This LED diode test is suggested by the Texas Instruments optoelectronics manual. The transformer current can be between 50 mA and 300 mA and the voltage between 6 and 9 V.
The circuit is from 1968, but we can use a modern one like the 741. The symmetrical power supply...
The circuit presented uses Linear Technology Operationals, having been found in a 1993 manual. The...
Found in American documentation from the 90s, this circuit triggers a buzzer with a Sonalert-type...