this circuit made from 3x7 seg displays that discount a desired time from 1 to 3 min .
a button in the left select the time ... also under the button there is a switch PLAY/STOP discounting .
The knob besides the 555 timer is to set a value of resistance to get a nearly discounting time depending on your smartphone CPU speed.
the circuit too complicated and not easy to make for the learners (2 hours of work ) ... so if you have a question , you are welcome :)
Components :
one timer
3 displays
3 converters
3 controlers
18 AND function (they was 26)
