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switch placed in noisy

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Member level 1
Apr 29, 2011
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Hi dear
I need to use a switch to check the state of a part but the line and switch are located in a long-distance and noisy place.

I adjust R1 to drain 10mA when switch S1 is closed and CD voltage is 3 volts. I'm confused to use 24V instead of 3V.
what`s your idea? is there any better circuit?and I donsyoura?Isthereanybeer∘uit→use?andIdont want to use an Opto coupler.

thank you


1) There is no beer.
2) You need a better translation tool.
3) 24 volts will destroy your microcontroller.
4) How much noise? What is the nature of the noise?
5) There is still no beer.
Change R2 to 2.2K and add a shunt 300 ohm from
the long wire's close-in end to ground. That will make
a safe-ish 3.2V from 24V which your switch can further
jam to GND when closed.

You might add some parallel C to your new resistor,
to both debounce and suppress HF radiated EMI

You might consider adding a cheap 3.3V CMOS logic
Schmitt trigger inverter as a "bouncer at the door",
socketed even, so that you can learn about the ugly
realities along and at the far end of the wire with less
pain. Then your uC will be guaranteed to receive logic
level signals and not latch up while you're messing.

What is the noise source ... what is the timing .... what is the exact problem?

24V ... and a voltage divider at the microcontroller side could give a benefiit
* ... if there is voltage noise capacitively coupled
* ... if there is ground bounce
* improves contact lifetime and switching quality by higher wetting voltage

Still I recommend a useful RC to reduce noise. Currently the RC tau is just 100us ... while you easily could go up to 10ms.
You also may use software filters.

If you need to use only passives, I would suggest a ferrite bead in series before the RC.
Higher voltages are typically used because the induced noise will be a smaller proportion of the signal. However, as pointed out in previous posts, you can only realistically go so high before causing other problems.

What is the distance that needs to be handled ? Whats producing the noise ?

Do you wish to stay wired versus wireless versus optical ?

What can you say about the return path, the ground path ? Conductivity,
media materials its made of......

Is the switch status a human safety issue ?

Regards, Dana.

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