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Voltage reduction question

C0st

Feb 11, 2018
3
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Feb 11, 2018
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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

Moderator
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Nov 17, 2011
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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:
upload_2018-2-11_11-44-7.png
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

Feb 11, 2018
3
Joined
Feb 11, 2018
Messages
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Thank you very very much, gonna try it soon. Won't the 5v fry the Rpi gpio?
 
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