Generating a short pulse of a Rising AND Falling edge

Does anyone have any ideas for a simple circuit which will generate a short pulse (order of a micro second or so - reason to follow) when a
signal goes high or low?
Reason is because my micro controller will either detect a falling edge or a high level. This means that my wheel encoder can only detect 1/2 the possible resolution. I was thinking of using something like a 555 circuit but I wondered if there was something better.
Thanks in advance
Mark
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Mark wrote:

Well others may have a better circuit, but I think an XOR circuit with the signal sent to one input with a delay. The other input would have zero delay. The pulse out would be as wide as the delay.
Cheers Bill K7NOM
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Mark wrote:

Can you use a syncchonous circuit (clocked)? D Flip Flop D Flip Flop __________ _________ in ---------| D Q |-----+------| D Q|---| | | | | | | ---|> | | --|> | | | | | | | | | | | | /Q |- | | | /Q |- | | ---------- | | --------- | | | | | clk _____|________________________| | _______ |__| EX OR | |_______________________| |---- out |_______|
Sorry about the ascii art... On either a rising or falling edge, you will get a pulse out that is one clock period long.
Be advised that using this on a quadrature encoder will probably give poor results unless you RC filter it heavily before squaring up the input signals with a comparator or Schmidt trigger inverter. Quadrature encoders are notoriously noisy.
Good Luck, Bob
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I use an encoder to timer IC from US Digital (LS7184). They are about $3 each and can be ordered directly from US Digital. http://www.usdigital.com/products/ls7183-ls7184/
These chips generate a pulse (width set by external resistor) and direction signal and can do this on 1, 2 or 4 edges (using both quadrature channels). It is much simpler than trying to do it with separate logic and is only a single 8-pin IC. The 4x resolution is useful as well.
They have other versions too which generate up/down pulses or with an inbuilt counter (parallel interface to micro)
-Daniel
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Mark,
I second Daniel's recommendation. I have had good luck with the LS7184 chips. I use the pulse to trigger an interrupt on an 8051, and the UP/DOWN signal to determine whether to increment or decrement the count. The chip also filters the inputs.
Jeff.
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All, Thanks for all the advice! Going to try the US Digital chip, looks easy to use.
Mark

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mark

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: Does anyone have any ideas for a simple circuit which will generate a : short pulse (order of a micro second or so - reason to follow) when a : signal goes high or low?
A monostable multivibrator (aka a one-shot) will do exactly that.
You can build it with logic (as others have suggested) get two in a single package in the 74xx123 . Either TI or Philips has an app note with example circuits, one of which is an edge detector (which is what you are looking for).
-Chris
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