Pulse Transition Detector for Leading AND Trailing Edges

transition detector

I need a simple pulse transition detector that provides an output pulse for BOTH the leading and trailing edges of its clock input square wave.

Can I use a non-retriggerable one-shot that is set up (by its external Rx - Cx values) to provide a desired-width output pulse, and that has both leading-edge and trailing edge trigger inputs? I was thinking of the 74HCT221 one-shot.

I'd like to trigger this at both its A (trailing edge detector) and B (leading edge detector) inputs with the same clock signal, and get a narrow output pulse on every leading and every trailing edge of the input clock signal.

Will this work, or will the 74HCT221 deny me the ability to "see" both edges of the incoming clock signal?

If this approach won't work, is there a simple modification (like buffering one of the inputs to achieve some delay) that will make it work, or is there a different, but simple approach I can use?

The application is a medical device which aids a patient suffering from foot drop, helping him to walk by lifting his foot at the proper time. A heel contact sensor detects when he comes off his heel, and also when he comes back down onto his heel, as he walks. The heel contact sensor contains an RF transmitter that "chirps" the current state of the heel contact sensor to a receiver embedded inside the nerve stimulator strapped to the patient's leg just below the knee. This stimulator applies an electrical signal to the deep peroneal nerve, lifting the patient's foot. The signal is removed, and the patient's foot is allowed to drop, when the heel contact sensor detects that the patient has come back down onto his heel.

Thus, if you picture the clock signal coming out of the heel contact sensor, it is a square wave at about .5 HZ, depending upon how fast the patient walks. There is therefore a need to detect both the leading edge of this signal, that is, when the patient comes off his heel (logical "1"), and the trailing edge, when he comes back down onto his heel (logical "0"), so the transmitter can "chirp" this data to the receiver.

Thanks for any help on this design problem!

NukerDoggie

pulse transition detector

xor gate

pulse edge detector

How does an XOR gate help me?

Added after 17 minutes:

Would I use an XOR gate like this?:

Pull one of its inputs always high. Tie its other input to the clock signal coming from the heel contact sensor. As soon as the heel contact sensor signal goes low, I'll get a high output from the XOR gate. That output can be used to trigger a one-shot that gives me the output pulse I'm after, triggered on the falling edge.

The other one-shot (the other half of the 74HCT221) can sit in place, triggered on the positive edges of the heel contact clock signal, to give me the desired pulses that signal the leading edges of the heel contact clock signal.

The separate outputs from the two one-shots can drive a two-input OR gate to give me one output line I can use to drive the transmitter "chirp" function.

How's that for taking the long way around! (Sorry, but IT.....COULD......WORK!!!!!)

Added after 1 hours 57 minutes:

However, a shorter route to the same end is simply to use 1/2 of the 74HCT221 as a leading edge detector using its input B, and the other half of the chip as a trailing edge detector using its input A, and combine the two outputs using a 2-input OR gate. This should provide an output pulse for every input pulse transition event.

So I can't see any real value in using an XOR gate. Care to explain why you suggested it?

edge detector in signal pulse

I said:

However, a shorter route to the same end is simply to use 1/2 of the 74HCT221 as a leading edge detector using its input B, and the other half of the chip as a trailing edge detector using its input A, and combine the two outputs using a 2-input OR gate. This should provide an output pulse for every input pulse transition event.

Click to expand...

This solution works very well. It provides a nice detection pulse of any desired width (desired width is set by the external timing RC network connected to each one-shot in the 74HCT221 chip) each time the clock input sees a transition.

[sarcasm] I want to thank everyone for all their help.