Voltage reduction question

[C0st]

I have an old alarm system I would like to connect to a raspberry pi,
the alarm system uses a voltage of 4.5v for digital 0 and 9v for digital 1
I need somehow to reduce 4.5v to close to 0 volts and 9v to 3.3v for digital 1 of the raspberry pi.
Voltage divider circuit with a zener is the best I could do but it doesn't achieve the close to 0
Any help would be appreciated.

 

[Harald Kapp]

Welcome to EP.

The quality solution is a comparator with 5 V output. Divide the input voltage down by e.g. a factor 4 (1.125 V low, 2.25 V high), set the treshold to ~1.7 V and you're set.

The cheap solution is a 5.6 V zener diode, transistor and a few resistors:

Note that this circuit inverts the signal 4.5 V in -> 5 V out, 9 V in -> 0 V out but that can easily be handled on the raspbery in software.

 

[C0st]

Thank you very very much, gonna try it soon. Won't the 5v fry the Rpi gpio?

 

[Harald Kapp]

Won't the 5v fry the Rpi gpio?

Right, I came up with this rather quickly. Of course, it the raspberrie's I/O is 3.3 V use Vcc=3.3 V.

 

[C0st]

Thank you so so much!

 

[AnalogKid]

100K from the base to the emitter?

ak

 

[Harald Kapp]

100K from the base to the emitter?

will not hurt