Circuit Diagram: RC Timer (With Op-Amp 741)
RC Timer, How it Works
The circuit uses RC circuit in combination with an Op-Amp Comparator to accomplish the charging characteristic of a timer. In the above diagram
a voltage is applied across the R1-C1 combination. The input for the Op-Amp is tapped at the point where R1 and C1 meets. This is the point where the positive terminal
of the capacitor gradually charges to the value of the power supply. As the capacitor C1 reaches the Op-Amp's reference voltage, the Op-Amp's output pin turns from High to Low, allowing current to flow
through resistor, R3, then through the LED (causing the LED to glow). The time delay can be controlled by changing the values or R1 or C1 or by adjusting the value of the trimmer
resistor. For a new timer cycle to occur, you must discharge capacitor, C1.