Hard to understand Input/Output GPIO multiplexing technique

Thread Starter


Joined Oct 18, 2021
Hi all,

I was taking a look at the attached circuit. It is the HMI that belongs to a commercial refrigerator. This is connected to a controller board that reads the switches and controls the LEDs. I can understand most of it, but I find difficult to understand how the controller board can read the Select Temp button, without activating the buzzer. Any ideas?




Joined Mar 30, 2015
Could you try to clean up the schematic so it's readable? You have too much white space, too many unnecessary wire bends, some things are drawn upside down, and the flow is very strange.


Joined Jul 29, 2016
on the micro controller you can make an IO pin an input as well as an output. So you can make the input high impedance to look for the status of the switch and when you want to run the buzzer you can sink to gnd. Based on how weak I am at circuit analysis I'd start there. I'm sure there is another explanation and I'm sure someone stronger in circuit analysis can help.