I've searched for a circuit that will turn a constant on switch into a momentary switch. I've checked google and all I find are the oppsit of what I am looking for. Anyone know of a circuit that can make it in to a momentary switch?
How would you want it to work? Latching switches have two positions, so would you want to produce a single pulse on one edge or on every event?
I would classify it, rather, as a flip-flop. Also, for this application, you would not need to explicity de-bounce since the RC value would be larger than the input "noise" duration.
Here is the image: I built one of these to use as a door sensor and it has lasted, so far, a little over 2 years without fault so it seems to be pretty stable.
Does above give pulses on both ON & OFFing the SPDT switch? A way I was doodling with: There's no debouncing, but it's possible none's needed: I can't find a figure for minimum pulse duration to turn an ATX system on, though changing the C1/C2 size gives plenty of adjustment. You could, of course, get rid of R2/C2 so turning the switch off did nothing. May be better with an XP system and also allows use of a SPST switch.
Yes, regardless of what state the switch is in, a change will initiate a puse with a given duration equal to the RC of the 555 IC timer. For the pulse duration, as long as it is long enough for the logic to "recognize" its pin as active low.
I can see all the NAND logic changes in simulation apart from any final pulse from the 7400, I guess it's a case where identical and ideal gates all switch in exactly the same time, no propagation or wire delays and don't make a noise. For the trigger, a pull-up resistor to 5V from one side of the switch, other side to ground, should be OK? And how long is this piece of string? So it should turn the system on without the monostable?
From the discription, looks like it -- "... this circuit supplies the 555 timer with a single low-going pulse..." This piece of string would be dependent on the speed that the logic on the other side is rated at switching. In this case, making the string as short as possible would be kind of illogical mainly because of the question "Why would you even want to do something like that?" It is not like you have something else waiting to use the circuit immediatly after this application. If you want a direct answer, I suggest the following: Use a stop watch and time how long it takes you to push and release the button to start the computer. Edit: Found the article.
I was going on the thought that if the pulse is long enough to trigger the 555 it could well be long enough to trigger the ATX system, which, if it has built-in debounce, should react to the initial brief pulse from a bouncy mechanical push-button. Also cuts the cost in half. Good idea for a few seconds doorbell or alarm though.
Hmmm... Confirmed by a better simulation program (Circuitmaker). I wonder if switch bounce will let it work both turning on & turning off.
I guess bouncing was what made it trigger when the switch changed states. Though... now that I think about it. I used the LS family with the switch tied to ground so the open state was really left up to the internal pull-up resistor to take care.
