While dissecting an extractor fan the other day, I came across a strange chip that initially looked out of place. However, upon further pondering I realised this was used to trigger the fan to stay on for minutes to hours of time as appose to the usual seconds of normal monostable circuits.
The chip used is a 'ripple counter', a 'binary up-counter' or a 'divider' circuit; it can have many names. The callsign divider makes the most sense in this context as we are dividing a frequency (or multiplying the time period) of an input.
What I have created in the picture above is a recreation of what I imagine is going on using a 7 bit divider with an astable input running at a low frequency. The ripple counter will count up in binary until the Q7 output is high, which will shunt the astable clock input to ground. The circuit will be frozen until the reset is triggered high, turning Q7 off and re-initialising the binary count from zero.
Q7 controls our digital output, in this example I have used a DC motor as that was what I was working with. Q7 is an active low monostable output so we need to invert it using a simple mosfet inverter circuit, the second mosfet on the right drives the motor at its desired current.
The ripple counter I have used is a 4024B 7bit counter, but to increase the possible time you could choose an IC with more Q outputs. Alternatively you do not need to use Q7 on this chip you could a lower output which will give you a shorter period.
The counter is incremented every time the clock gives a rising edge which means the monostable will be a function of the frequency period of the astable.
The equation to quantify this is (Astable period) x 2^(Qnumber) = Monostable period.