I am planning on building a ballistic chronograph with an arduino.
Is the timer on the arduino precise enough to get the speed of a bullet?
The answer to your questions depends on how accurately you want to measure the speed of the bullet.
The answer to your question really depends upon what you are going to use for sensors and what you think your sensor path will be.
If you use interrupts, or direct port manipulation, you can get down in the microseconds range for start and stop times.
Maybe something like this:
// need to flesh out the pre-set and setup code, declare variables, pinMode with internal pullups, etc.
// then the fun part
void loop(){
while{(PIND & 0b00000100) == 1){
// waiting for D2 to go low
}
// it went low - capture time
startTime = micros();
while{(PIND & 0b00001000) == 1){
// waiting for D3 to go low
}
// it went low - capture time
endTime = micros();
elapsedTime = endTime - startTime;
speed = distance/elapsedTime;
Serial.print ("time: ");
Serial.println (elapsedTime);
Serial.print ("speed: ");
Serial.print (speed);
Serial.println(" distance unit/microSecond");
Serial.println (" ");
Serial.println ("press reset to continue");
while(1); // One option: hang out until sensors are setup, and reset is pressed.
}
I was planning on using infrared lights and IR photoresistors for the sensors.
I need to be able to detect the speed of the bullet plus or minus 10 feet per second.
do the math.
how fast would the measurement have to be if the sensors were placed 1mm apart ?
how fast would the measurement have to be if the senors were placed 1 meter apart ?
if you calculate for an accuracy that is 1FPS, can work your way back. remember that calculated values are rarely the same as measured values. better to be 4 times more accurate than you think you need to be.
Yeah, I plan on putting the sensors a foot apart. The bullets should be traveling about 3000FPS. That would make the required accuracy about 1/10,000 of an inch.
I was thinking the sensor would be attached to the end of the barrel in a PVC pipe. This would cause everything to be lined up correctly, I think.
flyingfisch:
I was thinking the sensor would be attached to the end of the barrel in a PVC pipe. This would cause everything to be lined up correctly, I think.
Thats a good way to injure yourself. Even if you do manage to get it lined up, that PVC is going to shatter and melt from the pressure/heat.
The laser grid idea sounds cool. I have never seen one made like that, I kind of want to try it.
I was thinking shoot thru 2 strands of wirewrap wire, each break releasing a pin to change state.
JohnLincoln:
Combine CrossRoads wire break method with my zig-zag backwards and forwards approach to ensure the wire gets severed.
Wrap the wire backwards and forwards across a 'picture frame' with rows of small nails/pins around the edges.
You would only need the zigzag (nice idea BTW) at the far end of the range as the gun barrel can be positioned accurately enough to bread a single beam. No need then to keep replacing broken wires and if your using the same photo detectors both ends then there reaction times cancel out but a wire vs photo detector means compensating for the different reaction times.
I am working on a high accuracy timing device (>90ps resolution) that should easily adapt to this if more accuracy is needed.
As far as PVC at the end of the barrel goes, I saw some commercial designs that used a device at the end of the barrel as well as some DIY examples that did the same thing. Maybe they were using it on a spud gun or .22 or something? The commercial designs were only half-pipes though.
The wire break method sounds interesting, but if we do it that way I would not be able to fire through it repeatedly very well...
flyingfisch:
The wire break method sounds interesting, but if we do it that way I would not be able to fire through it repeatedly very well...
If you use the photodiode or phototransistor method, be aware that the conditions around the path at the first and second measurement station should be the same. You need to take into accont that the beam-break will be affected by the air between the transmitter and receiver of the beam. If the first station is in a PVC pipe or half pipe, and the other end is not, the pressure wave will be different, and may put you off by a few microseconds.
This is partivcularly problematic if your first beam is very close to the muzzle, and a 'plug' of high velocity air will preced the bullet, and will deflect the beam before the bullet arrives.
This clip is more about the Communications to an Android device through blue-tooth, but the hardware is Arduino based.
In this case it is timing a nerf gun, but the commentary says 3000fps is measurable.
