3 and 12 V.
In figures 1 and 2 we have two current drains that can be made with operational amplifiers and...
Great sensitivity for the circuit can be obtained with a Darlington amplification step for the...
The used diode is an unusual component today. We found this 12 V regulator in documentation from the...