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Simple Velocity measurement Board without yC

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LAN_403

Reaching

Tue Oct 31 2006, 09:30PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

okay, from time to time im into coilguns for a few months so i wanted a cheap and relative exact circuit to measure the projectiles velocity. i build several prototypes of this board but some of them had failures and bugs, but now, after some fiddling i got a nice small circuit which is exactly what i wanted.

the function is really simple.

you need two fotogates. one for the count impulse and the other for the stop impulse. a 1mhz crystal oscillator is switched via a rs flipflop through a cd4026 counter dekade when the start impulse comes in. when the projectile passes the second fotogate the flipflop resets and stops counting. now you have say 3476 counts on the display. thats the time in 1/1000000 seconds. depending on the distance between your fotogates you can calculate the speed in m/s or feet/second. in my case , the distance between those fotogates is 10cm so i have to calculate 100000/3476 or whatever is on the display.the resulting speed is 28,768m/s .. i wanted to build up this little circuit for those who dont have knowledge about microcontrollers . its just a simple but exact circuit, for sure nothing for those pro coilgunners. i have 2 versions of the board. the first one with 4 7 segment displays, the second with 6 7 segment displays. for sure you can use a 2mhz crystal instead of 1mhz to rise accuracy. and you can change the distance between the fotogates to say 1 metre or so to be even more accuratly. the 4 display version can only be used for relatively fast projectiles, from 10m/s up to say 300m/s with a relatively good accuracy. the 6 display version can display a whole second, so this is the better choice when youre not sure if your projectile is faster than 10m/s. you have to calculate a bit to get the formula you need to calculate m/s or feet/s for your distance.

here is a pic of the finished counter with a cigarrette lighter for size comparisation

Velocount2klein

and heres the link for the board layout. its made with eagle, dont know it seems not to popular here

when i finished the shematic i can place a link here of course

Marko

Tue Oct 31 2006, 10:38PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

It's cute!

Have you tested it yet for accuracy?

Brillant idea alltogether.

Reaching

Tue Oct 31 2006, 11:11PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

yeah, i tested its accuracy. it mainly depends on the distance between the fotogates. the more time to count the more accuracy. i have choosen 10cm cause i wanted it compact.. but if you want to measure very fast projectile with more than 100m/s you should add a faster crystal and enlarge the distance between the fotogates. for example. you have a 10cm fotogate distance and a projectile which is 62,5m/s fast you get a reading on the display of 1600 so thats 1,6mS for a distance of 10cm. the crystals accuracy is +-10 counts per second so you have 1610 counts worst case, so you get 62,11m/s which is -0,39m/s.. if you enlarge the distance to say 1metre you get 10 times the accuracy, but 10x1600 counts so you have to use the 6 display version. okay if you want to measure speeds which are supersonic the accuracy gets worse, but for normal rail and coilgun use this circuit is exact enough, and its cheap,compact and you dont need to use a YC.

i used the earlyer version of this circuit for months in my multistage coilgun research and made several tests with same settings, cap voltage projectile starting point etc and i got a really good reading of maybe +-40counts or so (maybe the voltage of the cap lost 2 volts or so) with the new version i had no problems. you can use any fotogate you want, it should have a positive output. the counter will do the rest. a 74hc14 shmitt trigger transforms the signal from the fotogate in a nice squarewave which triggers the flip flop,. the inputs are negative via a 2,2k resistor so you dont have to worry to toggle the flipflop when the fotogates are disconnected (the counter would go mad without these resistors, one failure in the earlyer version) with the small switch onboard you can reset the counter. the maximum frequency for the counter decade is 8 mhz at 15 volts (i use 5 volt so 2,5mhz is max for 5 volts)

Update

Finished the big version of this circuit, with 6 digits, .
Velocount3klein

with a fotogate distance of 1metre you can measure a velocity of 1000m/s with a tolerance of only 1%

(counts 1000 on 1 metre fotogate distance with 1mhz clock, failure count 10 counts per second) a another good thing. you can measure verly low speeds with an accuracy of nearly 100%. with a fotogate distance of 1 metre you can measure a minimal velocity of 1,000001m/s. when someone is interested in the 6 digit board layout i can add it here too

ragnar

Sat Mar 31 2007, 01:06PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Up from the grave, heya Reaching =)

I was googling for CD4026-based frequency counters, and came to this thread -- is there any chance you've got an Eagle copy of the six-digit board available?

CliffB

Sat Mar 31 2007, 10:54PM

Registered Member #614 Joined: Thu Mar 29 2007, 03:35AM
Location:
Posts: 3

I would also appreciate a copy of the diagram for the six digit counter. Do you know if there is a free way to just view an eagle layout? I am going to test the trial with your circuit. There is actually a version for hobbyists(non profit) at :

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