building a hand held tachometer

Technical Electrical
1

i do alot of work with small engines and i have been looking into buying a non contact tachometer but i cant find any for less than $200. I was wondering if it would be possible to build one useing a light sensor. I’m just uncertain as to how to convert the pulse from the light sensor into numbers on a display. Any help would be greatly apreciated.

Thanks,
Ben Van Selous

2

Ben,
There are number of circuits out there if you search for them. It is possible to use a light sensor with a light source. Once you get that far you need to filter the signal to cancel out the noise and then send that to a counter. Add to that a semi accurate time base and all you need to do is to interface your results to a display of some sort. When you add up all that is needed, one of those tachometers in McMaster Carr looks pretty nice.

3

Have you checked out the ones Harbor Freight has for $70? I have seen them on sale for $60 at times.

Photo Sensor:
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=41727

Digital Touch:
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=41726

4

Thanks for your help. mayby i will just buy one. I was just wondering if building one was a possibility.

Thanks,
Ben Van Selous