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Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
In order to build a speed trap with similar characteristics of the commercial ones with fast response, wide speed range i decided to design my custom project.
The sensor are photodiode based, with LM393 comparator, wired to have very fast response time and keep the error of maximum 1 LSB of the counts (200ns)
The counts are from 200ns to 3.35s (timeout). The sensors are 10cm spaced apart so, an hipothetical 1Km/s projectile would spend 100us +- 0.2 us time to travel it, giving a resolution of 0.2%
The timer uses hardware CCP1 and 2 modules (Capture/compare/PWM) so no software delays.
THe speedtrap works regardless of which is the first sensor crossed.
THe pic is programmed in picbasic pro and features, full floating point operations (sum,diff,multiplic,division,sqrt) (numbers with 32 bit mantissa and 8 bit exponent) with suffix notations , significant digits (automatic) management, automatic kinetic energy calculations . Shot statistics (avg, max, dev.st) with 4 memory slots, and single shot memory with 14 positions.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
I've got a bunch of 16f690's that I ordered thinking that the 18f series were all 16 bit. You're using a 20MHz clock, correct? If I remember right that gives an instruction cycle time of 200ns.
I know photo diodes are really fast, but aren't they really low level? Is that what the comparator circuit does? Trigger when there is a change in the diode output?
I've also never worked with LCDs. Are the pinouts pretty universal? I pulled a small LCD out of something commercial but it had a header on it that plugged into a main board, do you think I could drive this myself?
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
Turkey9 wrote ...
I've got a bunch of 16f690's that I ordered thinking that the 18f series were all 16 bit. You're using a 20MHz clock, correct? If I remember right that gives an instruction cycle time of 200ns.
Yes, 20MHz means 50 nanosecond cicle, since one istruction is 4 clock cicles, you get 200ns. For mobile point math you need 18 series Pics, but it isn't really necessary. you can use fixed point math,
Turkey9 wrote ...
I know photo diodes are really fast, but aren't they really low level? Is that what the comparator circuit does? Trigger when there is a change in the diode output?
THe photodiodes with the irradiation i use (100mA led driving) swing about 1V with 22Kohm load resistor, that's enough to trigger the comparator (with a bit histeresys), since is use 300mV threshold
Turkey9 wrote ...
I've also never worked with LCDs. Are the pinouts pretty universal? I pulled a small LCD out of something commercial but it had a header on it that plugged into a main board, do you think I could drive this myself?
The pinout is pretty much the same for alfanumeric LCDs based on HD44800 controller (almost all alphanum)
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PIC18F2550-based, hyperfast, 200 nanosecond resolution Projectile Speed trap
