Introduction: Cheap Water Leak Detector

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This instructable describes the modification of a cheap magnet based door alarm to a water leak alarm/detector.

My motivation were several overflows of water in our basement do to an unwilling water lifting unit/system. Naturally there is water everywhere and I am not aware. I figured some loud noise would be nice.

The hack itself is heavily inspired by squeaky leaks, a similar (yet not totally the same) projekt. Squeaky leaks has a fun documentation which I really recommend you to read: https://hackaday.io/project/167936-squeakyleaks

In the german hardware Store Bauhaus, I found a pack of 3 for the price of 6 euros and having above hack in mind, I decided to give it a shot and try to modify these into what I need. I only found out afterwards that Bauhaus also sells waterdetectors, but that would hat have satisfied my hacking desire.

Step 1: Analyzing the Existing Circuit

Here I need to make a huge shoutout to Bigclive (https://www.youtube.com/user/bigclivedotcom) where I learned a lot in terms of understanding circuits but also drawing out unknown pcbs.

I started by analyzing the existing circuit in order to figure out whether this hack was possible as well.

I have drawn out the circuit and figured out the datasheets of the components. The attached circuit diagram is simplified.

The pullup resistor of (measured 1,5 Megaohm) finally gave me the clue on how to modify the circuit afterwards. More on that in the next step.

I figured out that the stand by current is 3 µA (which totally corresponds to the 1,5 Mohm). This would mean that it would last some years on stand by.

The current when alarming is around 60 mA.

Step 2: Modifying the Circuit

Based on the knowledge that the resistor of 1.5 Mohm activates the circuit, I decided to only add one 47kohm resistor as a security percaution (I guess i could have left it out, but I wanted to limit the current if there would be a dead short circutit). The 47Kohm is only a order of magnitude. My rudimentary studies of the S8050 Datasheet indicated that it should be at least 20kohm.

I basically did the following three modifications:

  1. Removal of the 1.5 Mohm resistor
  2. Adding to either end of the former 1.5 mOhm resistor solder pads the follwing:
    1. 47kohm Resistor to the Base of the transitor
    2. just a wire to the positive Battery terminal.
  3. Adding a connection block for solder free addition of Electrodes.

The circuit now shows no (measurable) standby current (yay, Batterylife :)

The circuit activates the alarm by the pure touch of my hand. It also activates it by touching water from the tap. This is exactly what i wanted :)

Contact Electrodes:

I cut a bicicle spoke (assuming it is stainless steel) in two pieces and bent it into the shape seen on the picture.

The circuit can be tested just by dipping the eletrodes into water.