The timer, based on EXAR’s Type XR2240, is intended for use in photographic and printed-circuit departments, as a standby switch for TV and radio equipment, as a battery charging monitor, and similar applications. The timing can be set between 15 minutes and 24 hours 45 minutes with two BCD (binary coded decimal) switches.
The timing is programmed in steps, T, which are determined by the time Constant R1( C5 + C6)= 15 minutes. The BCD switches allow up to 255 such periods to selected. The timer functions are controlled internal switch bistable (flip-flop) and switch S1 and S2. To keep the period accurate, C5 and C6 must be carefully selected.
Diode D1 lights when the timer is switched on. The voltage at junction D1-R2-R3-R8 is then so low that T1 remains off. When the set time has elapsed, pin 10 of IC2 becomes logic high, whereupon the supply voltage exists at the junction. The LED then goes out. T1 conducts and relay Re] is energized, so that its contact changes over.
The supply voltage to the timer is stabilized by regulator IC1. Capacitors C1 -C4 smooth the supply voltage to prevent the timer being triggered by pulses on this voltage.