The configuration shown was found in an old Poptronics but can be easily assembled using a common calculator photocell. The relay is according to the supply voltage.
This low power circuit can be powered with voltages from 1.8 V. We find it in a 80s Hands On...
The voltage-gain for this amplifier is 10. Other operational amplifiers can be use and the Power...
Found in an American publication, this circuit applies to almost any operational amplifier. The power...