Magnethead494
Well-known member
For a multimedia project, I want to build a simple MOV camera to take to the track. I want to make a laser tripwire using a standard laser pointer and an LDR inside a tube. This tripwire is read by an mcu, which then puts a 5V circuit to the camera as the remote trigger by toggling a 2N2222 on the ground side of the trigger circuit. The camera is prefocused and pre-zoomed, so it shoots on the rising edge.
http://chdk.wikia.com/wiki/USB_Remote
The plan was to use an uno that I already have, and run it on 3 D cells, or a lantern battery with a 5V LDR. But I was told that there's a good chance that the 16MHz CPU speed would not catch a car breaking the beam at 160+MPH (I'm doing ideal calculations at 300MPH and realistic at 160MPH) on a reliable basis, and told that The Due would be better, but that the T3 might be able to do what I want.
By my figure, 1/16M is 6.25 x 10^(-8) and 1/48M is 2.083 x 10^(-8) of a second.
If the sensor is 3mm across, at 300 MPH (way over ballpark here) then that is 2.23 x 10^(-5) of a second ( 3 / 134,112 ). That's 1,073 cycles at 48MHz and 358 cycles at 16MHz.
Is that enough cycles to register the breaking of the beam?
I need to get the LDR, the hard part will be setting up the voltage divider to simulate a digital IO to activate an ISR. From what I've read, a sample unit can be anything from 2M to 200K unlit by laser, and 100-200 with the laser on it. So I just have to make a voltage divider to make a 0.7 (low) and 3.0 (high) volt output on the actual numbers.
My camera's Field of View is about 60 degrees at 5mm/28mm/f2.8 so I have a slightly narrow window to work with. My next battle is where to place the laser tripwire.
If I place the tripwire 30 feet before the camera, and assume the total reaction time from beam break to shutter is 1/10 of a second,
60 MPH: 88 ft/s = (30 - 8.8 ft ) / 88 = 0.24 s
90 MPH: 132 ft/s = (30 - 13.2 ft ) / 132 = 0.13 s
120 MPH: 176 ft/s = (30 - 17.6 ft ) / 176 = 0.07 s
160 MPH: 234 ft/s = (30 - 23.4 ft ) / 234 = 0.03 s
200 MPH: 293 ft/s = (30 - 29.3 ft ) / 293 = 0.002 s
that's the amount of offset delay required for a constant trigger position. In other words, 30 feet away would be perfect for pro-mods and fuel altereds, but the closer the car, the closer I need the tripwire to the camera. What I want to do is find a happy medium so that I get a shot somewhere near the finish line, before/after is fine for the most part (I'm going for close, not exact).
Each lane is 25 feet wide or so, so figure a 60 degree FOV, focused on the midstripe plus a 5 foot back-distance, makes a 30/60/90 triangle with a 30 foot long leg, and by trig, the short leg is 15 feet. So I have a 30 foot total viewing window to get within, 15 feet before the finish line and 15 feet after.
Based on that table above, I am going to want to bias the laser closer to the camera.
60 MPH: 88 ft/s = (18 - 8.8 ft ) / 88 = 0.10 s
90 MPH: 132 ft/s = (18 - 13.2 ft ) / 132 = 0.036 s
120 MPH: 176 ft/s = (18 - 17.6 ft ) / 176 = 0.002 s
160 MPH: 234 ft/s = (18 - 23.4 ft ) / 234 = - 0.023 s
200 MPH: 293 ft/s = (18 - 29.3 ft ) / 293 = - 0.038 s
So around 18 feet is a good starting point. Lots of trial and error will be involved.
But the big thing- is 48MHz enough? Or do I really need to go to a Due to capture a reliable beam-break?
http://chdk.wikia.com/wiki/USB_Remote
The plan was to use an uno that I already have, and run it on 3 D cells, or a lantern battery with a 5V LDR. But I was told that there's a good chance that the 16MHz CPU speed would not catch a car breaking the beam at 160+MPH (I'm doing ideal calculations at 300MPH and realistic at 160MPH) on a reliable basis, and told that The Due would be better, but that the T3 might be able to do what I want.
By my figure, 1/16M is 6.25 x 10^(-8) and 1/48M is 2.083 x 10^(-8) of a second.
If the sensor is 3mm across, at 300 MPH (way over ballpark here) then that is 2.23 x 10^(-5) of a second ( 3 / 134,112 ). That's 1,073 cycles at 48MHz and 358 cycles at 16MHz.
Is that enough cycles to register the breaking of the beam?
I need to get the LDR, the hard part will be setting up the voltage divider to simulate a digital IO to activate an ISR. From what I've read, a sample unit can be anything from 2M to 200K unlit by laser, and 100-200 with the laser on it. So I just have to make a voltage divider to make a 0.7 (low) and 3.0 (high) volt output on the actual numbers.
My camera's Field of View is about 60 degrees at 5mm/28mm/f2.8 so I have a slightly narrow window to work with. My next battle is where to place the laser tripwire.
If I place the tripwire 30 feet before the camera, and assume the total reaction time from beam break to shutter is 1/10 of a second,
60 MPH: 88 ft/s = (30 - 8.8 ft ) / 88 = 0.24 s
90 MPH: 132 ft/s = (30 - 13.2 ft ) / 132 = 0.13 s
120 MPH: 176 ft/s = (30 - 17.6 ft ) / 176 = 0.07 s
160 MPH: 234 ft/s = (30 - 23.4 ft ) / 234 = 0.03 s
200 MPH: 293 ft/s = (30 - 29.3 ft ) / 293 = 0.002 s
that's the amount of offset delay required for a constant trigger position. In other words, 30 feet away would be perfect for pro-mods and fuel altereds, but the closer the car, the closer I need the tripwire to the camera. What I want to do is find a happy medium so that I get a shot somewhere near the finish line, before/after is fine for the most part (I'm going for close, not exact).
Each lane is 25 feet wide or so, so figure a 60 degree FOV, focused on the midstripe plus a 5 foot back-distance, makes a 30/60/90 triangle with a 30 foot long leg, and by trig, the short leg is 15 feet. So I have a 30 foot total viewing window to get within, 15 feet before the finish line and 15 feet after.
Based on that table above, I am going to want to bias the laser closer to the camera.
60 MPH: 88 ft/s = (18 - 8.8 ft ) / 88 = 0.10 s
90 MPH: 132 ft/s = (18 - 13.2 ft ) / 132 = 0.036 s
120 MPH: 176 ft/s = (18 - 17.6 ft ) / 176 = 0.002 s
160 MPH: 234 ft/s = (18 - 23.4 ft ) / 234 = - 0.023 s
200 MPH: 293 ft/s = (18 - 29.3 ft ) / 293 = - 0.038 s
So around 18 feet is a good starting point. Lots of trial and error will be involved.
But the big thing- is 48MHz enough? Or do I really need to go to a Due to capture a reliable beam-break?