Watchdog

+12V p" 0 10n 12V 8OrnA max This timer automatically switches off equipment left operating unattended for more than about thirty minutes. The circuit operation is readily understood by following its power-on and time-out functions. Almost im- mediately after S4 has been de- pressed, relay contact rela closes to power Tn and the equipment con- nected to the mains outlet. This thus happens because the initial pres- ence of the + 12 V supply voltage in the circuit causes counter-oscillator ICI and set/reset (S/R) bistable Ni-N2 to be reset by means of a short, logic high pulse at the junction of RI and CI. The outputs of Ni and N3 go high and low respectively and T4 can energize Rel. So far for the power-on automatic hold function of contact rela. After being reset, ICI starts counting down its on-chip generated clock pulses which have a frequency of about 2 Hz. LED Di flashes at this rate to indicate the countdown con- dition. Note that S2 has been pro- vided to reset, i.e. disable ...