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Content 4/4

DIY water detector / leak alarm

Here is simple water detector you can build yourself. It can be used for many applications, e.g. leak detector for dishwasher and other water appliances, alarm that garden watering tank is getting full etc. It uses only basic components (NPN transistors, resistors and ceramic capacitors) and runs for years from a 9V battery.

Circuit operation

Water detector circuit diagram is shown below. The sensor is a pattern on the PCB, but also the screw holes are connected in parallel with the sensor. When water reaches the sensor, small current flows through it. The current is amplified by Q2 an Q1. R9 ensures the alarm stays off when there isn't water on sensor. You can increase the sensitivity by increasing R9 and reduce sensitivity by decreasing R9. Resistor R3 limits current to safe level if the sensor is short circuited.


Transistors Q3 and Q3 form an astable multivibrator (oscillator). The frequency is determined by R10, R11, C4 and C5 and is approximately 1 Hz. The purpose of this oscillator is to provide intermittent beeping sound, which is more easier to notice than a continuous sound. It also saves some power because the buzzer is active only 50% of the time. Transistor Q5 provides more drive current to supply power to the final stage.


The final stage is another oscillator. It's frequency is set to ca. 4 kHz to match the resonant frequency of the piezo sounder B1. If you use some other type of piezo buzzer, check the datasheet for the resonant frequency! If it is other than 4 kHz, adjust R7, R8 and/or C2, C3 to get the correct frequency. The piezo elements will work on wide frequency range, but the sound is the loudest on resonant frequency. Piezo elements don't like if there is constant DC voltage over them, but in this circuit it is not a problem since the power to the final oscillator is completely cut off when piezo is not making sound.

Building your own water sensor

I have designed a single side PCB which is you can buy from this page. When ordering, it is good to leave comment that it is single side board, with bottom copper, bottom soldermask and top silkscreen. Otherwise you could get additional questions about it. The Eagle design files are available to download from below


bjt_water_detector.zip   Schematic and board file, designed with Eagle 5.12



The component values are also shown in assembly drawing above and printed on the silkscreen. The BOM is also listed on this page. The component values aren't critical, but the values of R7, R8, C2 and C3 should be as specified to get loud alarm.

Using the water detector

This is easy. Just install the battery and fix it with cable ties, then place the board to location which you want to monitor. There are many possible ways how to mount the PCB:


-Use M3 standoffs to make 'feet' for the board so it stands upright, battery at top (see picture on top of this page, right side). The standoff screws also act as sensor as the screw holes are connected in parallel with the striped sensor are on backside of the board


-Different usage of standoffs is shown on left side of the image on top of this page. In this case, only the standoffs are acting as sensor. When the water rises up to striped sensor area, the whole circuit gets wet and probably fails to work. For added sensitivity you can add wires or stripes of metal to bottom of the standoffs.


-Hang the PCB from the hole on the top. For example on the wall with a nail.


-Connect wires on two screw holes with solder or screw. Then you can place the PCB on a dry place and run the wires to location you want to monitor. Pictures below show example of monitoring when watering barrel fills up. I have used solid copper wires, so they work as sensor and hanging hook at same time.



For more information, see the project web page http://kair.us/projects/bjt_water_detector/index.html

Bill of materials used in this project

1 kohm axial resistor
2
18 kohm axial resistor
6
1 Mohm axial resistor
3
10 nF radial ceramic capacitor
3
470 nF radial ceramic capacitor
2
2N4401 transistor TO92
7
22 mm piezo buzzer, for example Murata PKM22EPPH4002-B0
1
9V battery snap connector
1
150 mm cable tie
2
Aug 21,2020
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