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Circuits designed by David Johnson, P.E.
Last Updated on: Wednesday, October 18, 2017 05:04 AM

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Water Seepage Monitor

A while back I got a request from a company who needed a water seepage detector.  They wanted a device that ran on 24vac and detected water using a fully isolated circuit.  They wanted a circuit which switched off 24vac to a load when water was detected. The circuit below performs this function.  By changing the phototransistor’s position in the detector circuit, the circuit logic can be changed so the 24vac source can be turned on when water was detected.  This might be handy to activate a pump.

The sensor is made using two stainless steel probes.  The probes would be positioned as needed, so the water would come in contact with them when the water reached a certain level.  Since the detection circuit is isolated, the two wires leading to the probe could be located some distance away from the rest of the circuitry.
A 15v supply is developed using two capacitors connected to the 24vac power source.  The circuit is referenced to the two sources of the AC switching transistor Q1 and Q2.  The AC is first rectified and then filtered.  A simple zener diode limits the DC voltage to 15v.  A Schmitt trigger is used to form a 100KHz oscillator circuit which drives the primary of a small transformer.  A ferret bead with a 0.25 inch diameter forms the transformer core.20 turns are used on the primary and 10 turns on the secondary.  The secondary winding is connected to an optoisolator and to the two water detection probes, through a resistor.  The optoisolator has two infrared LEDs inside and a phototransistor.  When the two probes come in contact with water current flows in both directions through the isolator’s two LEDs.  Light from those LEDs is detected by the internal phototransistor.  The output of the phototransistor is connected to a second Schmitt trigger, which drives two n-channel FETs.  The transistors are wired in such a way that when turned on they conduct current in both directions.  The transistors can thus switch 24vac power to the load.  Using the transistor shown, the current should not exceed 10 Amps.

Click on Drawing Below to view PDF version of Schematic

Water Seepage Monitor designed by David Johnson, PE

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