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How to relieve analog part for digital input?

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carpenter

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I pardon myself in advance for a partially philosophical question.
Have me any device powered with 230V to 24DC and 5V DC , for standard home and office safety level, ground of device is connected to PE. In device is MCU and any digital input, we need to take them out of the device for general use, for example foot switch, for driving device with any PLC , etc.
I always wonder how to reliably implement such an input?
Any as on in the picture above or in the bottom or the other way?

FIO.jpg
 

Both ways will work but one side of P1/P2 is connected to ground or supply so you not have real isolation.
In general, it is safer to ground one side of an external connection than have both sides 'live' so the bottom solution would be better. However, with a small change in circuit you can still use the top circuit with P1 linking to ground to avoid the higher cost of an AC optocoupler.

Brian.
 

Put your optocoupler inside a diode bridge and a plain vainilla optocoupler.
It will be cheaper than an AC optocoupler, specially if the bridge is made with common 1N4148 diodes.
 

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