Presettable 99.9 Seconds Countdown Timer circuit

Thread Starter


Joined Jul 28, 2018
Hi! My name is Rex and a newbie electronic enthusiast.
I'm trying to create a presettable 0-99.9 seconds countdown timer that will drive a solenoid when the coundown reaches zero.

1. Set the BCD thumbwheel switch according to the desired starting countdown time. (00.1 - 99.9 seconds)
2. Upon pressing of the trigger switch,
the counter IC will be load the value of the above mentioned settings,
disable the input trigger switch,
and starts counting down.
3. Upon reaching zero,
a transistor will drive a solenod;
the load value is reset based on the setting in procedure 1;
and the tigger input is enable again.
4. Should the trigger switch is still pressed,
proceed to procedur 2-3.
Else if not, counter will wait until reigger switch is pressed again.

Main Componenet:
1x 555 timer IC
3x 74193 countdown counter IC (or equivalent)
3x BCD thumwheel switch
1x spst push button switch

I have attach a reference circuit.

Thank you very much in advance for all the help that will be contributed in this project.
Have a nice Day




Joined Jan 29, 2010
hi Rex,
Welcome to AAC.
If you want to use BCD switches for setting the count/time in 0.1s intervals, you need 3 BCD Down counters say 74LS192.
Also a 555 timer astable set for 10Hz.


Joined Mar 10, 2018
On power up do you want no false triggering to occur ?

Also you show no pullups or pulldown Rs for the BCD inputs. Eg. you have
floating inputs to counter when switch is open.

The counter/timer can all be done inside a UP, is that of interest ?

Regards, Dana.