At 03:03 PM 9/8/2011, Electron wrote: >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. That circuit is called a 'common-base' amplifier. As shown, it=20 doesn't apply bias to the sensor while the sensor is not generating a=20 signal. However, current flows from the collector to the emitter=20 when the input signal becomes about 1 Vbe drop below ground=20 (~0.65Vdc). Also note that the circuit clamps the input signal to=20 about 1 Vbe drop below ground. How much bias current is too much? nA? uA? mA? You can capacitively-couple the signal into the comparitor=20 stage. Just pick the RC time constant to be at least a decade below=20 your lowest input frequency. The real charm of using a proper comparitor is that the threshold=20 voltages remain constant over a wide temperature range. That is not=20 the case with the transistor circuits. dwayne >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 pul= se. > >> > >> > >> At 19.45 2011.09.08, you wrote: > >>> How about using one of the little SOT-23 op-amps now readily > >>> available as a comparitor? Set the threshold voltage to whatever you > >>> 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 rega= rds > >>>> (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 pick= up) > >>>> do you think I should add a second transistor stage to make=20 > 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 th= e > >>>> positive pulse too. ;) > >>>> > >>>> Of course I don't want to use any more transistors than=20 > stricly necessary, > >>>> but I wonder if the schmitt trigger of the PIC suffices, or if=20 > it's still > >>>> a good idea to add a second transistor buffer? > >>>> > >>>> Generally, to detect negative pulses do you bias a transistor=20 > accordingly > >>>> or use the "emitter to input, base to ground and output from collect= or > >>>> pulled up" scheme like on the URL link above? > >>>> > >>>> My solution (bias) seems too simple to be true, and makes me=20 > 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 > >>> > >>> -- > >>> 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 > >>> > >>> -- > >>> 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 > >-- >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 .