Photodetector/Transmitter

[andyjayem]

I'm trying to design a circuit to measure the speed of a spoked wheel spinning - using an IR transmitter and detector mounted either side of the wheel the spokes break the IR beam as they pass through. Ive got most of the circuit laid out, and the rest of the components I can find easily, but I'm having trouble finding a suitable IR transmitter and receiver. The distance the beam would have to transmit over would be about 30cm at most. Does anyone have any suggestions/advice? I've literally only started thinking about the project in the last couple of days so any suggestions are welcome!

Thanks all

 

[KMoffett]

Unless either the emitter or detector is very close to the spokes, there will be almost no change in light level as the spoke passes, with a 30cm separation. Perhaps with slit collimators on the end of long tubes, But then alignment gets very tricky.

Ken

 

[ericgibbs]

I'm trying to design a circuit to measure the speed of a spoked wheel spinning - using an IR transmitter and detector mounted either side of the wheel the spokes break the IR beam as they pass through. Ive got most of the circuit laid out, and the rest of the components I can find easily, but I'm having trouble finding a suitable IR transmitter and receiver. The distance the beam would have to transmit over would be about 30cm at most. Does anyone have any suggestions/advice? I've literally only started thinking about the project in the last couple of days so any suggestions are welcome!

Thanks all

I have seen on some bikes use a small reflective plate fastened to the spokes , near to the periphery of the wheel

Its used for riding at night to improve the chances of the cyclist being seen in the dark, perhaps you could modify one on these.

 

[Sceadwian]

A dot collimator with an optical laser source would work quiet well if what you need is to actually detect each spoke, alignment would be a piece of cake because the laser source is visible, the slit collimator would be required because typical red laser pointers have a pretty wide beam and diffusion and you'll need a beam smaller than the spoke thickness, two piece of board with pin holes in them will act as a collimator very well for a very tiny beam which should still have enough output to be easily detected using a photo transistor or diode.. Erics solution of using a small reflective patch towards the end of the wheel would be simpler to implement though.

Why do you want to detect each spoke passing?

 

[andyjayem]

I'ts a small experiment I'm going to try in order to see if I can use it to measure wire depth from a wireline unit instead of using an encoder. I don't expect it to work, or be any better even if it does, but hey ho.
I want to see if I can measure how much wire is spooling off the drum by measuring how many times the wheel on the measuring head turns as the wire passes over it. I guess I could try measuring it from the drum itself as it rotates, but like I say I only started giving it some thought yesterday.
I did think about using a laser as opposed to IR originally, but it's all self funded.....ah well

 

[KMoffett]

Picture of the wheel/drum arrangement?

Ken