Generating Long Time Delays

Generating Long Time Delays

Circuit Description

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.

Circuit diagram

Circuit

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