This circuit acts on a relay disconnecting the load when the input voltage exceeds the set limit. The transformer is 12 V secondary according to the relay used.
This circuit for frequencies from 1Hz to 100 kHz was found in the June 972 Radio Electronics...
This circuit generates short pulses. The transistor can be a 2N2222 or BC548 and the frequency is given by...
We found this filter in a Popular Electronics from 1993. The operational amplifier can be the 741....