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.
This circuit was obtained in the publication 101 Electronics Projects of 1979. The circuit can be...
This circuit was found in 80's Popular Electronics. Transistors are commonly used. The...
We found this circuit with 5.3V zener in a Philips publication from the 60s. The op amp could be a...