Using the ICM7250 timer integrated circuit manufactured by Maxim can be build a simple timer using few electronic components. The circuit has eight programmable inputs for connecting.
Units are set in BCD code at the four lower inputs pins 1 + 4, and tens to the upper pins 5-5-8. Thus, user can select any type between 00 and 99 units.
During each unit of time is determined by P1-R2-R3-C1. In this application have been set for unit a second.
On pressing switch S3, a flip-flop resets and starts the internal counter. Exit of IC1 (pin 10) then changes state (goes to "0"). After leak of fixed time units, output is again "1".
Voltage of the timer can be 2 -5 to 9 V. relay connected to the output is controlled by a power transistor, T1. The Darlington can switch currents up to 4 A.
The circuit itself consumes a current of 1 mA no-more.