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.