MrDEB

Been looking at revamping a Pinewood Derby finish line unit by using IR instead of a bright light and photo diodes or ?.
Problem is the present unit will false trigger sometimes due to ambient light etc.
planning on putting the IR emitters in the track and dectors in covered bridge.
Do I need to pulse the IR keds?
looking at some narrow angle leds from Reynolds electronics tsal7200
has a 17degree angle and 920nm?
Have seen lots of circuits that pulse the IR leds at 38gzh or so but is this nessary?
Also the dectors Reynolds has have 3 leads on them insteadof just 2?
want 12-14" disance from track to bridge.

jpanhalt

I checked the Reynolds site. The detectors shown are standard and have three pins that are power, ground, and output data. The site shows how to connect them.

I think you can assume they will require a modulated signal to work as intended, like the Sony, Vishay, other detectors.

From what I understand of your project, you want to detect which car passes the finish line first, and you have 6to 8 lanes to monitor. My advice would be to put a detector in each lane that the car would go over. Build the emitter and detector close together and depend on reflected IR light from the car as it passes over. This is a recent thread where I posted a simple PIC based detector.

http://www.electro-tech-online.com/m...pic16f628.html

There are many versions around.

Pinewood Derbys can get pretty competitive -- at least among the parents. I think the dispersion problems of using a single beam would be difficult to resolve, and in a close finish it would still be hard to determine which car finished first. Individual detectors can be wired to show only the first signal

MrDEB

The emitter will be in the track and the dector in a bridge that spans the width of the track.
Can I use the 3 pin dector as a simple switch (present setup uses photo diodes or phototransistors??) with 2 wires. The light source is a simple light bulb located in the bridge over the track.
IMO using IR emitters in the track and an IR dector overhead in the bridge located 12-14" above the track.
Never used IR emitter/dectors so am in the dark as to needing to pulse the IR leds etc. for this simple application.
Car breaks the IR beam.

jpanhalt

QUOTE=MrDEB]The emitter will be in the track and the dector in a bridge that spans the width of the track.
Can I use the 3 pin dector as a simple switch (present setup uses photo diodes or phototransistors??) with 2 wires. The light source is a simple light bulb located in the bridge over the track.
[/quote]

I think we need to standardize nomenclature. By emitter, I meant light source. In that context, the above statement is confusing with respect to the detector being in the bridge. If I wanted to use a single light source, I would put it in the bridge and have the detectors in the track, so you could know which lane won.

There are units made for such purposes as detecting paper feeds in printers (e.g., Vishay TCRT5000). They combine both the emitter and detector, are non-modulated, and are relatively inexpenive. One thought would be to put one in each lane, then use simple logic gates (the 2n2222's refered to below) or IC logic chips to determine the winner.

See: http://pdf1.alldatasheet.com/datashe.../TCRT5000.html

When my son was doing pine cars, I made simple timing gates using non-modulated IR emitter and detector, a few 2n2222 transistors, and a couple of hacked electronic stopwatches from a local discount chain. Basically, I pulled a lead from the start/stop switches. We easily measured times to 0.01 sec and used his test information to figure out the best place to put the weight. He built a one-lane test track.

Good luck. John

MrDEB

12 MM won't be high enough for a car to fit under?
12mm is aprox. 1/2"
thanks for info but ??

jpanhalt

The idea was under the car, not over. The cars must have a certain track clearance, right? The detectors would be built into the track and signal when the car crossed.

John

MrDEB

Ah yes grasshopper. GREAT idea but ambient light problems? and wouldn't each car need a reflective surface?

jpanhalt

Interference from ambient light probably depends on the sensitivity you want to attain. The device I built with my son was not bothered by ambient (indoor) light. The detectors do have filters to reduce interference from ambient light.

Ambient light is handled even better using the modulated format referenced earlier. But, if you reject the slight added complexity of that format, I don't know what the answer could be.

I would be skeptical that a single, non-modulated IR source in the bridge, 12 to 14" above the track, with detectors in each lane (how many lanes?) would work with the precision and absence from false detections that you need. A detector in each lane with an emitter in the bridge over each detector might work, though.

As for reflective surface, the bottom of the cars will be plenty reflective -- even if black, particularly if you go the modulated route.

John