Water-leak detector uses 9V batteries
Four comparators sound a buzzer when water is present or the battery is low.
Yongping Xia, Navcom Technology, Torrance, CA; Edited by Martin Rowe and Fran Granville -- EDN, May 27, 2010
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A previously published Design Idea describes a practical gadget that has the potential to save a lot with little investment (Reference 1). However, the circuit uses 120V line voltage and, as such, it is not that convenient for many applications. This Design Idea describes a portable water-leak detector that uses a common 9V battery for power (Figure 1). The circuit consumes less than 10 μA during detection mode, and a 9V alkaline battery has greater-than-500-mAhr capacitance. So one battery can last more than five years, which is equivalent to the battery's shelf life. When the battery voltage drops below 6.5V, the detector beeps to indicate that it is time to change the battery.
The design uses Maxim Integrated Circuits' MAX934, an ultra-low-power quad comparator with a built-in 1.2V reference. The chip uses about 6 μA. IC1A, R1, and R2 provide water-leakage detection. R1 is the water probe, which can be two bare copper wires wrapped in a sponge. R1 has high impedance when the sponge is dry, so IC1A's output stays high. Once the circuit detects the water leak, R1's value decreases to less than a few hundred kilohms, which forces IC1A's output low. Through D1, it makes the output of IC1B high.
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Either a wet probe or a low-voltage battery can force IC1B's output high, freeing a narrow-duty-cycle oscillator comprising IC1C, C2, R5, R8, and D3. The oscillation period is approximately 7 seconds, and IC1C's output is low for about 0.3 seconds. That low output allows a 2.4-kHz oscillator comprising IC1D, C3, and R9 to operate. When the circuit detects a water leak or the battery's power is low, the buzzer sounds for a fraction of a second every 7 seconds. In this way, the warning sound can last for a long time before the battery gets too low.
Resistors R6 and R7 increase IC1C's hysteresis, which lets you use a smaller value for C2. R10 and R11 increase IC1D's hysteresis to improve the sound frequency's stability. All capacitors are ceramic, ensuring low leakage current.
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Talkback
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With all due respect, this is an old concept. Basically it's an oscillator which is set in the "off" mode, then turned on via the use of a one-shot circuit. I have built a circuit just like this years ago. However, in lieu of a sensor, two pieces of wire can be connected -- one to the circuit, and the other to ground. Tens of megohms are recommended (to replace the 7.5M resistor if the detection of a man were to be created [the reason is for safety purposes]). Affix the two wires close to each other (obviously not touching, but a few millimeters apart). Once water droplets land in between the two wires and begin to create a large enough puddle, such that the puddle connects the two pieces of wire, the circuit will trip. Note that this can be done with merely ONE op-amp, if, for example, a state indication needs to be made (e.g., illuminate an L.E.D., or trip a pin of a micro-controller).
Will Compton - 2011-12-12 13:06:17 PST -
We are interested in this application www.flologic.com
Please contact us.
Charles R. DeSmet - 2011-29-7 11:37:57 PDT -
An entirely different article comes up when viewing this as a pdf.
Greg Dillingham - 2010-4-10 07:54:15 PDT -
Bloody annoying advertisement forced me to go away and do something else until it disappeared!
Rod Dalitz - 2010-1-6 13:44:10 PDT
