The timing circuit is built using XR2240 integrated circuit manufactured by EXAR. The circuit is intended for use in timing circuits in various applications such as TV and radio equipment for monitoring battery charge and similar photo applications. Timer can be set between 15 minutes and 24 hours and 45 minutes, with two switches BCD (binary coded decimal).
Timing can be programmed in steps that are detected time constant R1 (C5 C6) = 15 minutes. BCD switches allow selection of up to 255 such periods. Timer functions are controlled by an internal flip-flop (flip-flop) and the switches S1 and S2. Period to the accuracy, C5 and C6 should be carefully selected.
Tension common point of D1-R2-R3-R8 is so low that T1 remains blocked. After draining the time fixed pin 10 of IC2 becomes logical "1" so that the said common point is the supply voltage. LED turns off T1 and relay RE1 is activated leads and contact or switch