# Time delay dpdt relay using only pot, relay, and cap (discreet components and no transistors)

I am trying to design a simple time delayed relay circuit that uses only a potentiometer, 5V dpdt EM relay, a single sufficiently large capacitor, additional resistors (if needed), and buttons/switches, powered by a 9V battery. The schematic below works to simply turn on an LED while a button is pressed and keeps it on for a moment. I have only been able to extend the time by adding more caps in parallel. I believe there is a simple solution to this logic problem that has the relay triggering itself almost as an oscillator, but when setting up as an oscillator I still can only vary the time by adding caps. How can I arrange and wire the components mentioned to allow a time delay of a few seconds to minutes, dependent upon a single potentiometer?

I thought a toaster's circuit would work, but most I see use a 555 timer or a pair of transistors and other components. Is there a simple way to use an RC circuit alone as a timing circuit without the solid state components?

I know how to do this using transistors and ICs, but that is not the point. It is an experiment to show a possible use of capacitors and RC circuits without first teaching about solid state components, ICs, etc.

The time delay can be delay ON or delay OFF.

Thanks

simulate this circuit – Schematic created using CircuitLab

• Would it be possible to replace the relay with a different array of switches and a zener diode of appropriate reverse voltage? Commented Nov 27, 2015 at 20:45
• you're really trying to do it in the wrong way.. Commented Nov 27, 2015 at 20:46
• yes i know... this is more of an experiment then for a practical use Commented Nov 27, 2015 at 20:48
• "I know how to do this using transistors and ICs" then you should be able to work this problem out to an unsatisfactory answer. Commented Nov 27, 2015 at 22:40

I'd say forget it. There is a reason all those circuits you see use a 555. A practical capacitor does not old enough energy to power a normal relay for the period of time you are aiming at.

You might get it to work with a very very low current relay, maybe a 230V relay, with an idem capacitor. But that is exactly the circuit you don't want a novice to play with. (At least, if you want him to live long enough to rise above that level.)

The hold-on time will be in the order of the time constant given by the formula

T = R x C


So the value of C required will be given by the formula (approximately)

C = T / R


So, figure out your relay coil value and calculate C. The answer will be in farads so multiply by 1,000,000 to convert to μF.

One of the problems with this approach is that the voltage on the coil gradually drops off and the contacts slowly open. If there was any significant current flowing then the contacts will start to arc and heat and lifespan would be reduced.

one way to extend the time is put a resistor parallel to the switch

However: the resulting circuit is more vibration and supply voltage sentitive simple circuit

start with 4 times the resistance of the relay coil. lower resistances give longer extensions.