Electronics > Beginners
Sensing (interfacing) high voltage with arduino or any Microcontroller
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Mr. Scram:
How would using an optoisolator work? I'm probably misunderstanding something but I can't imagine that being anywhere near linear.
fourfathom:

--- Quote from: Mr. Scram on September 04, 2019, 10:03:32 pm ---How would using an optoisolator work? I'm probably misunderstanding something but I can't imagine that being anywhere near linear.

--- End quote ---

The OP said
--- Quote ---I have a project in mind which involves sensing high voltage dc on/off signals.
--- End quote ---

Linearity not required.
Mr. Scram:

--- Quote from: fourfathom on September 04, 2019, 10:10:34 pm ---The OP said
--- Quote ---I have a project in mind which involves sensing high voltage dc on/off signals.
--- End quote ---

Linearity not required.

--- End quote ---
As it turns out I was right while being an idiot.  ;D
ledtester:
Another reason to avoid using large pull-up/pull-down resistors is that the effect of the input leakage current of a digital input pin becomes significant. At room temperature the leakage current for the 74HC4050 is only a fraction of a microamp, but over the entire temperature range it can be as high as a couple microamps. 1 uA of leakage into 1 M ohm is 1 V which is dangerously close to the 1.35V maximum for a low logic level at an input.


--- Quote from: Laszlo on September 04, 2019, 02:47:20 pm ---Could anyone point me in the right direction regarding the 74HC4050, and how it should be driven from an optocoupler?

--- End quote ---

I would try something like this:



The idea is to pick R1 and D1 so that you get 2 to 10mA through the led when a high voltage is present on B+. You can create a higher voltage zener by putting two or more in series. You can also move R2 to the emitter to invert the output signal.

fourfathom:
If you aren't too particular about the on/off voltages, you can simply use a two-resistor voltage divider feeding the opto.  Say a 10K to the high voltage, and a 270 Ohm resistor to ground.  Connect the 10K/270 junction to the opto diode.  Opto current will start to flow around 40V and it will linearly increase to 10mA at 100V (roughly).  The 10K resistor will dissipate close to 1.5W at 120V input. so you will need a power resistor.  You can adjust the threshold voltage and opto current by playing with the resistor values.  Put a capacitor at the opto input if you want filtering.  If you use a high-sensitivity opto you can increase these resistor values significantly, reducing dissipation and current drain.

The zener circuits will give you a more abrupt on/off transition.

So another parameter that would be good to know is the source impedance of the high-voltage source.  How much current can you draw from these sources?
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