Practical (air) Pistol / BFTO

	Build your own stop plate [ Return to shooting index ] [ Build instructions ] [ Usage guidelines ] [ The future . . . ] [ Built to order ] last update : 20030420 (built to order, usage guide) UPDATE 7/10/02: the pair of plates I made were successfully used in the 29/9/02 BFTO pistol match, surviving Brocock hits at close (less than 1 meter) range. One set was also used at the British National PP competition 2002 where airsoft shot pick-up was a problem. /UPDATE An important part of PP is the timing of each stage. The main problem in detecting the 'last shot fired' is the low energy of all airguns (even worse for airsoft). There are a number of ways to detect last shot: audio timer (eg CED6000) microswitched plate piezo sensor plate 1: Audio timers Originally designed for use in full-bore PP competitions, these can often be adjusted to pick up even airsoft discharges (sensor must be very close to gun in this case). Advantages: accurate, self-contained, time per shot, starting sounds. Disadvantages: expensive (~£100), pickup problems, complex, external noise issues. 2: Microswitched plate Advantages: quite easy to build, cheap. Disadvantages: sensitive to angle of set-up, requires calibration, very sensitive to wind (no good outside). 3: piezo sensor plate Advantages: cheap, reliable, sensitive, highly wind resistant. Disadvantages: more complex to build, requires some electronics. IMHO, #3 is the most useful - it requires more 'engineering' then the other two but does not have the high cost of #1 or the sensitive nature of #2. Overview Impacts on a light weight aluminium plate are transfered to a pietzoelectric sounder which in turn produces a signal that stops a timer. Pietzoelectic devices incorporate a disk of ceramic that changes shap whenever a voltage is put across it - the reverse is also true, changing the shape of the ceramic generates a voltage. The trick is to mount the sensor in such a way as to pickup the slightest impact without having to worry about wind or slope of the ground. My current setup means that the slightest movement of the plate bends the piezo sounder, making it very sensitive to impact (so why not wind? - the sensor responds best to sudden movement, wind tends to be much more steady). Procedure Get the main components: stop plate, piezoelectric sounder, support material, suitable nuts and bolts. Drill/ grind two holes in the sounder's backing plate (the right size to take the bolts). Drill bolt hole in support material and assemble support structure. Bolt support to sounder to plate. Build circuitry. Test! Easy! 1: Main components Most are easily obtainable from a large hardware store (I got the piezoounder from Maplin, cat. YU87U). Choice of stop plate material depends on what you want to stop; I use 3mm aluminium for airguns (will stop even hard tin Brocock shots with no damage), I imagine for airsoft only you could use far lighter materials. UPDATE: 3mm aluminium is not quite up to lots of Brocock shooters, 2mm steel would be better. exVAT ea inc VAT timer NE19 rigid PP3 clip 1 £0.25 £0.29 FH98G SPDT Ultra Min Tggle 1 £1.01 £1.19 FH60Q Blk Push to Break Sw 1 £0.42 £0.49 M300R Min Res 300R 1 £0.06 £0.07 UK49D LED Green 5mm 2mA 1 £0.15 £0.18 UQ88V 1A 6V Hi Sens BT51 1 £1.27 £1.49 YN38R Power Supply Case 1 £1.69 £1.99 YW06G Chassis Phono Skt 1 £0.42 £0.49 JB24B banana plug red 1 £0.42 £0.49 JB25C banana plug black 1 £0.42 £0.49 FK84F HP buzzer 1 £2.29 £2.69 KC91Y T2 box 1 £1.18 £1.39 XR89W 4 core alarm white 1 £0.25 £0.29 235-5059 desk version digital timer 1 £20.99 £24.66 FF71N sub-min toggle K 1 £1.27 £1.49 sensor YU82D Piezo Xducer 50/2.8 1 £0.67 £0.79 QH30H C106D SCR 1 £0.67 £0.79 HH04E Line Phono 1 £0.42 £0.49 UK20W hyperbri LED Red 5mm 1 £0.67 £0.79 cable HH01B Phono Plug Screened 2 £0.67 £1.57 PB60Q Loudspeaker Reel 0.25 £9.78 £2.87 £45.00 With the exception of the desk timer the list references Maplin codes and prices (timer is RS ). You may want more of the 4-core cable (to reach from ear height to the ground, for example). If you already have a shot timer with an external input (like a CED6000) this cuts down the kit list, dropping the price to about £14. 