[sdiy] Rectifier for logic level pulse?

[Magnus Danielson]

Tim Parkhurst wrote:
> On Mon, Sep 28, 2009 at 6:00 PM, Harry Bissell <harrybissell at wowway.com> wrote:
>> Hmmm... rectifying the pulse might be tough... how wide is the pulse ?
>>
>> If its a square wave it would be easy with passive components, if its a narrow pulse
>> its quite tough. One side would have to reset very quickly.
>>
>> H^) harry
>>
> 
> My original intent was to work with a square wave, or even perhaps
> with the relatively slow pulse you'd get from a manual push button.
> The solutions suggested here seem to generally work by inverting the
> pulse, and getting one positive-going pulse from the original input,
> and the other positive-going pulse from the inverted version. I can
> see how this would work, but I could swear that I had seen a passive
> solution that used a few diodes to full wave rectify the
> negative-going pulse (prolly should have stated that at the
> beginning). I mean, if you can rectify AC to produce a positive only
> waveform, couldn't you do the same with the negative pulses you get
> from the cap in a differentiator? Still, if adding an inverter is the
> simplest solution, then that's hardly the end of the world.

Another solution would be to use a simple RC-delay and an XOR-gate. 
First use an XOR-gate just for buffering purposes. Then feed one of the 
inputs directly and another from a RC-lowpass filter. It works like this:

1) Consider the initial state of logic '0', the low-pass filter would 
have low output and the output XOR gates sees '0' and '0' and outputs '0'.

2) A rising edge comes in, that let's the output see '1' directly while 
the RC-lowpass just haven't moved much, so it looks like a '0' so then 
'1' and '0' produces an output of '1'.

3) After a while the RC-lowpass has charged up so it looks like a '1' so 
then the output XOR sees '1' and '1' to output '0'.

This gives an output blimp on the rising edge... what can the the 
falling edge do for us?

4) The falling edge comes in and the output XOR gate sees '0' and '1' 
since the RC filter is still pretty well charged, so the XOR output 
becomes '1'.

5) The RC link discharges and eventually it looks like a '0' for the 
output XOR gate and that makes the inputs look like '0' and '0' and thus 
produces '0'.

Thus, gives an output blimp on the falling edge.

Both the rising and falling edge triggers the output blimp.

Should do the trick. A 74HC86 should work.

Cheers,
Magnus