Accurate 24 Hour Timer Circuit

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.