The transistors of this circuit found in an English 1980s documentation may be the modern BC548 equivalents. The circuit is battery powered and the earphone must be crystal.
We found this configuration in a 1974 technical documentation. It can test both TTL logic circuits and...
This circuit emits an alert sound (interval beeps) if the car lights are left on and the ignition key...
This is an amplifier that uses a pickup coil placed next to the telephone set and the listening is done in...