Rain Gauge as a rain switch

Tipping-bucket rain gauges can be used directly as a sensor to trigger a pump or other device when rainfall is detected.

The rain gauge works by sensing the tipping of the bucket using a magnetic (non-contact) reed switch. This switch is very small and light and cannot be used to switch a heavy load directly. Thus an external relay should be used which is rated sufficiently for the load in question. The relay should be connected so as to latch in the 'on' position when it is triggered, because the rain gauge switches will be opening and closing regularly as rain continues to fall. The relay needs to be fast because the rain gauge contacts close for only 10mS - the relay must pull in and hold by this time in order to latch properly.

One possible suggested circuit is given below. One set of relay contacts is used to provide the latching behaviour, and the other is used to switch the load - in this case a pump which is normally running - detecting rainfall disengages the pump. A manual reset switch is used to unlatch the relay and restart the pump when necessary.

An electrician or electrical contractor familiar with the pump or other load in question should be consulted about a suitable relay.

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