2: Drill holes in the sounder's backing plate The piezo material itself is very fragile, so it is mounted on a thin brass disk. Cut two holes in the brass to take the mounting bolts. This needs to be done carefully, too much vibration will break the (delicate) electrical connection to the ceramic - once this happens the sounder is ruined. I recomend you epoxy the contact wires to the plate - this makes the whole thing far more robust. The best way I have found to cut the holes is to use a router bit on a minidrill, this eliminates any bending forces likely to be applied during drilling. 3/4: Assemble support structure/mount sensor As the disk itself will probably break (the ceramic spall off) under excess twisting force, you need to support the plate so that edge hits do not damage the sensor. Construct a cruciform frame, using foam to support the weight of the plate. Note that this will not affect the operation of the sensor as it is very sensitive to motion (tapping the sensor with a finger nail provides enough motion to trigger the latching circuit). Images show components (yes, I know the first pic has a square plate, it's from the second one I made) and completed structure (note mounting method for piezo disk): 5: Build circuitry UPDATE: circuit design has been changed (thanks Ewok!), it's even slightly simpler than before! Basically, replace the 130ohm resistor with the relay and up the supply voltage to 9V (loose the other SCR). See new diagram, below. For a different approach to the circuit (using a 555 timer for automatic reset, also good for 'hit sensitive' plates with no timing), see the Weston Electronics site here . This is probably the most demanding part of the project, requiring some facility with electronic components and a soldering iron. Two circuits are needed, one on the plate and one to actually do the timing. The components are speced for a 9V supply. A power switch and a push-to-break reset switch will also be useful (power must be interupted to deactivate the latch circuits). Keep the wire connected to the gate (g) as shot as possible - it acts as an anntenna (locking the SCR into it's 'on' state). The plate side circuit can be constructed using terminal block. Please forgive my lack of correct component symbols - too much effort when you're drawing with Paintbrush! A veroboard layout map and/ or photos will be posted eventually. If you have a CED6000 just remove the DPDT switch and buzzer (note that the CED requires a normally closed switch circuit, not normally open like stopwatches) Plate side circuit.
Top	Useage Guidelines The plate can be mounted at any angle from vertical to horizontal. Do not tilt the unit forwards (puts too much strain on the sensor)! I recommend cable-tieing the cross member to a suitable bar or bolting the whole thing to an upright post.You could also glue the plate directly to the foam, 'ruggedizing' the unit (that's Ewok's idea). The system is not quite wind immune, gusty wind will set it off. The best way to stop this is to increase the amount of foam padding, completely supporting the plate. One problem with the system is the high sensitivity of the sensor - a hit on the mounting post will often be enough to trigger the unit. It may be possible to stop this by shock isolating the plate/ sensor from it's mounting post, but the easiest way is to put a barrier in front of the post.
Top	Future plans At the moment the system is a bit fragile (sensitive to miss-handling). An ideal setup would be to epoxy the piezo disk straight to the metal plate - much more robust! This should be possible with more sensitive circuitry (if you are just a Brocock shooter you can do this anyway, the hit produces enough vibration to trigger the existing circuit). It would also be possible to make a full size IPSC target this way . . . fully automated scoring anyone?
Top	Build to order I have been asked several times if I will build plates for people - the answer is 'I suppose so' (the construction is not that hard, do it yourselves - I'll even offer tech support!). Below is the price list: Timed plate with stopwatch: £60 (£49 for timer unit, £6 for sensor and £5 for a 25m cable). Timed plate for use with CED6000: £25 (£14 for timer unit, £6 for sensor and £5 for a 25m cable). Postage extra, and you have to find/ build your own plate and stand.
Top	History 20030420 'Build to order' section added. Usage guide modified. 20030419 Circuit diagram corrected.