Re: 2 pulses from 1
From: Fred Bartoli (fred._canxxxel_this_bartoli_at_RemoveThatAlso_free.fr_AndThisToo)
Date: 03/09/05
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Date: Wed, 9 Mar 2005 14:02:29 +0100
"Terry Pinnell" <terrypinDELETE@THESEdial.pipex.com> a écrit dans le message
de news:ivnt219o2uqv0aui047q2h3p9vkcu6d111@4ax.com...
> This morning I breadboarded both Fred Bloggs' final circuit
> news:4224845A.6080401@nospam.com and Fred Bartoli's. Both work fine
> thanks.
>
> The latter is especially attractive because of its delightfully low
> component count, as well as its novelty. My test input was a clean
> square wave from a function generator, so of course the circuit might
> need a front-end clean-up in practice. However, the outputs would
> probably be fine without further squaring up, as their leading edges
> were fast.
>
> But how does it work please?! What function does that left hand PNP
> serve, with its gate permanently at Vdd?
>
Hi Terry,
don't forget its emitter :-)
There are 3 basic circuits to build around a BJT : common emitter, common
base and common collector (emitter follower).
Google for those and you'll find all the needed information.
As you've seen, there are 2 transistors, the right one in common emitter
configuration, the left one in common base.
10K
___
.-|___|-+------+-----VDD
| | |
1K 1n | | |
12V ___ || | A | |<
>-|___|--||--+-------|----| Q2
pulse || | | |\
| | |
>| | |
Q1 |-----' |
/| |
| |
Set <-----+ +-----> Reset
| |
.-. .-.
| | | |
R1 | | | | R2
'-' '-'
| |
| |
=== ===
GND GND
Due to the base emitter junctions, the A point is clamped to +/- 0.6V around
VDD.
On rising edge, Q2 is reverse biased and the capacitor current is injected
into Q1 emitter => positive pulse on the set output.
On falling edge Q1 is reverse biased and the capacitor current turns Q2 on
=> positive pulse on the reset output.
The 1K resistor obviously limits injected current.
R1 needs to be > 1K*VDD/(12-VDD-0.6).
BTW, since those transistors drive a regenerative circuit (your NOR SR
latch) you don't need squaring after the transistors: the hysteresis is
built into the SR latch.
-- Thanks, Fred.
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