er, Shouldn't the 10k resistor be to the +ve line? As shown, there's nothing to turn the transistor on. RP On 17/03/2008, Jinx wrote: > > I assumed analog detection is the right way to go.....or is it ? > > Alan, I use a similar arrangement to measure propellor speed on > light aircraft, with a couple of differences > > Inside the propellor head, approx 200mm diameter, are 6 strips > of adhesive aluminium foil on a painted black background. At > 3000rpm this means 300 pulses/sec into b0 of a 16F628. INT > is the detection flag, and it works very well. The sensor collector > is pulled up to 12V (plane battery). Output from the resistor - > collector junction is normally high, and low over the foil. Pulses > go through a minimal filter and clamp, as shown > > If you want to use an analogue input (ADC or comparator) I'd > suggest a low-high window to eliminate the false positive from > the transition. The difference between light and dark should be > distinct enough to do that successfully > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > > > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist