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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number.

Tank water level circuit with hysteresis

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This circuit simulates a water tank float valve in that it has the hysteresis missing in other electronic level sensor circuits. It delays the pump from coming on every time the water level drops by only a small amount below the top probe as water is drawn from the tank.

This circuit uses a 555 (IC1) to drive a solid-state relay but a standard relay could be used instead to control the pump. Three stainless steel rod probes are cut to lengths as required and are hung from the top of the tank. The longest, reaching to the bottom of the tank, supplies a common positive voltage into the water with the current limited to about 50µA via a 330Ω resistor.

The middle probe, connected to pin 2, acts as the pump start trigger when the level drops below it. The top (shortest) probe, connected to pin 6, provides the pump stop signal when the level rises to it. Three-core flex can be used to connect between the tank probes and the controller.

With a full tank of water, all the probes are covered and the output from pin 3 of IC1 is low. As the water drops below the middle probe, pin 3 goes high and turns on the pump. The pump continues running until the water rises to the top probe at which point pin 3 goes low and turns off the pump. The distance between the middle and top probes sets the hysteresis. The advantage of this circuit is its simplicity and reliability.

The basic circuit has been in use for over 25 years for pump control on farms, using 2-pair underground telephone cables over distances of 100 metres between the tank sensor and pump controller. The 15V zener diodes were fitted after a lightning strike on the farm took out the 555 but they cannot guarantee zero damage in the event of another strike.

Peter Robertson,

Walkerville, Vic. ($50)

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