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Hi, I am Tony I0JX

This is quite a common trick, but there still are people unaware of it, so why not repeating it here?

24 - 28 V relays are quite easy to find in the surplus market, most often at good prices. They work very well at 12 - 14 V using the circuit shown below.

The idea is based on the fact that all relays, once activated, need a much lower voltage to remain active.

When you close the activation switch, the relay gets nearly 24V, i.e. the sum of the power supply voltage and that of the charged electrolytic capacitor. After a while the capacitor discharges and the voltage across the relay goes down to nearly 12 V, which is well sufficient to keep the relay active.

Play with resistor and capacitor values to best suit the characteristics of your relay.

That's all.