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 120 V 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 9 V battery for power (Figure 1). The circuit consumes less than 10 μA during detection mode, and a 9 V 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 6.5 V, the detector beeps to indicate that it is time to change the battery.
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| Figure 1. | IC1A and IC1B determine the conditions for sounding the buzzer, and IC1C oscillates to provide the trigger for IC1D. |
The design uses MAX934, an ultra-low-power quad comparator with a built-in 1.2 V 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.
IC1B, R3, and R4 form a low-voltage detector. When the water probe is dry and the battery voltage becomes lower than 6.6 V, the voltage on IC1B’s negative input is less than 1.2 V. Because the reference voltage is 1.2 V, IC1B’s output changes from low to high. So when the probe is dry and the battery voltage is higher than 6.6 V, IC1B’s output is low, which forces IC1C’s output high, and IC1D’s output stays low.
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.
Reference
- Tregre, Jeff, “Doorbell transformer acts as simple water-leak detector,”
