This audio oscillator is from a 1957 Raytheon publication. The tone generated depends on C2 and C2 and the handset must be high-impedance magnetic. Equivalent transistors can be tested.
This circuit uses a TL071 to differential amplify the signals from a microphone and thus reduce...
We found this circuit in a 1995 publication. It makes use of infrared LEDs of the type existing at...
This circuit is from September 1984 Radio Electronics. The frequency can be changed by changing C1 and...