The circuit shown in Figure 1 can be used as a timed sensor, activating a load at time intervals ranging from a few seconds to more than 30 min, depending on the components used in the RC network. For a 1,000 ?F capacitor and a 1 MΩ resistor, the time interval can be extended to up to 15 min.
Figure 1 - Timed sensor (555 + NPN).
The output transistor in this block is an NPN BC548, meaning that the relay is on when the 555 is set with the output in the high logic level. You can use other transistors, as described in other articles in this site, to drive high-current loads directly. The circuit can be powered from 5 to 12 V supplies.
