In Texas — at least around Houston — we don’t have basements. We do, however, have bilges. Both of these are subject to taking on water when no one is paying attention. A friend of mine asked me what I thought of an Instructable that showed how to make a water sensor using a few discrete components. The circuit would probably work — it relied on the conductivity of most water to supply enough current to a bipolar transistor’s base to turn it on.

It is easy to overthink something like this, so I told my friend he should go with something a little more old-fashioned. I don’t know the origin of it, but it is older than I am. You can make a perfectly good water detector with things you probably already have around the house. My point isn’t that you should (or shouldn’t) construct a homemade water sensor. My point is that you don’t always need to go to the high-tech solution.

On the other hand, this is Hackaday, so I’m sure you want to know how to hack a water sensor out of common household items. The picture probably tells you the story anyway, but if not, read on.

What Do You Need?

The heart of the water sensor is a spring clothes pin. You also need two flat metal pieces. I’ve seen it done with pennies but you could probably use a couple of washers or pieces of scrap metal. I’ve even seen it done with aluminum foil, but I don’t recommend it. There’s one critical piece left: an aspirin. You could probably use some other things, but it has to be something hard enough to keep the clothespin open, but will also dissolve when in contact with water.

You can figure out the rest. You connect wire to the metal contacts, make a sandwich with the aspirin in the middle of the contacts and clamp it together with the clothes pin. If detecting water isn’t your thing, you might enjoy [American Hacker’s] video (see below) that uses the same idea to detect when a door opens.

More Simple Sensors

Anti-static foam, wire, Plasti Dip for an analog pressure sensorAnti-static foam, wire, Plasti Dip for an analog pressure sensor

The aspirin and clothes pin trick is just one way to make a simple sensor. Conductive foam works well as a pressure transducer (especially if you use a little Plasti-Dip to seal it). A lot of sensors use the property of another component (like this temperature sensor). Foil seems to be a common component, too.

Many times, a component made to create something can also sense it, as odd as that sounds. For example, LEDs can act as light sensors. A speaker can work as a microphone (or, you can rip the magnet out of it, steal a relay coil, and make a magnetic speed sensor).

Rant On

If you think about it, the point I’m making is one we often see in the comments for Hackaday posts. No, not “That’s not a hack.” I’m thinking more of the latest Raspberry Pi project that turns a light on when it gets dark that will elicit a lot of comments about how you could do that with a 2N2222 (or an op amp, or a 555, or whatever your weapon of choice is).

Generally, we don’t mind projects like that. People don’t need a program that prints “Hello World!” but it is a good way to get familiar with a programming language. By the same token, sometimes doing a simple project with an Arduino, a Raspberry Pi, or an FPGA is more about getting familiar with the development environment and how to apply the tool.

On the other hand, your LED blinker doesn’t need a 2 GHz CPU with 32 GB of RAM running an RTOS. My point with these sensors is the same: there are times you really do need a sensitive, precise sensor. Most of the time you need a lot less. If you aren’t going for an educational project, take some time to think about if you are using a shovel to put sugar in your coffee.

Your Turn: Homemade Sensors

There are lots of ways to make simple sensors. Your turn. What’s your favorite do-it-yourself sensor? Drop a note in the comments and let us know what sensors you’ve hacked out of improbable things.