Yes, you can design a circuit to detect negative pulses with just a single supply, just shift up the input signal. Isaac Em 8/9/2011 17:04, Electron escreveu: > 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 transit= ion >>> 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 necessa= ry, >>> but I wonder if the schmitt trigger of the PIC suffices, or if it's sti= ll >>> a good idea to add a second transistor buffer? >>> >>> Generally, to detect negative pulses do you bias a transistor according= ly >>> 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 .