This circuit is from an August 1984 Radio Electronics magazine. It monitors the state of a RAM backup battery. Resistor R1 determines the battery charge.
Found in Popular Electronics from the 1980s, this circuit is intended for switching filters with...
The BA1404 integrated circuit (not very easy to obtain) consists of a complete stereo FM...
This transmitter uses an electret microphone and has a few tens of meters in range. The coil L1 is...