Glowing pressure plate

Question

I need to build a pressure plate to go on the front of a prop to be used in Airsoft.

The idea is this plate is both an target to be shot at and a visual representation of "health".

My initial thinking is along the lines of a translucent perspex circle mounted on springs in front of micro switches. It could have RGB LEDs behind it that can be changed from Green to Red. The electronics behind all of this are already handled as part of a larger project by an Arduino.

The problem for me lies in the physical design of this plate. Specifically, how to mount the plate in such a way that the pressure will reliably be detected and the plate will return to "off" position. Also, which types of switches to use. I was thinking maybe micro switches with long arms for leverage?

Maybe rather than micro switches some kind of membrane would be better? But would that be able to withstand getting shot at by airsoft weapons? Impact force on the plate will be <= 1 Joule from a 6mm plastic BB.

Any help gladly appreciated!

Answer 1

The usual method of doing this would be to use a Piezo buzzer as an impact sensor. Something like this:

When struck, these buzzers will produce a voltage spike. This is exactly what they do in most electronic drum kits. What you'd do is have a heavy backer board (3/4" plywood would be overkill, but suitable), and your lexan/plexiglass/whatever target. Between the two are these piezo buzzers. Depending on the size of the target you might be OK with one, or you might need more than one buzzer per target. You'll have to experiment with different mounting techniques, but I'd start with simple double-sided tape (not foam tape).

The output of the buzzer will be a spike that goes positive and negative, and could be a fairly high voltage (if a hand hits it). You will want to feed that signal into an ADC, while protecting the ADC from the voltage spikes.

You'll also have to experiment to see if this signal needs to be amplified before going to the ADC. I doubt it, but keep it in mind. This will vary depending on the buzzer you choose and the construction of the target.

While this sounds like a lot of work, it does offer you some advantages. The biggest is that if you have a target with 3 or more buzzers you can do a form of triangulation and figure out where on the target it was hit. That way the target can react differently depending on if you got a bullseye or not.

I also suggest that for experimentation you get a piezo and connect it up to an o-scope. Just hit the thing and you can see how it reacts.

Answer 2

If you want to use microswitches, you could buy some with lever arms and extend them by using some heat shrink tubing to attach a few inches of cable tie. Like this:

Then you could mount the plate on springs. (btw those white bumpers on the front of the robot in the pic are spring mounted cabinet rollers. Something like that might work for mounting it.)

You could mount the switch outside of the perimeter of the target, and you'd have plenty of leeway with how far back the target was pushed b/c of the flexibility of the plastic lever on the switch.