Push button switching circuit

Question

The circuit is designed to control the power to a load via a single momentary push button switch.

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K1 and K2 are miniature DPDT relays with dc12v coil resistance of 1.2k ohm. K2 coil is time delayed via the external resistance and capacitance.

If the push button is pressed momentarily K1 should turn on and stay on. When the push button is held longer than certain time constant, K2 will turn on and drop the K1.

The question is how do I determine proper values for R and C if the required long press is about 1.5 second?

Simple RC time constant doesn't seems fit because of the influence of the K2 coil resistance.

• JIm Dearden: If k2 coil is 1.2k then R1 is going to be in the region of about 100 - 200 ohms otherwise K2 will never get enough voltage to turn on. This would give a value of about 200 Farads for the capacitor - far too big.

Answer 1

A possible solution

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By using a transistor (connected as an emitter follower) the value for R1 can be increased because the transistor provides current gain. This means the capacitor value can be decreased to a practical level.

Note that the diodes are placed across the coils to prevent back emf when the coil is turned off.