Electronic – Pulse Low on High Signal

mosfetremote controltransistors

I am trying to build a circuit that disconnects the output for a brief time when power is connected. However I want the circuit to remain connected after power is removed.

I can achieve the first bit with a RC circuit connected to the base of an NPN transistor pulling the gate of a p-channel mosfet to GND. This works well because my input power is a clean vertical uptick. Meaning the voltage is at one instant zero and at the next 5V.

I have tried keeping the circuit connected using two NPN Transistors connected to the gate of the same p-channel mosfet. The two transistors are in the form of a inverter(not gate) signaled by the input power. This works well if the signal quickly goes from 5V to GND. My signal however has a gentle downward slope to zero. I have tried using a voltage divider and a comparitor to speed up the signal but neither has been fast enough.

Is there any way I can briefly disconnect my output when the input goes from LO to HIGH but ignore the change from HIGH to LO? This would leave my circuit connected at all times except for a brief moment when power is connected.

Edit:

Thanks to @EM Fields I was able to figure this out. I slightly edited the circuit Fields provided.

enter image description here

Here is the Spice simulation,

enter image description here

It shows the main input and the main output.

Best Answer

Try this:

enter image description here

and here's the LTspice circuit list if you want to play with the circuit:

Version 4
SHEET 1 880 680
WIRE -320 64 -432 64
WIRE -272 64 -320 64
WIRE 48 64 -272 64
WIRE 128 64 48 64
WIRE 176 64 128 64
WIRE 272 64 176 64
WIRE 432 64 368 64
WIRE 48 80 48 64
WIRE -320 112 -320 64
WIRE 176 112 176 64
WIRE -272 176 -272 64
WIRE -240 176 -272 176
WIRE 48 176 48 144
WIRE 48 176 -16 176
WIRE 432 224 432 64
WIRE -320 240 -320 192
WIRE -272 240 -320 240
WIRE -240 240 -272 240
WIRE 176 240 176 192
WIRE 176 240 -16 240
WIRE -320 304 -320 240
WIRE -272 304 -272 240
WIRE -240 304 -272 304
WIRE 288 304 288 112
WIRE 288 304 -16 304
WIRE -432 320 -432 64
WIRE 176 336 176 240
WIRE 288 336 288 304
WIRE 128 368 128 64
WIRE 128 368 -16 368
WIRE -432 448 -432 400
WIRE -320 448 -320 368
WIRE -320 448 -432 448
WIRE 48 448 48 176
WIRE 48 448 -320 448
WIRE 176 448 176 400
WIRE 176 448 48 448
WIRE 288 448 288 416
WIRE 288 448 176 448
WIRE 432 448 432 304
WIRE 432 448 288 448
WIRE -432 512 -432 448
FLAG -432 512 0
SYMBOL Misc\\NE555 -128 272 M0
SYMATTR InstName U1
SYMBOL res -304 96 M0
SYMATTR InstName R1
SYMATTR Value 1meg
SYMBOL cap -304 304 M0
SYMATTR InstName C1
SYMATTR Value 1µ
SYMBOL voltage -432 304 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
WINDOW 3 24 104 Invisible 2
WINDOW 0 7 105 Left 2
SYMATTR Value PULSE(0 12 1 10m 100m 10)
SYMATTR InstName V2
SYMBOL cap 192 336 M0
WINDOW 0 -37 30 Left 2
WINDOW 3 -34 60 Left 2
SYMATTR InstName C3
SYMATTR Value 10n
SYMBOL res 160 208 M180
WINDOW 0 47 73 Left 2
WINDOW 3 35 45 Left 2
SYMATTR InstName R2
SYMATTR Value 100k
SYMBOL pmos 368 112 M270
WINDOW 0 25 1 VLeft 2
WINDOW 3 73 95 VLeft 2
SYMATTR InstName Q1
SYMATTR Value FDS6575
SYMBOL res 416 208 R0
SYMATTR InstName LOAD
SYMATTR Value 100
SYMBOL cap 64 80 M0
WINDOW 0 -37 30 Left 2
WINDOW 3 -41 60 Left 2
SYMATTR InstName C2
SYMATTR Value 100n
SYMBOL res 272 432 M180
WINDOW 0 47 73 Left 2
WINDOW 3 35 45 Left 2
SYMATTR InstName R3
SYMATTR Value 1000
TEXT -424 480 Left 2 !.tran 15 startup
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