Thanks, but the impulse will be very similar to half sines, first negative
then positive. Duty cycle of input signal is like:

3% negative half sine, 4% zero volt, 3% positive half sine, 90% zero volt

frequency of this whole cycle will be ~ 10Hz to 250Hz

timing precision is extremely important, that's why I think I must use a
couple of transistors per part of pulse detector, to get very sharp edges
at the output.

One thing that worries of my design is that it feeds back about 440mV at
the pickup, which is an inductor and will probably not like it. I can
bypass with a capacitor, but then the timing will be affected. :(

Perhaps I should really use the "emitter to input, base to ground and
output from collector pulled up" scheme like on the URL link above, so
I get rid of the capacitor (not shown in my schematic), but then I will
have to add a 3rd transistor.. :( Ain't there a way to get a good gain
from the "emitter to input, base to ground and output from collector
pulled up" solution? That stage has no gain at all.

Cheers,
Cesare


At 16.36 2011.09.08, you wrote:
>On Thu, 2011-09-08 at 15:45 +0200, 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
>>=20
>> Power supply is single and +5V.
>>=20
>> I have designed my own circuit, using just one transitor biased this
>> way (file my-circuit.GIF in attachment).
>>=20
>> However on the Net I found this other circuit:
>> http://www.transmic.net/XR400/use_negative_first.jpg
>>=20
>> 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..).
>
>You mention it's a negative "pulse". In that case if you are really
>starved for parts you should be able to get something working without
>any transistors.
>
>I just tried a quick sim with something that you might be able to fine
>tune to work for your particular situation (you didn't mention the
>negative pulse width, duration or frequency, so I've made assumptions).
>It uses 2 resistors and a cap. I've attached a screenshot of the sim.
>
>TTYL
>
>
>
>
>--=20
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