contact mic trigger circuit - UK Vintage Radio Repair and Restoration Discussion Forum

adibrook · 25th Jan 2007, 4:33 pm

I need a circuit to close a connection when a sheet of plywood or other material is struck by an air rifle pellets. I'm screwing around with shooting stuff and getting pictures of the impact again.

I got pretty good results using another technique. I just connected the two wires to a couple of pieces of foil seperated by about 2mm, and put the target (apple) behind this. As the pellet passes through the foil, it closes the circuit momenterely. By the time my laptop processes it and actually takes the frame, the pellet shoudl have hit the apple. However, it didnt work! I UNDERestimated modern electronics. I thought the pellet would travel about 2 meters ish in the time it takes the photo to be taken. But it was surprisengly less, so the bullet didnt even hit the apple when the frame was taken.

The problem with this method is, that the foil ''trigger'' targets work only once, maybe twice. And they are annoying to make.

So, i thought of a completly new method. I will use a sheet of thin plywood with a contact mic stuck to it. When the pelelt strikes the wood, a contact mic stuck to it will pick up the vibration, and trigger the photo. by that time the pellet woudl ahve allredy gone through the wood and hopefully hit the apple.

I can use a headphone epoxied to the wood as a contact mic. My friend does this alot and for musical purpuses it works quiet well.

At the moment i'm thinking that it needs a high gain preamp, either an op-amp or a descrete amp. Of course, distortion doesnt matter here (i aint gonna be playing mozart through it!). Then a diode (possibly) and this connected to the trigger pin of a 555. The output can trigger a FET or bipolar transistor across the contacts.

So...as soon as the pellet hits the wood, the 1st positive pulse from the contact mic is amplified, and triggers the 555 monostable, which triggers a picture to be taken.

Any ideas people?

I'm not sure how sensetive the 555's trigger pin is, but maybe its possible to run it off the mic directly.

Thanks.

25th Jan 2007, 6:01 pm

Hi
Have a look at the following link
http://www.hiviz.com/tools/triggers/triggers2.htm
The circuits are simple but they do work.

arjoll · 26th Jan 2007, 10:42 am

Quote:

Originally Posted by adibrook

At the moment i'm thinking that it needs a high gain preamp, either an op-amp or a descrete amp. Of course, distortion doesnt matter here (i aint gonna be playing mozart through it!). Then a diode (possibly) and this connected to the trigger pin of a 555. The output can trigger a FET or bipolar transistor across the contacts.

Sounds plausable, use the op amp as a comparator, triggering the 555 as a monostable to get the length of pulse you want. Your 555 should be able to drive a relay directly. From memory a 555 triggers on a low pulse (<= 1/3 supply) so you might have to put an inverter in there, say a 4049.

Someone can probably suggest something better, or with a lower chip count though

Biggles · 26th Jan 2007, 1:54 pm

You may have to play around with mic amp gain levels, as you don't want the circuit to trigger from extraneous noises, i.e. from the air rifle firing. An ultrasonic transducer may be worth trying as the pick up device, or you could experiment with a strain guage bridge circuit, triggered by the flexing of the wood. A cmos version of the 555 may be more sensitive for direct input triggers, but could be slower to respond.

Biggles.

adibrook · 27th Jan 2007, 5:34 pm

I'v dug around and found a HCF4001B quad NOR chip. That should work as an inverter.

I came up with this circuit, but havnt actually built it yet.

http://img.photobucket.com/albums/v4...ook/triggg.gif

The mic will be a normal in-eat type headphone stuck to the sheet of material.

In theory, any noise picke up by the mic will turn into positive and negative pulses. The 20K pot sets the op amps bias, so when the positive pulses are less than the voltage on the wiper, nothign will happen. However, as soon as a positive pulse comes along thats bigger than the voltage on the wiper, the op amp will amplify it by its open loop gain, and the oputput will go to the NOR gate. The NOR gate holds the trigger of the 555 high untill the output of the op-amp reaches logic 1, and then the NOR gate's output goes logic 0, and that triggers the 555. This makes the FET conduct and trigger the camera.

Or have i misunderstood something?

BTW, the two wires that go to the camera, one is connected to the cameras ground, and the other is at 0.65v, as measured by mi AVO minor. I presume 0.65 is logic 1 for the camera's circuitry, and the switch (here repalced with a FET) brings it to logic 0. Would a Jfets on resistance be sufficiantly low to replace a normal push button switch?

Thanks.

Merlin · 29th Jan 2007, 4:26 pm

How about setting up a strong electro magnet near the apple? As the pellet passed through it maybe you could pickup a change in EMF or something- plus your view wouldn't be obscurred...

adibrook · 30th Jan 2007, 12:56 am

The pellet is lead. I'm not sure what effect (if any) lead has on magnets.

However, the good thing about the contact mic idea is that i can allways use a sheet of see thru plastic. That way i can aim at stuff behind it, and not rely of estimating the trajectory.

I'v made some progress tho. It seems that the camera is quiet happy being switched by an ordinerly BC108. Which is familiar to me and i have a plentiful supply of.

