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Light LED and being detected by MCP23S17 IO Expander

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ehabzaky

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Dear All,

It may be very basic question but I need help to understand it please.

I have a human sensor which simply send 3.3 volts output once a human movement is detected. I'm connecting this output directly to a MCP23S17 to detect this signal and further process it through Raspberry PI. Up to here everything is OK.

I want to put an LED to give visual indication of human sensor action. When I connect the output of the human sensor to an LED with a resistor it was lighting OK but the MCP23S17 no longer detect the signal as per the following diagram. It seems that the current is going through the LED path only.

inputled.png

Could someone guide me how to have both LED working on signal and MCP23S17 detect it?

Regards,
 

It is easy to guess that the MCP23S17 only detects a high reading if the voltage goes above a certain threshold. I believe when the led turns on, it draws a few mA, which pulls the sensor wire to a lower voltage.

It will help if you install a transistor to detect the signal. Then the transistor turns on the led by means of current drawn from the power supply.

Or, does the MCP23S17 have a spare pin that you can connect the led to, and not interfere with the sensor reading?
 
Hi,

You need to read you sensor's datasheet.
Especially "output high current" and "output high voltage". Maybe you find a chart for this.

Then you will see if it is able to drive your LED while and your input.

Klaus
 
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