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Fluid Depletion Sensor

All materials and content contained on this page are the exclusive intellectual property of Andrew R. Morris and
may not be copied, reproduced, distributed or displayed without his express written permission.

This circuit detects the absence, rather than the presence of a conductive fluid. This requires the use of AC, rather than DC for sensing, in order to prevent an electrolytic process from corroding the electrodes. The AC is produced by a sensor oscillator, consisting of Q2 and Q3. Q4 and Q5 comprise an oscillator which turns the beep oscillator (Q6 and Q7) on very briefly, about once every 10 seconds when the sensor oscillator stops. This is to conserve battery power during the alarm condition.

Q1 is a low battery detector. It causes the circuit to start beeping when the battery voltage drops to about 1 volt. The circuit will continue beeping until the battery voltage drops to about 0.7 volts. Q1 can be deleted if you donít need low battery detection.

The electrodes are just small wires or pins that lose contact with the fluid when it drops below a certain level. Although I used it in a direct connection, you may be able to change the values of C1 and C2 to capacitively sense the presence of the fluid. You may need to increase the oscillator current by lowering the values of R3 through R6.

When itís not beeping, the entire circuit draws less than 140uA off a full 1.5 volt battery. This circuit was built onto a piece of perf-board about the size of the larger 37 cent US postage stamps.

More:  Liquid Level Sensors
Click on Schematic to view PDF

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