At 22.54 2011.09.08, you wrote: >Yes, you can design a circuit to detect negative pulses with just a >single supply, just shift up the input signal. But this has the same defect of my circuit, i.e. it will bias the pickup, with unpredictable results.. I may use a bypass capacitor and remove DC, but then the timing will vary with input signal frequency. :( I am afraid I will have to use the "emitter to input, base to ground and output from collector pulled up" scheme (is there a better name for it? :) = ) but I gotta verify if it biases the input signal.. I think it shouldn't. Else I must use a dual supply, but that will be hell with current design and constraints. Luckily there are simulators like LT Spice. :O And breadboards, too. :D Besides biasing, on a single supply what other techniques are there to detect a < -2V without biasing the input signal? (loading it to ground is fine of course). Cheers, Mario > >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 regard= s >>>> (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 transi= tion >>>> 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 necess= ary, >>>> but I wonder if the schmitt trigger of the PIC suffices, or if it's st= ill >>>> a good idea to add a second transistor buffer? >>>> >>>> Generally, to detect negative pulses do you bias a transistor accordin= gly >>>> 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 suspiciou= s. :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 --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .