The post explains a simple 24 hour precision timer circuit using just a couple of CMOS ICs, which can be used for switching ON or OFF any mains operated load, with any desired delay between 1 and 24 hour interval.
The uniqueness of the long interval timer circuit is proven as it delivers switching duration up to 24 hours and predominantly beneficial for various applications.
How it Works
When S1 is pressed, Re1 will be energized and the timer will initiate. You may notice the position of P1 dictates the period of the timing interval.
The capacitance of C2 permits 12 hours tops but if you twofold the capacitance, the timing of the interval increases proportionally. Thus, it is very suitable for use in applications that control a NiCd battery charger.
The timer will reset and Re1 becomes deactivated when S1is pressed at any time during the sequence. FF1 works as a debouncer circuit for S1.
When the switch S1 is pressed, FF1 will channel a logic high pulse to the clock input of FF2 which clips. After that, IC2 begins counting as its reset state is over.
Simultaneously, T1 is applied with a positive logic level and Re1 is energized. When the timing interval is over (when the counter output Q13 becomes high), FF2 is reset and Re1 will be deactivated as a result.
To get the precise 24 hour duration of the timing interval, you must briefly use counter output Q3 rather than Q13 to reset FF2.
Using the provisioned component values, the interval must be regulated between 3 and 45 seconds.
Then, divide the required relay-on time by 1024 and set P1 correctly. Afterwards, attach the R input of FF2 to Q13 again, press S1 and use Re1 to power the related device to a predetermined duration.