MCP23S17 port expander driving ULN2803 issues

[crocu]

Hello,


I'm experiencing problems to drive relays with MCP23S17 over ULN2803.

The MCP23S17 is 3.3V powered.

When i set an output of MCP23S17 at high state : "1",
i can properly measure 3.3V voltage on the MCP23S17 pin but the voltage measured at the input of the ULN2803 falls down and relay can't be driven.

Do you think MCP23S17 (when 3.3V powered) can't drive ULN2803 (common pin set to +5V)

Should i have to place a level converter : MAX3002 between MCP23S17 and ULN2803 to shift the levels ?

If so will a MAX3002 or 74LVC07A deliver enough current to drive ULN2803 ?

Attached below the schematic i use so far.

Thank you for your help,

 

[panic mode]

3.3V level should be fine, this is TTL level "1". input current is only about 1mA
but your schematic does not show GND connection of ULN2803 (pun9).
without it, ULN inputs are floating and this would explain your observations.

 

[crocu]

Hello

You are right the GND pin was not soldered properly, even it was looking clean.

Now it works, thanks very much for your help.