Hello Fernando,
I am using PCF8575(sparkfun breakout board) with ULN2803.
My config is as follow:
PCF8575
SDA > pin 20 (I did not use resistor because I have supposed that they are on the breakout board (R1,R2,R3))
SCL >pin 21 (same remark)
Gnd > Gnd Arduino
VCC >+5 Arduino
A0,A1,A2 > Gnd (weld between central pad and right pad (at the back of the breakout board )
P00-P05 > PIN 1-5 ULN2803
ULN2803
Pin 1-5 > P00-P05 PCF8575
Pin 9 > Gnd Arduino
Voltage check on output pin PCF8575(P00-P05) gives 4.82 (timer ON) or 0V (timer OFF).
Regarding those values,I can suppose that my PCF8575 is working properly.
My problem is that once I connect P00-P05 to pin 1-5 of the ULN2803,voltage drops to 1.7 V on input pin ULN2803 and to 0.44V on output pin (voltage drop between input and output is correct I think) but why does the voltage drops to 1.7V on input pin with the consequence that it's not possible to trigger a relay with 0.44V on output.
Could you please help me
Once again thank you very much for your help and for the superb project that you offer to us
Best regards
Jean-Yves