This circuit is from a 1972 Motorola publication. The used transistor can still be found, but it can be used equivalent. The circuit is powered by the amplifier with which it must operate.
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...