This filter configuration used in telecommunications uses 4 Jaumnann crystals that are interesting for low and medium frequencies. The circuit is from an 80's documentation.
The two previous circuits show the INA185 a current sensing amplifier in basic applications. This...
This metronome configuration with the 555, quite common was found in a 90s Hands On Electronics. The...
This bidirectional circuit was found in an American documentation from the 80's. The LEDs are...