Level shifter required

[RonH]

I'm in need of a level shifter from 15V to 5V.

We used to use 5V on our limit switches but after a while we started to have the switches fail, they were not cleaning themselves with any arcing.
We are now redesigning our controller and have an opportunity to fix this problem.
I have a Supply voltage of 15V that I wish to pull up the limit switches to but my uC is an Isolated 5V, I thought about using an optoisolator with a CD4050 to shift the level but I ran into a problem on powering the 4050, if I power it with my isolated 5V then I'm no longer isolated and I can't use the 15V then my outputs will be 15V and blow the Opto. I don't want to add another DC-DC just for this one device.

Can anything think of a simple solution to my problem?

Thanks

Ron

 

[k7elp60]

Ron,
I may not understand your problem fully. I would still use opto couplers.
Use a resistor in series with the input to the 15V. Connect the output transistor collector to the uC +5V line thru a pull up resistor and when the limit switch is on the output of the opto would be low. I would also add a resistor across the input terminals of the opto coupler for noise or transcient protection.
Ned

 

[RonH]

I did think about this as well but if my math is right then I will need an 820 Ohm 1W resistor I could go with a 1/2W but that doesn't leave much overhead.

It could just be me but id didn't seem the like the right way to go about it.

Ron

 

[k7elp60]

I was looking at a typical opto that needed 10mA forward current on the input for transistor saturation and my calculations were for 900Ω or standard value of 820Ω. There are darlington output opto's that would require less forward current on the input. That might be a solution.

 

[creakndale]

Single Darlington optocoupler in 8 pin package: 6N139
Dual Darlington optocoupler also in 8 pin package: HCPL-2731

creakndale