Last year, I wrote about a problem that a friend of mine was having with a bouncing switch input. I searched the web, found the circuit at right, and proceeded to breadboard it for him. We put the scope on it, and it seemed to work great. A couple of weeks later, however, he called and said that when he tried using it in his math camp class, it didn’t work at all.
A couple of days ago, I got an e-mail from my friend, saying that it was time for math camp and could we take a look at the debounce circuit again. This time, we connected the circuit to the input of a 74LS640 transceiver, as he did last year. Sure enough, it just didn’t seem to switch at all. When I put the scope on it, I found that with the switch closed, the voltage on the output only got down to around 3 V or so.
I’m kind of confused by this. Is the IC input actually sourcing enough current to keep the output at 3 V?
What we ended up doing is wiring up a transistor switch on the debounce circuit output. That works just fine, but seems kind of kludgey to me.