Bathroom Water Heater Timer

Electronic switch ES1 connects the frequency determining capacitor to the input of clock oscillator N1. The logic level that exists at the Dm terminal of IC; is shifted towards output Q at a speed that is defined by P1, which enables defining fixing times between roughly 1 and l0 minutes, 9 minutes being a commonly used value. When the star ·r but

ton is pressed, S ”R (set-reset) bistable toggles, and LED Dt lights. A logic l is written into the shift register with the aid of a positive pulse transition applied to terminal CP. After 64 clock pulses from the logic high level is available at the out- ` put of the shift register, and p enables oscillator N4 to drive i piezoelectric buzzer Bzi. The LED is turned off shortly after the START button is pressed, because the bistable is reset by the CL. OUT pulse from ICL The timer is conveniently fed from a 9 V battery, and should not consume more than about 10 mA. When, after developing, photo- graphs are immersed in the fixing bath at irregular intervals, it becomes difficult to observe the correct fixing time for each of these. When the fixing interval has lapsed, the timer provides a short beep. The circuit is composed of a 64-stage shift register which is l loaded with zeroes on power up, because it lacks a reset input. This problem is solved by the present timer, which is capable of "remembering" up to 32 immersion times, and automatically provides a signal when a photograph is to be taken out of the bath.

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