Hello,
I am using a PCF8574 IO Expansion Board I/O Expander I2C-Bus Evaluation Development Module with the Arduino and I am able to trigger HIGH and LOW in LED light. the observation is that the LED light is very low. using multimeter:
what is the reason behind that?
my application I would like to trigger a 5v relay using this extender.
In what circuit do you measure? Do you measure HIGH or LOW?
It has "pseudobidirectional" pins - it has only two states, using Arduino terminology OUTPUT HIGH and INPUT_PULLUP. It cannot source much current, only sink.
It is tge pcf8574 module
We already know this. How is it connected? What code is used? What do you measure? Do you have photos of your setup?
Thank you Smajdalf for your reply.
connections Arduino and the PCF8574
Connections PCF8574 with LED light
the code
#include "Arduino.h"
#include "PCF8574.h"
// Set i2c address
PCF8574 pcf8574(0x38); // address defined as per the I2C scanner
void setup()
{
Serial.begin(9600);
delay(1000);
// Set pinMode to OUTPUT
pcf8574.pinMode(P0, OUTPUT);
Serial.print("Init pcf8574...");
if (pcf8574.begin()){
Serial.println("OK");
}else{
Serial.println("KO");
}
}
void loop()
{
pcf8574.digitalWrite(P0, HIGH);
delay(1000);
pcf8574.digitalWrite(P0, LOW);
delay(1000);
}
measurments
I hope that is clear
See post #2
Solution;
Wire led plus resistor from Vcc to pin of the 8674. Note that the logic will be reversed so LIW will switch the LED on.
That is strange. When P0 is HIGH the measured value is too low. For P0 LOW it is too high. Did you try it without the LED, with nothing connected to P0? What about other pins?
Not clear, I think. Your words have confused @Smajdalf.
I think what you meant to say was that
You have not attempted to measure the voltage on P0 when it is set to 0 or LOW.
These readings are as expected, as already mentioned. The pcf chip is not faulty. Driving LEDs brightly requires more current than a pcf pin can source. However, it can sink more current than it can source, so if you connect the led+330R between Vcc and P0 it should be brighter (but will be off when P0 is HIGH and on when it is LOW.
A relay module may require less current to trigger it than a led requires because it may have a transistor on-board. However, some relay modules have an opto-isolator, which will require more current, like a led (an opto-isolator contains a led). If you can wire the relay module so that sinking current from the opto-isolator triggers the relay, that will be fine.
No they are not. PCF8574 should source 100 uA. I don't recall if type of LED was mentioned but even a red LED of reasonable size and quality should have about 1.5 V forward drop at 100 uA (as well as some light output).
thank you for passing by and for your input. Please be patient with a noobie 
if you see the code, in the loop section, am LED light will be on and off every 1 second. I am aware that PCF is doing the opposite.
I have tried to remove the loads (LED+330R) and measure the voltage directly from the pin. the result is 5v. however, when I connect the load again drops and the LED as low light. to test the LED, I connect it +ve side of the LED to the 5v and it works fine
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