negative voltage to positive passive conversion

[Tim_B]

Hi Chaps

I have a problem as always, I need to be able to monitor 4 lines. They all share a common grnd and can have 3 states in reference to the common line.

Ov
+9
-9

I need to recognise these states in a MCU. 0 is easy and so is +9 but the -9 is giving me problems. Especially as I must consume no power doing so. So a couple of opto isolators are not an option as the transistor outputs will draw current and as the device is battery powered and needs to run for a year +. The signals will wake the MCU from its slumbers.

 

[Nigel Goodwin]

You really need to give more details, is there really a three state line?, and if so why?.

 

[kchriste]

If the MCU has an ADC, then use that. Bias the input to the ADC and use a voltage divider so that it's input is always between 0&5V for a 9 to -9V swing. Which MCU are you using?

 

[Tim_B]

Ok lets try again

I have 4 outputs from a device

1
2
3
4
common

The outputs can have 3 states 9v, 0v and -9v. I'm not sure how I can make that any clearer.

As I said in my original post

Especially as I must consume no power doing so. So a couple of opto isolators are not an option as the transistor outputs will draw current and as the device is battery powered and needs to run for a year +. The signals will wake the MCU from its slumbers.

What that means in real simple speak

1 +9v is easy a simple r/r divider will do
2 0v is easy as I do nothing as this is the default state
3 -9v is the hard bit
4 I cannot use any current consuming devices eg opto isolators
5 I have to be asleep all the time and wake via interrupt on change
"The signals will wake the MCU from its slumbers" that means no ADC!

 

[kchriste]

3 -9v is the hard bit

Do the -9V the same way as the +9V; with a divider, except put the "low end" of the divider to the MCUs +5V supply. Use high value resistors to minimize current drain or put a 6V zener in series with the resistor connected to the -9V signal so there is zero current draw at 0V input. Since there are no 3 state inputs on any MCUs that I know of, you'll need 8 IO pins to detect 3 states on 4 lines.

 

[Tim_B]

kchriste

Thank you very much!

 

[mneary]

Divider does not meet the requirement of "consume no power".

Maybe if you define "no power" there would be other useful suggestions.