PCF8575 and ULN2803 [SOLVED]

[Jean Watrelot]

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

[Fernando Garcia]

Hi!

If your module is like this:

Exists only the pads to resistors. Need add resistors of 10 K.

When the ULN is connected is normal have 1.5 - 1.7 V.

What's the model of your relay board?

It have inverted logic, I mean turn on with LOW and turn off with HIGH signal?

Best regards.

[Jean Watrelot]

Hello again Fernando,

This is well the board that I use.
Following your answer, I have added 2 10K resistors.
This is the link to my relay board.
http://www.ebay.com/itm/8-Channel-5V-Re ... 1317483666.
May be my problem comes from the way to connect this relay board.
Thanks again

Jean-Yves

[Fernando Garcia]

You can test it connecting IN1 - IN8 directly to GND.

Check if the jumper is in right position (JD-VCC / VCC) .

Here's the basic circuit for this board.

Moreover for this quantity of relays I think better use an external power supply connected between JD-VCC and GND.

[Jean Watrelot]

Thank you Fernando,

Just finished rewiring my breadboard using opto-isolation for Arduino and an external power source connected to Gnd and JD-VCC.
Everything works fine.
I can now continue the design of my PCB ( I test each part of ferduino separetly on a breadboard and if it's OK I convert it to my PCB project).
Thanks again

Jean-Yves

[Fernando Garcia]

Glad to hear!