This simple circuit can work with either 4.5 V or 6 or 9 V if a 1k resistor is added in series with the set of LEDs. The circuit is from an old French publication.
This well-known circuit triggers a relay when light striking the phototransistor is cut off. The...
In this circuit the transistor switches when the voltage reaches the pre-set value in the trimpot. The...
This rectangular signal generator was found in an Electronics World of 1966. The transistors are...