pcf8574 random key presses indicated

Other Hardware LEDs and Multiplexing
1

I am using a pcf8574 board from waveshare to give me 5 key inputs via i2c.

I wired up initially without pullup resistors and was getting intermittent indications of key press and wrong values , so, I stuck in some 10k resistors and provided 3.5 volts to the circuit. I was still getting odd indications so I thought maybe 5v.

It seemed to be ok for a good while and then I was getting odd key presses again, if I turn power off and wait for 10 min and then turn it all back on its good again, until its not.

So I suspect that I have not got the correct resistors or ......

#include <PCF8574.h>
#include <Wire.h>
#include <TimedAction.h>
  
PCF8574 PCF_38(0x38);  // add switches to lines  (used as input)

uint8_t expander = B00100000;
 
TimedAction timer = TimedAction(100, check_switch_multi);
 
int val;

void setup()
{

	Serial.begin(9600);
 	PCF8574::PCF8574(expander);    
 	Serial.println("\nTESTPCF8574\n");
}

void loop()
{
	timer.check();
}

void check_switch_multi()
{
	for (int sw = 0; sw < 5; sw++)
	{
		val = PCF_38.read(sw);   
		if (val == 0)  
		{
			Serial.print(sw);
 			delay(200);
		}
 	}
}
/* ************************************************************** */
void check_switch_single()
{
	val = PCF_38.read(0);  // get the value for pin i
	Serial.print(val);
}

/*
expected serial output of pressing keys in order of 0123401234  

Opening port
Port open

TESTPCF8574

0123401234    

BUT occasionally I get random key presses the can start and then just continue 

01234012340000000000000000000000000000000000000000 ...

0 with no key pressed  

sometimes pressing 3 will illicit a 0  in the output

turn off the device wait 10 minutes turn it back on and its working ok again


*/

pcf8574.jpg
pcf8574.jpg997×501 162 KB

2

Problems you might be facing
-Loose connection
-if resistors are incorrect check through multi-meter.
-use proper power supply
-your arduino might be lagging depends on your board.

3

are 10k resistors the right type/value ??

is perhaps the usb cable from the pc to the Arduino not powerful enough to supply the voltages ??

why would it work ok for 10 - 15 minutes and then not, I thought it might be getting too much voltage ??

4
  1. yes
  2. no
  3. probably loose connections.
5

thank you both very much

Some days passed and I built a second keypad very carefully ( I thought) and I had the same problem, so I thought its gotta be something else, so I stuck it together on a bread board, turned it on and not a problem, doesn't miss a beat.

Seems I can't solder to save my life.

breadboard_wins.jpg