Picture below is an automatic timeout circuit we recently made for a rainwater collection system.
This is for use within a rainwater toilet flushing system with 3000 litres of rainwater storage capacity and a standard header tank in the loft with a ballcock. The pump is a 12V pressure activated pump of the type common in motorhomes and boats. As the toilets are flushed, the ballcock drops in the loft tank and the pump automatically switches on to refill the tank with water from storage butts in the garden.
If the rainwater supply runs out then the pump will run dry okay, but if it is left running dry for too long it will burn out. Therefore a device was required which would automatically cut the power to the pump if it was detected as having run for too long – more than five minutes. (It was not possible to use float switches to detect low water levels in the water storage vessels due to their location).
Handily the chosen pump outputs a 12V signal while it is running, so this can be used to monitor the status of the pump without the need to detect the increased current draw through the cables when the pump is running or the sudden dip in battery voltage when the pump switches on etc.
The device we made is fitted with a 10A rated relay with NC and COM connections – therefore when the relay is not energised, NC and COM are shorted out inside the relay and the pump receives power. When the relay is energised, NC and COM are open and so the pump receives no power.
When the pump is detected to have started running, a timer starts. If this timer gets to five minutes then the relay is energised cutting the power to the pump to save it from burning out. An LED is used to let the user know that this has happened and they system has to be powered off and then on again to reset it after the pump has been checked and the water stores replenished.
As the rainwater storage is also sometimes used to water gardens from a connected tap, the device also has a manual override. Pressing and holding the button for one second triggers the override and then the pump can be run for as long as the user wants until the override is cancelled with the button.
If you require any kind of rainwater pump and/or monitoring system, email firstname.lastname@example.org with details of your requirements.