Generating a short pulse of a Rising AND Falling edge

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

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

I use an encoder to timer IC from US Digital (LS7184). They are about $3 each and can be ordered directly from US Digital.

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

Mark wrote: : 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

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.

All, Thanks for all the advice! Going to try the US Digital chip, looks easy to use.

Mark