Fliptop Timer

This circuit shows how a 4017 CMOS decade counter can be used to build a timer circuit. Pushbutton S1 will discharge capacitor C1 through resistor R2. When S1 is released, C1 will charge up through R1 causing a rising edge at the clock input of IC1. This causes the output Q1 to go high (to the supply voltage). Current will flow through R4 and LED D2 will light. At the same time C2 will begin charging through preset P1 and R6.