Generating long delays of several hours can be accomplished by
using a low frequency oscillator and a binary counter as shown below.
A single Schmitt Trigger inverter stage (1/6 of 74HC14) is used
as a squarewave oscillator to produce a low frequency of about 0.5
Hertz. The 10K resistor in series with the input (pin 1) reduces
the capacitor discharge current through the inverter input internal
protection diodes if the circuit is suddenly disconnected from the supply.
This resistor may not be needed but is a good idea to use.
The frequency is divided by two at each successive stage
of the 12 stage binary counter (CD4040) which yields about 1 hour
of time before the final stage (Q12) switches to a high state.
Longer or shorter times can be obtained by adjusting the oscillator
frequency or using different RC values. Each successive stage changes state
when the preceding stage switches to a low state (0 volts), thus the
frequency at each stage is one half the frequency of the stage before.
Waveform diagrams are shown for the last 3 stages. To begin the delay
cycle, the counter can be reset to zero by momentarily connecting the
reset line (pin 11) to the positive supply. Timing accuracy will not be
as good as with a crystal oscillator and may only be around 1 or
2% depending on the stability of the oscillator capacitor.
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