I am building homeautomation board with MCP23017 and Raspberry. All input and output are protected by PC847 opto-isolator. I use 24V for swiches med 3.9 Kohm resistor on anod of PC847 (6,5 mA) . Katod are connected to 0

My question is: why can I measure 1,8 V between PC847 anod and noll pole of my Finder transformator when switches are off? And when I switch one input with eg 3 different switches I measure 3*1,8V?

 

I am building homeautomation board with MCP23017 and Raspberry. All input and output are protected by PC847 opto-isolator. I use 24V for swiches med 3.9 Kohm resistor on anod of PC847 (6,5 mA) . Katod are connected to 0

My question is: why can I measure 1,8 V between PC847 anod and noll pole of my Finder transformator when switches are off? And when I switch one input with eg 3 different switches I measure 3*1,8V?

Post your schematic. Also, let us know how you are measuring the 1.8V? Is it with a digital meter in volt mode or a digital meter in diode test mode.