I'm looking into making some arduino controlled reactive targets for an Airsoft shoot house. Basically, these sensors will be mounted onto human-shaped targets that represent either hostages, or hostage takers. I need some way to determine when a target is struck by a BB. I am wondering if something like the KnockSensor tutorial, or this vibration sensor: Piezo Vibration Sensor - Large - SEN-09196 - SparkFun Electronics. would work if I attached them to a roughly 12" x 12" thin metal plate. My concern is that the chaos of the shoothouse (airsoft guns shooting, people shouting, pyro displays going off) would cause the sensors to give false readings. Can anyone give me an idea of how sensitive these two types of sensors are?
A piezo knocksensor when connected to an arduino can be programmed to do different functions depending on its reading as an analog input.
Example. If the sensor reaches maximum threshold (1024) it could light an LED.
If the sensor reads (500-600) it could sound a buzzer.
In your case you would want to make sure that the bb has hit the target so you will need to do some testing with the sensors to find out what values will trigger your output.
Remember that analog inputs read from 0-1024 (0v-5v) accordingly.
I've made piezo touch/tap sensor work. To each lead attach parallel 2 diodes. Ground the diode that 'points in' and connect the other to an NPN grid for both. Connect collectors to 5V through some resistance, I used 2.2k IIRC, and connect the emitters to pins, I used digital and counted microseconds the pin was high to determine how hard the touch/tap was. Using 2 pins let me see the tap and the release separately. It took a very light touch to not register, with wind you might want to adjust the circuit to make a dead zone -or- in software just ignore what doesn't look like a BB hit.
I think that is very feasible. I have a cheap toy dart gun target that uses piezo sensors to pick up where on the target a dart has hit, so it clearly can be done.
wizdum:
My concern is that the chaos of the shoothouse (airsoft guns shooting, people shouting, pyro displays going off) would cause the sensors to give false readings. Can anyone give me an idea of how sensitive these two types of sensors are?
As others have posted, this project should be feasible with either type of sensor. However for the concern about possible false positives, this is something you will just have to experiment with your shoothouse because a lot of the relevant variables are going to be specific to the location. Yet, this is probably something you can avoid with proper calibration of signal thresholds by means of software, hardware, or both.
Another idea (not specific to preventing false positives, but it could help there too) is to use multiple sensors per target and compare the signals from each. That way not only will the system detect a hit on the target, but also be able to estimate the location. The accuracy will depend upon a lot of details like how many sensors, how they are arranged on the target, their sensitivity, etc..., but this type of system can be very effective. As I recall a similar system was used in the rifle and pistol target shooting events during the London Olympics, though for most of us the time and expense necessary to develop a system that percise and accurate might be impractical.
BTW, the piezos I have are the kind from the buzzers. I'm pretty sure that people do use them as pickups for acoustic instruments.
Here's 2 buzzers and 1 just the disc, all $1 or less. If you're going to need multiple sensors per target and multiple targets then you might as well save a few $.
They're even cheaper in multi-packs direct from Hong Kong. How about 10 discs for $1.70?
http://www.dealextreme.com/p/zx-18t9a1-18mm-piezo-transducer-sound-discs-w-leads-10-pcs-147962?item=14
Thanks for the help guys. I'll probably pick up a Piezo disc and start experimenting with different thresholds. Might take it out to the range and see how it reacts to a pound of tannerite, that should be a significant enough "boom".
Try tapping it with your finger first.
I got started after seeing this:
I got a high sensitivity vibration sensor working nicely, and is able to detect vibration while avoiding spurious false positives. More in my blog at Internet of Things: How to Avoid False Positives with a High Sensitivity Piezo Vibration Sensor