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 transition >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 .