ICs 555 and 4017 are well-known to electronics hobbyists. Presented here is a very interesting circuit of an LED stop watch using timer 555 and decade counter 4017 ICs. This stop watch is able to monitor time for one second to 60 minutes. It can be used in sports, laboratories etc.
IC 555 (IC1) is the heart of the circuit. It is used in a stable multivibrator mode. The special feature of this mode is that if this pin 4 of IC1 is kept at a high logic, it produces clock pulse normally but as soon as pin 4 gets low level logic, it stops to generate clock pulse and the output stays at low level.
In this circuit, pin 4 of IC1 is wired to ground through a 10 KΩ resistor and push-to-on switch SW1 is provided across the positive supply and pin 4. As soon as switch SW1 is pressed, IC1 starts generating clock pulses and the whole circuit gets activated.
AT each clock pulse, IC2, IC3, IC4 and IC5 start counting which is indicated by glowing LEDs. When switch SW1 is released, IC1 stops generating clock pulses and all counter ICs stop counting. The positions of all glowing LEDs will show how much time (pushing time of SW1) has elapsed.
IC2 counts for 1 to 10 seconds, IC3 counts for 10 to 60 seconds, IC4 counts for 1 to 10 minutes and IC5 counts for 10 to 60 minutes. Preset VR1 should be adjusted so that IC1 produces one full clock cycle in one second.
Resistor (all ¼-watt, ± 5% Carbon)
R1, R2, R4, R5 = 10 KΩ
R3 = 22 KΩ
VR1 = 100 KΩ
C1 = 10 µF/16V
C2 = 0.01 µF
IC1 = NE555
IC2 – IC5 = CD4017
SW1 = Push – T0 – On switch