Better options then an Optocoupler?

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The-Kevster

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Hi all!

I have a 32V switch which I want to use as an input into a RaspberryPi. The power supply is rated for 28V but I measured it at 32V. It drives a large motor so I am guessing it will drop to 28V when in use but just in case am planning for 28V-32V :grin: As the Pi will only accept a 3.3v input and is running from a separate power supply I thought using an Optocoupler might be the best idea.




As I have 6 switches in total I want to use 3 2-channel optocouplers.
http://uk.rs-online.com/web/p/optocouplers/7613552P/
**broken link removed**

The issue is to bring 32V down to 1.5V <60mA I would need to use a 560k 2W (at least) resistor which seems very excessive.

Is there a better option or will using such a high resistor be perfectly fine?

Thanks for any help!
Kev
 

Work out the value of the resistor that is the load for the transistor, from that you can work out how much current the transistor needs to take to make the bottom end of the resistor equal to 0V. Now for your opto isolator there is a transfer ratio which is the current in the opto diode compared to the collector current, this is typically 10 (can be <1 to >100). this is then the current that has to flow through your opto diode. Now work out the 32V feed resistor. Plucking figures out of the air, I would start off at 100K for the 32V feed and 10K collector load. Connect them up and see what the collector voltage is, if its less then .5V thats it! , if its more, reduce the 100K / increase to 10K.
Frank
 

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