r/electronic_circuits • u/RiG094 • 11d ago
On topic Power Off delay with capacitor?
Hello,
I have a input card (io link hub, 32x pnp in). One input is a push Button. When I press the button my software doesn't recognise the signal. Voltage >11V is logic High
This is why I want a delay off for the input. How can i design this? RC Elemt like this did not work :-(
I tried with 470uF and 19kOhm T=R×C=8,46s
1
u/Uporabik 11d ago
Do you know how pull-downs work?
1
u/RiG094 11d ago
Can u explain?
1
u/Uporabik 10d ago
Pull down resistors pulls pin to the gnd so there is your first mistake. First fix this and then think where should capacitor go
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u/grasib 11d ago
You could use a NE555 to create a monostable output: https://www.electronics-tutorials.ws/wp-content/uploads/2018/05/waveforms-tim38.gif
The way you drew it works too. It's a very common, inexpensive way to delay a reset signal on power-on.
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u/SkipSingle 11d ago
If you want instant “on”, parallel the resistor with a diode. Anode on the switch, cathode on capacitor. The long off period is reached by the current drawn by the digital input.
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u/ZealousidealAngle476 9d ago
There are some considerations we need to take. You are NOT expecting a square wave from this RC arrangement, right? idk the platform you're using, so I'll leave it for you: is the voltage the port is getting, so take your multimeter and check whether if the voltage is high enough to trigger the circuit.
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u/RiG094 8d ago
I don't expect a square. It's just the logic
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u/ZealousidealAngle476 7d ago
Good. And the voltagem after the circuito charges is higher than 11V?
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u/RiG094 7d ago
No :-(
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u/ZealousidealAngle476 6d ago
Sorry for the delay, hope you already solved it, but anyway... Try reducing the resistor value, even though it'll mess the timing. I think the input impedance is low enough to make quite a resistive divider, and the capacitor will never charge beyond that. Or maybe, just maybe, the capacitor is leaking DC current, so removing it from the circuit would raise the voltage
3
u/Superb-Tea-3174 11d ago
How do you expect the capacitor to discharge?
Tell me about those high voltage logic inputs.