Examples of sensors include reed switches and microswitches that can be activated by very short pulses. This is useful when circuit to be powered up needs to remain on for a longer period of time to work efficiently.
The proper value of the capacitor depends on the time the relay remains on, and also on the relay’s resistance. Typical values for relays between 50 and 1,000 ohm in are the range of 100 to 4,700 µF. Observe that the relay will be on for a time interval that begins when S1 is pressed and ends when the voltage falls to the minimum level that will keep the contacts closed (not when it falls to 0 V).
Figure 1 – Timed relay circuit