So the FET can be replaced by a BC108 in the schematic.

25th Jan 2007, 4:33 pm	#1
adibrook Retired Dormant Member Join Date: Dec 2004 Location: Weymouth, Dorset, UK. Posts: 377	contact mic trigger circuit I need a circuit to close a connection when a sheet of plywood or other material is struck by an air rifle pellets. I'm screwing around with shooting stuff and getting pictures of the impact again. I got pretty good results using another technique. I just connected the two wires to a couple of pieces of foil seperated by about 2mm, and put the target (apple) behind this. As the pellet passes through the foil, it closes the circuit momenterely. By the time my laptop processes it and actually takes the frame, the pellet shoudl have hit the apple. However, it didnt work! I UNDERestimated modern electronics. I thought the pellet would travel about 2 meters ish in the time it takes the photo to be taken. But it was surprisengly less, so the bullet didnt even hit the apple when the frame was taken. The problem with this method is, that the foil ''trigger'' targets work only once, maybe twice. And they are annoying to make. So, i thought of a completly new method. I will use a sheet of thin plywood with a contact mic stuck to it. When the pelelt strikes the wood, a contact mic stuck to it will pick up the vibration, and trigger the photo. by that time the pellet woudl ahve allredy gone through the wood and hopefully hit the apple. I can use a headphone epoxied to the wood as a contact mic. My friend does this alot and for musical purpuses it works quiet well. At the moment i'm thinking that it needs a high gain preamp, either an op-amp or a descrete amp. Of course, distortion doesnt matter here (i aint gonna be playing mozart through it!). Then a diode (possibly) and this connected to the trigger pin of a 555. The output can trigger a FET or bipolar transistor across the contacts. So...as soon as the pellet hits the wood, the 1st positive pulse from the contact mic is amplified, and triggers the 555 monostable, which triggers a picture to be taken. Any ideas people? I'm not sure how sensetive the 555's trigger pin is, but maybe its possible to run it off the mic directly. Thanks.

25th Jan 2007, 6:01 pm	#2
jmshep Guest Posts: n/a	Re: contact mic trigger circuit Hi Have a look at the following link http://www.hiviz.com/tools/triggers/triggers2.htm The circuits are simple but they do work.

26th Jan 2007, 1:54 pm	#4
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: contact mic trigger circuit You may have to play around with mic amp gain levels, as you don't want the circuit to trigger from extraneous noises, i.e. from the air rifle firing. An ultrasonic transducer may be worth trying as the pick up device, or you could experiment with a strain guage bridge circuit, triggered by the flexing of the wood. A cmos version of the 555 may be more sensitive for direct input triggers, but could be slower to respond. Biggles.

27th Jan 2007, 5:34 pm	#5
adibrook Retired Dormant Member Join Date: Dec 2004 Location: Weymouth, Dorset, UK. Posts: 377	Re: contact mic trigger circuit I'v dug around and found a HCF4001B quad NOR chip. That should work as an inverter. I came up with this circuit, but havnt actually built it yet. http://img.photobucket.com/albums/v4...ook/triggg.gif The mic will be a normal in-eat type headphone stuck to the sheet of material. In theory, any noise picke up by the mic will turn into positive and negative pulses. The 20K pot sets the op amps bias, so when the positive pulses are less than the voltage on the wiper, nothign will happen. However, as soon as a positive pulse comes along thats bigger than the voltage on the wiper, the op amp will amplify it by its open loop gain, and the oputput will go to the NOR gate. The NOR gate holds the trigger of the 555 high untill the output of the op-amp reaches logic 1, and then the NOR gate's output goes logic 0, and that triggers the 555. This makes the FET conduct and trigger the camera. Or have i misunderstood something? BTW, the two wires that go to the camera, one is connected to the cameras ground, and the other is at 0.65v, as measured by mi AVO minor. I presume 0.65 is logic 1 for the camera's circuitry, and the switch (here repalced with a FET) brings it to logic 0. Would a Jfets on resistance be sufficiantly low to replace a normal push button switch? Thanks.

29th Jan 2007, 4:26 pm	#6
Merlin Retired Dormant Member Join Date: Apr 2006 Location: Shrewsbury, Shropshire, UK. Posts: 67	Re: contact mic trigger circuit How about setting up a strong electro magnet near the apple? As the pellet passed through it maybe you could pickup a change in EMF or something- plus your view wouldn't be obscurred...

30th Jan 2007, 12:56 am	#7
adibrook Retired Dormant Member Join Date: Dec 2004 Location: Weymouth, Dorset, UK. Posts: 377	Re: contact mic trigger circuit The pellet is lead. I'm not sure what effect (if any) lead has on magnets. However, the good thing about the contact mic idea is that i can allways use a sheet of see thru plastic. That way i can aim at stuff behind it, and not rely of estimating the trajectory. I'v made some progress tho. It seems that the camera is quiet happy being switched by an ordinerly BC108. Which is familiar to me and i have a plentiful supply of. So the FET can be replaced by a BC108 in the schematic.