Everyone loves water gun fights in the summer. But you can always make something better by adding electronics to it. So I designed a set of water sensors and buzzers that will beep when they are hit with the spray of a water gun. This lets you play a game similar to laser tag but with water.

And if you were wondering, no it won’t shock you or electrocute you.

Project Steps

The Control Circuit

Here is the circuit that I designed for this project. It is a slightly modified version of a 555 timer circuit in monostable mode.

The output of the circuit is controlled by the voltages at pins 2 and 6. The resistors that are connected to these two pins set the initial voltage HIGH (roughly the same as the supply voltage). When water sprays the connectors between pin 2 and ground, the voltage at pin 2 drops below the reference threshold (1/3 of the supply voltage). This causes the output at pin 3 to go HIGH and turns on the buzzer for a few seconds. How long the buzzer will remain on is determined by the values of the capacitor and the resistors connected to pin 6. With the values that I used (shown in the circuit diagram), the buzzer will sound for about 2 seconds. Increasing the values of the resistor or the capacitor will increase this time. Decreasing these values will decrease this time.

So basically, when water is sprayed on the sensor, the circuit beeps for a few seconds.

Water Sensor Design

The kind of water sensor that I chose to use in this project is just a series of wires running parallel to each other. When a drop of water hits two adjacent wires it conducts electricity between them and triggers the control circuit. The wires that I used for this are made from paperclips. I chose these because they are relatively sturdy and will hold their shape.

First, take the paperclips and unfold them so that they are straight. Then decide on how you want the sensor to set up. For prototyping, I just took the wires, bent them into a “U” shape, and stuck them in adjacent slots on a breadboard. For the final configuration, I bent them into smaller U’s with sharp angles similar to staples. Then I soldered them to a piece of perf board.

Prototype Everything on Breadboards

It is always a good idea to prototype the circuit on a breadboard before soldering a circuit together. This will give you a change to test it out and make any necessary changes.

Solder the Circuit onto a Piece of Perf Board

Once you are satisfied with how the system is performing, you are ready to solder everything together. I soldered my circuit onto a piece of perf board. I like to use perf boards when soldering together prototypes because they are very versatile and let you make connections in any way that you want.

Seal the Board of the Water Sensor

There are a lot of exposed wires on the water sensor. If a drop of water connects any of them the buzzer will sound. If a drop of water gets stuck between them, then the buzzer will stay on and be really annoying. So to help prevent this, I sealed most of the board to insulate it.

First I taped over the portion of the sensor that I want to remain exposed. This was the top face of the wires sticking out of the board. Then you can use any kind of spray-on sealer or spray paint to coat the rest of the board. Be sure to get thorough coverage. You want to cover up all the exposed metal.

Once it has completely dried, check to see if it needs another coat. Once you are satisfied with the coverage, remove the tape.

Mount Everything Inside a Project Enclosure

Now all you have to do is mount all the parts inside a project enclosure. You need to make one hole in the top of the enclosure for the buzzer. You also need to cut a slot in the side of the housing for the wires that go to the water sensor.

I attached the circuit to the inside of the housing with hot glue. I also mounted the batteries inside the box. Then I closed it up and screwed on the lid.

Finished Water Tag System

​Now your water tag system is complete and ready to try out.

There are a lot of ways that you can use this system. The simplest way is to simply put the control circuit inside your pocket and attach the sensor to your clothes. You can do this with a safety pin, a binder clamp or a fabric arm band. I just glued a binder clamp to the back of the board and clipped it on my shirt. You can also set up the sensor on a stationary platform like a target shooting game.

At times the buzzer might stay on after being activated. This means that a bead of water is stuck between wires on the sensor. To fix this just brush it with your hand or blow on it. This should knock off the offending drop of water and allow the system to reset.