555 flood alarm

Basement Flood Alarm Circuit

flooded room

Even though floods don’t always rate high on our list of concerns, if we get an early warning about one, we should take steps to help prevent as much damage as possible, including dangerous substances within the floodwaters that could enter our homes. The circuit of the basement flood alarm introduced below is designed to sound off like an alert signal generator when water gets where it doesn’t belong. Power by a compact battery, this alarm will provide an early alert that can help you ward off a damaging flood in your basement or crawl space. The circuit works on the simple, sound, and extensively proven principle of electrical circuits. The flood alarm has an aqua-sensor with two sensor probes. When the sensor probes make contact with water, the water completes the electrical circuit, which allows an alarm to sound.

The entire circuit can be created on a medium-sized perfboard. The electronics and battery should be housed in a small prototype enclosure (made waterproof using epoxy adhesive). However, for mechanical reasons and to simplify construction, the aqua-sensor (formed from two stainless steel nails protruding through the case) should be kept at a distance from the electronics (see author’s prototype). The finished system can be wall-mounted at ground height or at a suitable height above ground as per individual need.

wall-mounted prototype

(author’s wall-mounted prototype with enclosure built from salvaged aquarium accessories)

The electronic is designed for use with almost any low-voltage piezo sounder. It’s basically a low-frequency, astable oscillator driving an active piezo sounder and built around the ubiquitous, eight-leg 555 timer chip. The BC547 transistor drives the astable oscillator (by extending positive rail of the power supply to the rest of the circuit) only when water is detected by the aqua-sensor probes (also see the “detection-sensitivity controller” preset pot). In the author’s intended application, it was not possible to power the circuit from the AC line, so the circuit is designed to operate from a standard 9-V battery.

555 flood alarm


  • R1, R2, R3 = 1K ¼ w
  • R4 = 18K ¼ w
  • R5 = 180K ¼ w
  • P1 = 1M


  • C1, C2 = 1uF/25v
  • C3 = 10nF
  • C4 = 10uF/25v


  • IC1 = NE555
  • D1 = 1N4148
  • T1, T2 = BC547
  • LED1 = Red/10mA


  • BAT = 9V PP3 Battery
  • BZ1 = 5V Piezo-Sounder

Tinker Tip

  • IC1 astable multivibrator (AMV) runs at a frequency (F) near 3.8 Hz (duty cycle 52%). The period (T) of the continuous square wave output is set by components R4-R5-C2 (here, it is around 264 ms). As this is not very critical, other timings may be obtained by choosing appropriate RC components to suit your convenience
  • In the prototype (tested with value of P1 locked to 330K), no-load output available across points A and B is around 120 mA at 7 V when water presence is detected by the aqua-sensor. With some skill and patience, you can replace the 555-IC portion with a dedicated tone generator circuit using Holtek chips, like the HT2812D.

Tinker Tip


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