Here's a plan similar (if not the same) as the one I posted on here before the crash. I use it to switch off a washing machine when a barrel outside is full of waste water. This barrel gravity feeds via a hose onto the lawn.
I didn't use the bottom probes as I wasn't using a reservoir tank. Instead I took pin 13 on the 4001 IC low (to ground). Parts for this were least that $10, excluding the power supply.
This will do what you what, is a fun little project and fairly easy.
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