dgordon42
Posts: 754
Joined: Tue Aug 13, 2013 6:55 pm
Location: Dublin, Ireland

MagPi 48 P50 circuit diagram 5V into a Pi?

Thu Jul 28, 2016 2:37 pm

The circuit diagram on page 50 of the MagPi issue 48 shows the outputs of an LM339 connected directly to the GPIO's of a Pi. The LM339 is powered from a 5V source, so it will output 5V to the Pi.
A Pi's GPIO pins are not 5V tolerant.
This will damage or destroy a Pi, or am I missing something?

Thanks,
Dave.

gordon77
Posts: 3693
Joined: Sun Aug 05, 2012 3:12 pm

Re: MagPi 48 P50 circuit diagram 5V into a Pi?

Thu Jul 28, 2016 2:57 pm

I thought that at first but looking at the datasheet

http://www.ti.com/lit/ds/symlink/lm339.pdf see sect 8.2

it appears to be an open collector output you pull up / down as required to suit.

dgordon42
Posts: 754
Joined: Tue Aug 13, 2013 6:55 pm
Location: Dublin, Ireland

Re: MagPi 48 P50 circuit diagram 5V into a Pi?

Thu Jul 28, 2016 3:10 pm

gordon77 wrote:open collector output
That's what I was missing.

Thanks,
Dave.

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