# Simple circuit to convert a toggle switch turning on into a short pulse

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?

UPDATE

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)

• Does it suddenly change to 17V? And what does it change from? I feel a circuit diagram may be of some use here. Apr 27 '15 at 13:51
• Sounds like you need a RC high pass filter. Apr 27 '15 at 13:58
• Its a battery based power supply, so the voltage can be between 11 and 17V depending on the state of the battery.
Apr 27 '15 at 14:52
• Another way to state it would be a bit of circuitry that gets powered with between 11V and 17V. It needs to emit a high pulse for a few mS on power up and then keep the signal low until power is lost (and also not consume too much current itself once the pulse has gone low)
Apr 27 '15 at 14:55
• I think you need a monostable multivibrator. Apr 27 '15 at 15:57

step 1. connect a bleeder resistance to the o/p terminal of Toggle Swith and GND--say 10K 2. connect 0.01 microF cap. in series with 47 K res. and connect cap terminal to o/p of toggle switch and the resistance to GND. Connect the ckt input to the node of cap. & resistance. VTIngole

Here's an easy way to do it using a 555:

and here's the LTspice circuit list if you want to run a simulation.

Version 4
SHEET 1 880 1140
WIRE -480 576 -624 576
WIRE -352 576 -480 576
WIRE 0 576 -352 576
WIRE -352 640 -352 576
WIRE -240 640 -352 640
WIRE -32 640 -176 640
WIRE -480 656 -480 576
WIRE -352 720 -352 640
WIRE -320 720 -352 720
WIRE -32 720 -32 640
WIRE -32 720 -96 720
WIRE -64 784 -96 784
WIRE -624 800 -624 576
WIRE -592 800 -624 800
WIRE -480 800 -480 736
WIRE -480 800 -528 800
WIRE -624 848 -624 800
WIRE -592 848 -624 848
WIRE -480 848 -480 800
WIRE -352 848 -480 848
WIRE -320 848 -352 848
WIRE 48 848 -96 848
WIRE -480 880 -480 848
WIRE -624 896 -624 848
WIRE 0 912 0 576
WIRE 0 912 -96 912
WIRE -352 976 -352 848
WIRE -64 976 -64 784
WIRE -64 976 -352 976
WIRE -624 1008 -624 976
WIRE -480 1008 -480 944
WIRE -480 1008 -624 1008
WIRE -32 1008 -32 720
WIRE -32 1008 -480 1008
WIRE -624 1072 -624 1008
FLAG -624 1072 0
FLAG 48 848 OUT
FLAG -592 848 +12
SYMBOL Misc\\NE555 -208 816 M0
SYMATTR InstName U2
SYMBOL voltage -624 880 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
WINDOW 3 24 104 Invisible 2
SYMATTR Value PULSE(0 12 1 .01)
SYMATTR InstName V1
SYMBOL cap -496 880 R0
SYMATTR InstName C1
SYMATTR Value 2µ
SYMBOL res -464 752 R180
WINDOW 0 -50 67 Left 2
WINDOW 3 -59 34 Left 2
SYMATTR InstName R4
SYMATTR Value 470k
SYMBOL diode -528 784 R90
WINDOW 0 -38 25 VBottom 2
WINDOW 3 -34 28 VTop 2
SYMATTR InstName D4
SYMATTR Value 1N4148
SYMBOL cap -176 624 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C2
SYMATTR Value 0.1µ
TEXT -600 1040 Left 2 !.tran 15

• Many thanks, although I think it would need to be a SE555 for the +17V max voltage. I've not tried this as I think my simpler capacitor + transistor solution will do the job well enough for my needs on this project.