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Topic: speed of photodiode (Read 1 time) previous topic - next topic

calvingloster

I'm building a lap timer for our motocross track. The whole circuit will be signaled by a laser switch which is a laser diode shining onto a photodiode. This will give the signal for my arduino to start and stop timing etc. So I have a few concerns, firstly will the photodiode change its resistance when the laser light is taken away for a few split seconds. U can imagine if a bike comes flying past its going to block the light for a very short time.

I set up a basic circuit with a battery, an LED and a photodiode. I shine the laser on the photodiode and the LED glows brightly. When I take the laser away the LED dims but slowly. So I'm concerned when the bike rides past the signal of the trip circuit will not change at all? Any suggestions? Or am I missing something?

RthuR

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Nov 12, 2013, 11:53 pm Last Edit: Nov 12, 2013, 11:55 pm by RthuR Reason: 1
This may not be entirely relevant, but I built a ski timer which followed the same principles. I ended up using an infrared gate, the datasheet from the manufacturer specified the response time though. I ended up using  this. I believe the response time was 10ms, so if you use speed = distance / time you should be able to find out the maximum speed that it will detect (distance being the width of a rider / length of a bike).
If I recall correctly, assuming a skier had a leg width of 15cm, the max speed was 15 m/s or about 50 km/h)

DVDdoug

Speed shouldn't be a problem.   Check the specs, and I'll bet the photodiode works at MHz rates.   In fact you might have to slow it down, or take several readings in a row to make sure it doesn't get falsly triggered by a bird, or flying piece of dirt, etc.

I think we need to see a schematic...  You've got a laser and a photodiode, so what's the LED got to do with it?

It probably will take some experimentation.   You have to make sure the sensor is not turned-on  by sunlight (or other light), but that it's always turned-on by the laser, and turned-off when the laser is blocked.  At outdoor events, of course the lighting will change through the day.

And in a tough environment like this, you might want to have a 2nd redundant system running in parallel, or at least one or two people with stopwatches.


MarkT

Quote

I set up a basic circuit with a battery, an LED and a photodiode. I shine the laser on the photodiode and the LED glows brightly. When I take the laser away the LED dims but slowly.


Then there's something wrong with that circuit, or you've overheated the photodiode,
expect switching times measured in microseconds.  With the correct circuit using
reverse-biased photodiode you can go even faster.

Or are you using an LDR and calling it a photodiode?  Photodiodes are usually
made of silicon (although an LED can be used as a photodiode)
[ I won't respond to messages, use the forum please ]

polymorph

An LED and a photodiode? What part number is that photodiode?
Steve Greenfield AE7HD
CET Consumer Electronics and Computer
Please don't read your attitudes into my messages

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