I'm thinking of trying to make an electronic scoring system for 10m air pistol/rifle use.
The commercial units (quite expensive) use, I believe, 3 x IR lasers to triangulate the position of a pellet passing through the plane of the 3 lasers. The precision needs to be sub-millimetre, probably 0.1mm or so. Measuring at a distance of maybe 100mm. The pellets will be passing the lasers at around 160 m/s.
Is this something that I can realistically expect to achieve, and what lasers/sensors would do the job?
Some… I built an Arduino-based replacement for an old dashboard clock using a GPS module for time, stopwatch/lap timing, and average speed. But I didn't program from first principles, rather I copied, pasted and adapted other people's code. SO that's about my level… pretty good at bodging things together, but I probably don't fully understand what I'm doing.
If I have a laser diode with a Vf of 5.6V and a 12V power supply, what would be the value for the current limiting resistor for a laser current of 70mA.
If not you may have some difficulty doing what you want
If the project is for your personal use, stay with paper targets and a proper backstop.
If the project is for a group or a non-profit, consider asking the commercial manufacturers for a donation; maybe for some consideration such as "Sponsored by..." etc.
Building a home unit for sub-millimeter accuracy is going to involve a considerable investment of engineering time.
Now, you could outfit a rifle/pistol with a 1-3 mW laser pointer inside the barrel and construct a rather nice electronic target of phototransistors (or LEDS used as optical sensors) and a reasonable cost.
otoh, without googling anything, I think that will depend on the resistance of the laser diode. But assuming the diode resistance can be ignored, we need a resistor that will ensure 70mA current flowing through diode+resistor given a 12V supply. I think that Vf is the voltage drop across the diode when operating with current in the forward direction, so 6.4V needs to drop across the resistor. V=IR, so the resistor value would 6.4/0.07, which by mental arithmetic is around 90.
So 90 Ohms. I'm sure that's either very close, or completely wrong.
However, a resistor for a laser diode is not appropriate except for those $1 cheap-butt pointers. Lasers utilize "laser driver circuit" which is an active current limiting circuit rather than the passive resistor. The more advanced circuits for lasers can be rather complex: LASER DIODE DRIVER BASICS – Wavelength Electronics (teamwavelength.com)
It would have to be suitable for use in club competitions, so a laser pistol wouldn't do the trick – but I can see you're probably right that it might be easier
I wondered about the precision also - haven't found a specification for that for the project, but it's clearly good enough for a lot of shooting clubs which is a good sign.
As an avid air-rifle and air-pistol (CO2) enthusiast, the most serious problem with laser simulations is the weight of the gun and the lack of realistic recoil. There are few good solutions without approaching the firearm manufacturers.
