Post by thread:Optical pickup

<x-flowed>I am making a no contact tachometer using a pic for a model airplane and am trying to find a reflective sensor for the pickup. The ones that I have looked at have to be to close ~.150 inches and I would like to be several inches away. Does any one know of any any sensors? Thanks ______________________________________________________ Get Your Private, Free Email at http://www.hotmail.com </x-flowed>

At 11:37 PM 10/25/1999 -0500, you wrote: >I am making a no contact tachometer using a pic for a model airplane and am >trying to find a reflective sensor for the pickup. The ones that I have >looked at have to be to close ~.150 inches and I would like to be several >inches away. Does any one know of any any sensors? Did one already. You don't need to be reflective. You can simply use a photo diode (or an LED - somebody will explain how this works) to sense the light. The diode simply gets connected across an op amp, and voltage changes according to light. This is why, btw, things like the Tower tach tell you NOT to use them indoors with flourescent lights. They see the lights pulsing and report THAT as the RPM (try it - it works!) Ask private if you have any questions. I can't share everything, but the basic ideas are good. You also want an AGC on the output (a cap & resistor) to sense the changes. Andy ================================================================== Eternity is only a heartbeat away - are you ready? Ask me how! ------------------------------------------------------------------ spam_OUTandyTakeThisOuTrc-hydros.com http://www.rc-hydros.com - Race Boats .....andyKILLspam@spam@montanadesign.com http://www.montanadesign.com - Electronics ==================================================================

I remember from my RC days that the commercial tachs for airplanes could be used a few inches away. They also read light through the prop, not reflected from it. Geoff Child wrote: > I am making a no contact tachometer using a pic for a model airplane and am > trying to find a reflective sensor for the pickup. The ones that I have > looked at have to be to close ~.150 inches and I would like to be several > inches away. Does any one know of any any sensors? > > Thanks > > ______________________________________________________ > Get Your Private, Free Email at http://www.hotmail.com

You can use a reflective label on the prop, or just reading the light intensity difference through the prop. To improve that technique, just attach to the prop a label half white, half black, so it will helps to create a discrimination of light / dark even in different ambient light conditions. Wagner. Brian Kraut wrote: {Quote hidden}> > I remember from my RC days that the commercial tachs for airplanes could be > used a few inches away. They also read light through the prop, not reflected > from it. > > Geoff Child wrote: > > > I am making a no contact tachometer using a pic for a model airplane and am > > trying to find a reflective sensor for the pickup. The ones that I have > > looked at have to be to close ~.150 inches and I would like to be several > > inches away. Does any one know of any any sensors? > > > > Thanks > > > > ______________________________________________________ > > Get Your Private, Free Email at http://www.hotmail.com -- -------------------------------------------------------- Wagner Lipnharski - UST Research Inc. - Orlando, Florida Forum and microcontroller web site: http://www.ustr.net Microcontrollers Survey: http://www.ustr.net/tellme.htm

'Look' at the problem a different way - you are shining a light onto a reflector and measuring when you get a little / lot back. In ambient conditions there is already a *LOT* of light around so you have to get close to to be able to pick up the *SMALL* increase when the reflector passes. So - how can you get more light? 1. use different light to the ambient - ultra violet is good (otherwise your TV remote wouldn't work in a brightly lit room). 2. increase the concentration of the light onto the reflector with a focusing system (don't forget to re-calculate the focal length of your selected lens from 'natural' light to UV wavelengths). 3. eliminate the ambient light by collimation (put a shade tube over your reciever to make it highly directional). Bye. > {Original Message removed}