Sensor For Tachometer

Hello all.

I'm currently finishing an HNC in Mechanical Engineering. As part of the course, I am designing a wireless tachometer. In hindsight, I should have chosen something that is not so electronics based - my knowledge is not great. I work for a Industrial Fan manufacturer so I thought it would be good to make something that I could use at work.

Anyway, I found a circuit for a 7-segment LED counter. http://www.aaroncake.net/circuits/counter.htm I've got the parts to make this and I'm ready to crack on. The part that I am unsure about is the sensor... I've found a sensor from a local stockist. **broken link removed**
This only has a 120mm range though, with a peice of 200x200 white paper. I'd like a lot bigger range, Pepperl & Fuchs make a 2 metre range one but this is expensive. It has to be the diffused type as I cannot mount a reflector on a fan blade. Ideally the target area should be less than 100x100mm... Anyone know of anything? As these are to be used to measure the speeds of fans up to 2.5M diameter, I would like one that would work from 2 metres away - although to be honest I would be happy with one that will only measure a few inches - this would not fail my project I would just not use it for measuring fans, it'll just be used occasionly for lathes. I'd also like to be able to read speeds of up to 3000RPM, although again this is only a wish.

The main bit that I am unsure about is connecting it to the circuit. I know it will be easier to connect a N/O type but I can only really see ones that are PNP or NPN that are suitable.. Would this be hard to connect to my circuit?

Thanks if anyone can help, I would rather not join a forum and ask a question straight away but hopefully I'll be able to help someone else on a part of the forum somewhere in return!

Pete.

I take it we are talking about the fan speed. one way is to use a magnet on what is rotating and a coild that it passes CLOSE to that will get curent thru it when the magnet goes by. in this way it can be used to trigger a circuit measuring the delay between passes unfortunately I can't be of much more help but t i think you also need to be more spcific

I think he wants to measure at a distance (120 mm is too close). I know there was an IR tach project in servo mag last year. here is a thread discussing it **broken link removed** the design is pretty old (msi logic) but could pretty easily be updated to a microcontroller based solution (mostly for the pulse counting and display driving)

The non-contact tachs I've seen use a beam of light and a piece of reflective tape on the rotating shaft. It could use an incandescent lamp or laser (like the ones I've used) or IR like philba suggested. A Google search for "optical tachometer" returned many hits for commercial-made tachs.
JB

*edit* Here's a couple links for inexpensive optical tachometers (I don't know the quality though) from a company here in the US called Marlin P. Jones & Associates. One uses a laser ($50US), the other one (probably) a lamp ($40US).
https://www.mpja.com/directview.asp?product=15160+TE
https://www.mpja.com/directview.asp?product=15161+TE

Thanks for the replies so far guys, I appreciate it.

Thunderchild - I like the magnet idea. I don't think its practical for this appllication though as at higher speeds (3000rpm) on a large diameter the magnet would put it off balance by quite a lot I think. I was hoping to use a defracted light photoelectric sensor so no need for reflectors etc.

Reading my first post again, it is a little bit unclear. Basically, I'd like to build a tachometer with this circuit: https://www.aaroncake.net/circuits/counter.htm for the counter. I felt this would be a good choice at first but now I'm not sure sure but I have built this part of the circuit now anyway. I did try a circuit with a PIC but I had trouble programming it and unfortunatly the university could not assist. I was originally aiming for a 2 metre sensing range (to test industrial fans for example) but I'm swaying towards a much shorter distance as the sensors are a lot cheaper.

I was wondering if anyone has any ideas on running a photoelectric sensor? Any range is ok, I'm just unsure how I'd wire a PNP or NPN sensor to this circuit. Would a N/O sensor be easier? To be honest, the tachometer does not have to work accuratly or even properly - thats all for the report to cover. I'd just like it to sense something if its possible - I'm thinking that by using my original circuit, it will count up and up (total revolutions) rather than RPM.

Any Ideas??