Interfacing 4x4 keypad using 5 arduino pins

Projects General Guidance
1

Rummaging in my old electronic stuff I found a 74HCT157 (Quad 2:1 mux) and immediately crossed to my mind to use it in my current 4x4 keypad interface that uses 8 pins of my uno board...and save 3 pins. It sounds easy but I need help here figuring out how to tell the Arduino that read 4 pins with the select pin S of the 74HCT157 in LOW, save that status and then that read the same 4 pins with the select pin S of the 74HCT157 in HIGH and save that another status and finally put them together. I made a schematic showing the circuitry between keypad, mux and Arduino. (see picture above below). Thank yo for your help!

2

This won't work. All I inputs to the multiplexer are undriven, all times, irrespective of key presses.

3

You could use an I/O expander like the MCP23009 as it has programmable I/O. Just add some pull-ups.

4

Other than the input to the keyboard you need to tie the NOT Enable to low.

Mark

5

Finally, I finished my small project to integrate a 4x4 keypad with Arduino uno using only five pins. At first I had planned to implement it using a 74HCT157. After some failed design attempts, I moved to the idea of use a 4052 but even as it resolved the rows writing, it took another IC: a 74HC165 for the columns reading. Here my raw code. I don't like it ‘as is’ but it works!:

#define PIN3  3
#define PIN4  4

int latchPin = 5;
int dataPin = 6;
int clockPin = 7;
int input;

void setup() {
  pinMode(PIN3, OUTPUT);
  pinMode(PIN4, OUTPUT);
  pinMode(latchPin, OUTPUT);
  pinMode(dataPin, INPUT);
  pinMode(clockPin, OUTPUT);
  Serial.begin(9600);
}

uint8_t shiftIn(uint8_t dataPin, uint8_t clockPin, uint8_t bitOrder) {
	uint8_t value = 0;
	uint8_t i;

	for (i = 0; i < 4; ++i) {
		digitalWrite(clockPin, HIGH);
		if (bitOrder == LSBFIRST)
			value |= digitalRead(dataPin) << i;
		else
			value |= digitalRead(dataPin) << (7 - i);
		digitalWrite(clockPin, LOW);
	}
	return value;
}

void loop() {
  
  setMUX(LOW, LOW);
  digitalWrite(7, HIGH);
  digitalWrite(5, HIGH);
  input = shiftIn(6, 7, LSBFIRST);
  digitalWrite(5, LOW);
  
  switch (input) {
    case 14:
      Serial.println("1");
      break;
    case 13:
      Serial.println("2");
      break;
    case 7:
      Serial.println("3");
      break;
    case 11:
      Serial.println("A");
      break;
  }
  delay(10); 
  
  setMUX(LOW, HIGH);
  digitalWrite(7, HIGH);
  digitalWrite(5, HIGH);
  input = shiftIn(6, 7, LSBFIRST);
  digitalWrite(5, LOW);
  
  switch (input) {
    case 14:
      Serial.println("4");
      break;
    case 13:
      Serial.println("5");
      break;
    case 7:
      Serial.println("6");
      break;
    case 11:
      Serial.println("B");
      break;
  }
  delay(10); 
  
  setMUX(HIGH, LOW);
  digitalWrite(7, HIGH);
  digitalWrite(5, HIGH);
  input = shiftIn(6, 7, LSBFIRST);
  digitalWrite(5, LOW);
  
  switch (input) {
    case 14:
      Serial.println("7");
      break;
    case 13:
      Serial.println("8");
      break;
    case 7:
      Serial.println("9");
      break;
    case 11:
      Serial.println("C");
      break;
  }
  delay(10); 

setMUX(HIGH, HIGH);
  digitalWrite(7, HIGH);
  digitalWrite(5, HIGH);
  input = shiftIn(6, 7, LSBFIRST);
  digitalWrite(5, LOW);
   
  switch (input) {
    case 14:
      Serial.println("*");
      break;
    case 13:
      Serial.println("0");
      break;
    case 7:
      Serial.println("#");
      break;
    case 11:
      Serial.println("D");
      break;
  }
  delay(10);  

}
  
void setMUX(uint8_t a, uint8_t b)
{
  digitalWrite(PIN3, a);
  digitalWrite(PIN4, b);
  //delay(1000);
}

I can post here the connections between the two ICs and the Arduino if requested.

6

Palliser:
Finally, I finished my small project to integrate a 4x4 keypad with Arduino uno using only five pins. At first I had planned to implement it using a 74HCT157. After some failed design attempts, I moved to the idea of use a 4052 but even as it resolved the rows writing, it took another IC: a 74HC165 for the columns reading. Here my raw code. I don't like it ‘as is’ but it works!:

Congrats. What about it don't you like?

Palliser:
I can post here the connections between the two ICs and the Arduino if requested.

Please do. It's always good I think to see how lots of different approaches can be used to resolve the one challenge.

Geoff

7

Here the schematic showing interconnection between the 4052, 74HC165 and Arduino uno. Notice that it is only used half of the capacity in both ICs.