Hope that title makes a bit of sense. Basically, I’m building a device that incorporates a pressure-sensitive pad. Well, I can get a foam-rubber pad pretty easily, but I’m not sure what one would use to sense the pressure applied to said pad. The pad is going to be tapped on by hands, mallets, or other implements – it’s not going to be hit very hard. The actual pressure sensor doesn’t need to be extremely accurate, either; if I can manage, say, 15+ different levels of detection, this would be fine.
I’ve searched many places for things like “pressure sensors”, and while I’ve come up with many results, I’m not sure if this is what I’m looking for. Most of the components that are called “pressure sensors” seem to be for measuring fluid pressure; most of them seem to come with socket inputs, rather than having some sort of pressure-plate. I’ve also come across components for measuring force, and for measuring load, but I don’t know if these are what I’m looking for.
Finally, many of the sensors I’ve come across are clearly designed for fairly high-end applications (stock car racing, industrial applications, etc), and are priced as such. I’m looking for something cheap, really; under $100 would be preferable, and the cheaper the better.
So, hobbyists, professionals, and anyone else who knows about this sort of thing: any suggestions on what to use for this application? Any recommendations on where/what to buy?
True, a kitchen scale probably would do the job pretty nicely.
Forgot to mention one thing, though: it needs to be able to take readings quickly, for an indefinite period of time. I actually have a large number of kitchen and bathroom scales here, but most of them invoke a “tare” function before each measurement – which takes at least a second or two. I’d need something that would, say, let you tare it once, and then keep that tare until you turned the device off.
Measure the output of the scale’s strain guage directly. Maybe differentiate with a capacitor to remove the d.c. component (i.e. perform your own “tare” function).
A fairly inexpensive (if somewhat inaccurate and insensitive) pressure transducer can be made from a piece of antistatic foam of the type used to ship and store electrostatic sensitive semiconductors (if you live in the US, you can get a 5" x 5" piece at a local RadioShack), some copper foil and cellophane tape. Cut a piece of the foam and two pieces of foil to the size you want for your transducer. Solder a wire lead to each piece of foil, then put the foil pieces and the foam together like a sandwich with the foam in the middle, and wrap the assembly with the cellophane tape. Don’t wrap it too tightly, just enough to hold it firmly together. Now you’re done. you can monitor changes in pressure with an ohmmeter that has a 10-100k range, the higher the pressure, the lower the resistance. happy experimenting!
If its impacts you’re trying to capture (like drumming), you could use a little microphone in the rubber pad and feed the output to a 4 bit digital converter (15 levels + 0). The gain could be adjustable with a potentiometer, so you could tweak your range to trigger with more or less pressure.
Electret microphone is a good idea, but you’d need an op-amp to amplify the signal. Really nothing fancy though. It would be helpful if we knew what you wanted to do with the output.
Correct me if I’m wrong, but it sounds to me like you’re trying to measure the magnitude of a mechanical impulse. Traditional strain gages (and most pressure gages utilize a strain gage sensing element, BTW) are usually not well suited for performing this task. I would instead investigate the use of a microphone (as suggested by jnglmassiv) or accelerometer. And does anyone know how those electronic drum heads work? Perhaps that’s the best technology to use?? They seem to be fairly inexpensive nowadays.
Keep in mind that microphones and crystal-based accelerometers are dynamic transducers, In that they only respond to signals that change over time. This makes them well suited for impulse measurements. If, however, you also need to measure a static force, then we’ll have to come up with something else.
A primary consideration is how much travel do you want on the sensor arm because that will determine the construction. If you can use higher travel distance then it is trivial to couple a spring and a potentiometer but if you need a travel distance measured in mils then you need something else.
A static force transducer can be built quite easily. Imagine a double sided PCB with two copper tracks running parallel, one on each side. As the PCB flexes, one track stretches and the other one contracts so that their resistance varies. If you have these two resistors as part of a bridge you can then amplify the difference. In practical terms these transducers are made with tracks which come and go lengthwise several times in order to amplify the effect. You can make one with regular PCB or with resistor alloy wire glued to a board. The principle is quite simple but you need to know enough electronics to design the circuitry.