Choosing the right sensor

Hi Chaps
A project im working on is a target range for airsoft and its coming along quite nicely. The target is raised by a heavy duty servo. the target will either be wood, sheet metal, or perspex, so in either case it will be a solid mass. The part I am struggling with, is choosing the best sensor to detect when the target has been shot. Exact location is not important, just an acknowledgement that it has been hit.

I have tried vibration sensors and it does work to some degree however its not reliable, as it does not always detect it depending on where the target is struck. I do have an active vibration sensor also which i am yet to try though suspect the results will be similar.

The velocity of the BB is low, travelling at around 300FPS and only weighing 0.2g so as you can imagine hitting a 3 ft long solid surface (silhouette of upper body) wont generate that much of a vibration regardless. keep in mind these rifles are designed for shooting people in a safe manner, so their forces are very low (however i can personally testify that you do know that you've been shot. lol had some impressive bruises, and lost two teeth to the sport)

I have toured with the idea of using an electret microphone, mounted tight against the targets surface on the rear to detect the frequency of the sound of the BB striking the target. it should give quite a loud crack so as long as i could tune out the other frequencies or perhaps the lower dB threshold so i could eliminate background noise triggering it

Accelerometers to detect shock... though i don't think there would be enough shock to gain a good reading.

I would welcome everyone's thoughts on what they feel would be the best way?

I haven't tried doing it, but I would guess that a sound-based solution would be the easiest approach to get working. I suspect the effectiveness would be very dependent on the material of the target - a target consisting of a thin rigid sheet would probably conduct sound better than a chunk of plywood, for example.

If you can't get that working perhaps it would be possible to put a rim round the outside of the target and stretch a film over it so that it was held just clear of the surface - so that it acted like a drum when the shot landed.

Yes thinking sound is the way to go. the second skin is a good idea, similar to a snare drum. Il keep that in mind if i don;t have any joy with the microphone

If you can record the impact on the target with a decent recorder and take the recording to a lab that has spectrum analyzer or play it into an Android phone with an audio spectrum analyzer app installed and find out the frequency (+/- x hertz) then you can design a very narrow band-pass op amp active filter that only amplifies sound in that band and put a threshold detector comparator circuit on the back end to trigger a 555 one-shot to send a signal to the arduino .

http://sim.okawa-denshi.jp/en/OPtazyuBakeisan.htm

A thought I had was a force sensor behind the target. Not positive it would work, but with a couple placed strategically, you should be able to even sense where a shot hit. Is this range indoors or outdoors?

raschemmel , thanks. It was a narrow band filter i was thinking of, though would like to try and avoid adding extra electronics to it if possible, and utilise a sensor interfaced directly to the arduino.

I had considered the force sensor, though figured it would be a fail, as there wouldn't be any movement of the target itself, unless it would detect tremors. Even so, with the vibration sensors i have tried with this method works great so long as the target is hit by the sensor.. not so great if on the outer edges for instance. Trying to find a more reliable solution to what ive tried. Think il have a play with some microphones and see what i can come up with there. its both indoors and outdoors. Different configurations on how the target is raised, revealed.

I'll bet a shatter sensor (automotive alarm breaking glass detector sensor sold in auto supply stores) would be perfect. I bet the frequency is similar. You might try one of those. They are what many people put on their cars if they live in a bad part of town.

Thats a good suggestion. could always experiment with the material of the target . sheet perspex may well be up there

Just put a car alarm at the target end in a safe place and put the crush sensor in the target zone behind something and arm the alarm. When it goes off it can flash some Headlights/tail-lights and a horn until you disarm it with the remote control.

lol not exactly the angle im going for but wey for thinking out the box :wink:

Well I don't think you are looking at it from the right angle. What's supposed to happen is when the alarm goes off you yell "hey !
Someone's stealing my Car ! That [blankety-blank] and you open fire on the target. It's what I call "value added..."

I have found using a piezo element with a weight on it and op-amp / comparator pair for detection works very well. More details in my blog at Internet of Things: High Sensitivity Vibration Sensor Using Arduino.

What function does the weight perform ?

I found gluing a weight to the piezo element using epoxy increased the vibration waves physical stress on the piezo element, effectively amplifying the electrical signal out of it, which is further amplified by the op-amp before being fed to the comparator for detection. This enabled the piezo element to detect much smaller vibrations. For example, using this circuit I am able to easily detect someone walking across a hardwood floor on the other side of the room. More details in my blog at Internet of Things: High Sensitivity Vibration Sensor Using Arduino

So basically it's a spin off of the soup-can telephone on a string ? (Progresso Soup TV add)