Timer chip 1CM7250 from Maxim enables a very simple timer to be designed. The IC has eight programmable inputs for connecting to say two thumbwheel switches. Units are set in BCD code on the four lowest inputs. pins 1-4, and tens on the highest, pins 5-8. Thus, the user can select any time between 00 and 99 units.
The length of each time unit is determined by P1-R2-R3-C1. In this particular application time units of one second have been assumed. Other units are easily selected by a few small changes.
According to the relevant data sheet, the maximum overall resistance value of the timing network must not exceed 22 MΩ. while the value of the capacitor must not be smaller than 10 pF. Note that if longer units are set, capacitors with small leakage currents must be used.
When switch S 3, is pressed, an internal bistable is reset and the counter is started. The output of IC1 (pin 10) -then changes state (becomes low). After the elapsing of the set number of time units, the output becomes high again.
The supply voltage for the timer may be 2-9 V, in spite of the data sheet stating that up to 16 V is permitted. In the prototype, the output was permanently high with supply voltages of 9.5–16 V.
The relay connected to the output is switched by a power transistor, T1. ThiDarlingtonon can switch currents of up to 4 A. The circuit itself draws a current of only 1 mA.