Multiple Voltage inputs for Digital Input pins

[Lara2022]

Hello All,

I have a requirement of designing a 4 channel Digital Input section wherein the digital inputs shall be capable of accepting digital signals ranging from 5 to 24V DC, but the catch here is before applying digital signal, the max voltage level for digital input pins shall be selectable through switch. For Example. if I have sensor whose output is 12V, then I set the digital input max voltage level to be 12V using switch, if a signal more than this value is applied some indication has to be there. I done simple 24V digital input schematic with optocoupler isolation but im not able to get any reference for the above requirement. Please help me out

 

[Nigel Goodwin]

You need to provide more information, EXACTLY what is the project? - why do you want it manually selectable?, and why do you want an indication if it's too high?.

 

[Diver300]

I suggest that you have a comparator. One input of the comparator needs to be connected to the digital input and the other input connected to a switchable voltage. If the comparator operates, the digital input is too high or the setting is too low.

That needs a comparator that can handle up to 24 V, which might be difficult. You could divide the input voltage by 10 with a potential divider, and then compare to smaller voltages, for instance 0,5, 1.2 and 2.4 V to correspond to 5, 12 or 24 V inputs.

A circuit like that will not give any indication of the voltage setting being wrong if the digital input happens to be off. An input at 0 V could be a 0V / 5 V digital input that is off, or it could be a 0V / 24 V digital input that is off.

 

[danadak]

Then consider using an A/D to measure the channel and see if its conforming to
the max input. So question is why does it occur that you have a known sensor
tied to a known input, with a known range, exceed its own specs ?


Regards, Dana.

 

[tumbleweed]

but the catch here is before applying digital signal, the max voltage level for digital input pins shall be selectable through switch.

That sounds like a recipe for disaster unless you protect the circuit from overvoltage... "Oops, forgot the damn switch again"

 

[eTech]

Use an optocoupler at the input. Then no input voltage selection is needed.

 

[schmitt trigger]

^^^^^ This is exactly how I would do it.
Of course, plain-vanilla optocouplers are not the fastest devices in town. One can find, for a price, faster logic optocouplers.