Simple 5 to 20 Minute Delay Timer Circuit

The article discusses a simple 5 to 20 minute delay timer circuit for a specific industrial application. The idea was requested by Mr. Jonathan.

The Request

Good evening,

While trying to figure out a solution to my problem on google, I came across your posting at http://homemadecircuitsandschematics.blogspot.com/2012/05/simple-delay-timer-circuits-explained.html

Because you posted your email address there, and have been responsive to requests, I thought I’d take a shot at getting your help with a circuit design.

I'm trying to figure out how to build a better Sous Vide controller.  The main problem is that my water bath has a very high hysteresis, and when heating from colder temperatures will overshoot about 7 degrees from the temperature at which power is terminated.  It is also very well insulated, with a gap between the inner and outer vessel which makes it act like a thermos jar, because of this it takes a very long time to decline from any excess temperature.

My PID controller has an SSR control output and a relay alarm output.  The alarm can be programmed as a below limit alarm with an offset from the set-point.  I can use a five volt supply I already have for my circulation motor to run through the alarm relay and drive the same SSR the control output is driving.  To be on the safe side and protect the PID controller I'll add a diode to both the alarm voltage and the control voltage to prevent one output from feeding back into the other.  I'll then set the alarm to stay on until the temperature rises above the set-point minus 7 degrees.  This will allow the PID tuning to be adjusted without having to account for the initial temperature ramp-up.

Because I know that last few degrees will be achieved without any power input, I'd really like a way to delay any recognition of the control signal for about five minutes after the alarm shuts off, as it will still be calling for heat.  This is the part I've yet to figure out the circuitry for.  I’m thinking of a normally closed relay in series with the control output, which is held open by the alarm signal.  When the alarm signal is terminated, I need a delay on the order of five minutes before the relay returns to its ‘off’ normally closed state.

I would appreciate help with the delayed off portion of the relay circuit.  I like the simplicity of the initial designs on the page, but I get the impression they wouldn’t handle anywhere near five minutes.

Thank you,

Jonathan Lundquist

The Design

The following circuit design of a simple 5 to 20 minute delay timer circuit can be suitably applied for the above specified application.

The circuit employs the IC4049 for the required NOT gates which are configured as voltage comparators.

The 5 gates in parallel form the sensing section and provides the required time delay trigger to the subsequent buffer and the relay driver stages.

The control input is acquired from the alarm output as indicated in the above description. This input becomes the switching voltage for the proposed timer circuit.

On receiving this trigger, the input of the 5 NOT gates are initially held at logic zero because the capacitor grounds the initial trigger via the 2m2 pot.

Depending upon the 2m2 setting, the capacitor starts charging up and the moment the voltage across the capacitor reaches a recognizable value, the NOT gates revert their output to logic low, which is translated as a logic high at the output of the right single NOT gate.

This instantly triggers the connected transistor and the relay for the required delay output across the relay contacts.

The 2M2 pot may be adjusted for determining the required delays.

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