finish line judge- pinewood derby

I’m looking for help getting started building a finish line judge for pinewood derby for our local cubscout pack. Most commercial setups use phototransistors and infrared LEDs for sensing circuitry. I’d like to do the same since the components are cheap. I need a simple high/low type circuit to wire into the arduino that will let me sense if the beam between LED and phototransistor has been broken for each lane. After that, I can probably code the rest to determine which of the 3 lanes broke their beam 1st, 2nd, and 3rd and show an appropriate output.
Any help that can be given with the circuitry would be greatly appreciated.

I am doing a slightly similar project but mine is for a track that is currently 10cm wide and has mini electric dragsters racing down it. I however opted for the laser and LDR solution so that the width of the track could be changed without too much difficulty in the future.

What kind of width track are you talking about? For the photodetector I would expect that you would have to use an analogueRead for them which would be: 5v - Sensor - Gnd and an analog read pin from between the sensor and the ground pin to an analog pin on the arduino. depending on what emitters and detectors you use, you may be able to have a digitalRead of low or high but don't count on it.

Then the code for it should be relatively simple but just post it up if you have any other issues.

Mowcius

Thanks for the reply. Sounds like the simple "common collector" configuration? One schematic I've seen uses a schmitt trigger after the sensor for signal conditioning to TTL levels(high/low). Any experiences here?

Also, as for the setup I'm trying to build, I'm not really concerned with showing finish times as much as finish [u]order.[/u] So for a 3 lane track, I'd have 3 different beams looking at a fixed distance top to bottom somewhere in the 4 to 5 inches range.

Thanks again for any help.

One schematic I've seen uses a schmitt trigger after the sensor for signal conditioning to TTL levels(high/low). Any experiences here?

Sorry, no experience there...

Also, as for the setup I'm trying to build, I'm not really concerned with showing finish times as much as finish order. So for a 3 lane track, I'd have 3 different beams looking at a fixed distance top to bottom somewhere in the 4 to 5 inches range.

Maybe an infra-red imitter/receiver combe would be best for that. If you had an infra-red LED shining up from the bottom of the track and a receiver close to it pointing up then when the car went over, the light would reflect off the bottom of the car and fall on the detector/receiver.

Mowcius