Sorry, I don't understand: as I wrote the power come from a single +5V supply, will the comparator allow me to set the threshold voltage to -2V? Because the first thing I thought about was using a comparator, but without a dual supply, I thought it wouldn't work for the negative pulse. At 19.45 2011.09.08, you wrote: >How about using one of the little SOT-23 op-amps now readily=20 >available as a comparitor? Set the threshold voltage to whatever you=20 >want (near 0V, I assume) and add as much hysteresis as needed. > >Tiny and inexpensive. > >dwayne > > >At 07:45 AM 9/8/2011, Electron wrote: > >>Hi all, >>I need to make a circuit that will let my PIC digital pin detect if >>an input signal (from a reluctor pickup) is smaller than about -2.0V >> >>Power supply is single and +5V. >> >>I have designed my own circuit, using just one transitor biased this >>way (file my-circuit.GIF in attachment). >> >>However on the Net I found this other circuit: >>http://www.transmic.net/XR400/use_negative_first.jpg >> >>I have simulated it, and I think it's inferior than mine in all regards >>(uses two transistors rather than one, edges are not sharper, etc..). >> >>However I don't have a very big experience on analog electronics and >>I wanted to ask Your valuable opinion. >> >>Moreover, for this app (detect a negative pulse from a reluctor pickup) >>do you think I should add a second transistor stage to make the transitio= n >>between 1 and 0 sharper at the PIC digital input? >> >>I can't just invert the input from the pickup as I have to detect the >>positive pulse too. ;) >> >>Of course I don't want to use any more transistors than stricly necessary= , >>but I wonder if the schmitt trigger of the PIC suffices, or if it's still >>a good idea to add a second transistor buffer? >> >>Generally, to detect negative pulses do you bias a transistor accordingly >>or use the "emitter to input, base to ground and output from collector >>pulled up" scheme like on the URL link above? >> >>My solution (bias) seems too simple to be true, and makes me suspicious. = :P >> >>Thanks! >>Mario >> >>-- >>http://www.piclist.com PIC/SX FAQ & list archive >>View/change your membership options at >>http://mailman.mit.edu/mailman/listinfo/piclist > > >--=20 >Dwayne Reid >Trinity Electronics Systems Ltd Edmonton, AB, CANADA >(780) 489-3199 voice (780) 487-6397 fax >www.trinity-electronics.com >Custom Electronics Design and Manufacturing > >--=20 >http://www.piclist.com PIC/SX FAQ & list archive >View/change your membership options at >http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .