I need to convert a permanent high signal into a short high pulse.
I've got a design problem where I have a toggle switch that connects to a variable +11V to +17V power source, providing a signal to turn the system power on. I need the ability for the system microcontroller to override this signal and turn the power back off. There are lots of ways to achieve this of course, but in this application we need to retro fit some existing systems, so the ideal solution is a simple bit of circuitry that converts a 11V to 17V high level signal into a short high pulse of a few mS. The signal then needs to go low and stay low until power/ the high input is lost.
I can do it with a low power voltage regulator and PIC10 microcontroller but it seems there should be a much simpler way with say a capacitor and bit of clever transistor trickery?
In response to the idea from vijay ingole and comment from Chris Johnson below I've come up with this solution:
The series 1uF capacitor C1 lets through much more of the AC pulse from the switch turning on than a 10nF, and the 1uF capacitor to ground C2 stores a decent amount of charge. Because the lowest input voltage is 11V, even though the voltage seen across C2 is much lower (it peeks at a bit over 5V), its plenty to stay above the transistors 0.7V base threshold for a decent amount of time. The large capacitor values also provide pretty good debouncing of the switch closure.
I think this is a great solution and thank you vijay for the idea. Please anyone shout if you can see any problem with it but I don't think it breaks any transistor good practice rules. Here's what it looks like on the scope (yellow=C1+C2 junction, blue=output with 1K pull up added)