Hi everyone,
I'm working on a project that involves reading an 8x8 pressure sensor matrix (though my circuit is designed for 16x16, only the first 8 rows and 8 columns are currently connected). My setup uses shift registers (74HC595) to control the rows and a multiplexer (CD74HC4067) to read the columns. The circuit and the readings work fine when I press a single point, a single row, or a single column. However, I'm facing a strange issue when pressing multiple points which i've been trying to fix for weeks now.
The Problem:
When I press three points in the matrix that form a square (e.g., A12, A21, and A22), the fourth corner of the square (A11, in this case) gives very high readings, almost as if it's being pressed. This behavior persists regardless of which square I press in the matrix.
For example:
Pressing A12, A21, and A22 causes A11 to also register as high, even though it’s not being pressed.
Pressing B23, B32, and B33 causes B22 to register high.
I think the issue is not cross talk because the extra sensor that gives high readings is giving very high readings, like when it is pressed. for example the other 3 sensors I press will give 600 en the forth corner I don't press gives 550 (where it would give 30 or somthing when it would be cross talk).
My Setup
Microcontroller: Arduino Uno
Shift Registers: 74HC595 to control rows (send)
Multiplexer: CD74HC4067 to read columns (receive MUX)
Sensors: Pressure-sensitive resistors connected at each matrix intersection.
The rows are activated one at a time using the shift register, while the MUX reads the analog values for each column.
What I've Tried:
- Pull-Down Resistors: Added 10kΩ pull-down resistors on the column lines to prevent floating signals.
- Signal Damping: Added 100Ω resistors in series between the shift register and the rows.
- Delays for Stabilization: Added small delays (10 microseconds) between setting a row high and reading the columns.
//sent shifter
int SER_Pin = 2; // defining arduino pin 8 for serial output shift register1
int RCLK_Pin = 4; // defining arduino pin 9 for clock signal shift register1
int SRCLK_Pin = 5; // defining arduino pin 10 for clock signal shift register1
int OE_Pin1 = 3; //pin for controling high impedance state shift register 1
int shift = B1; // Bite for shifting shift register
// Pin-definities voor de receive MUX (kolommen)
const int receiveMuxS0 = 6;
const int receiveMuxS1 = 7;
const int receiveMuxS2 = 8;
const int receiveMuxS3 = 9;
const int receiveMuxSIG = A0; // Analoge pin voor het ontvangen
// Array om de druksensorwaardes op te slaan (8x8 matrix)
int pressureValues[8][8];
void setup() {
// Zet seriële communicatie op voor de monitor
Serial.begin(115200);
// Zet de MUX selectie pinnen als output
pinMode(SER_Pin, OUTPUT);
pinMode(RCLK_Pin, OUTPUT);
pinMode(SRCLK_Pin, OUTPUT);
pinMode(OE_Pin1, OUTPUT);
pinMode(receiveMuxS0, OUTPUT);
pinMode(receiveMuxS1, OUTPUT);
pinMode(receiveMuxS2, OUTPUT);
pinMode(receiveMuxS3, OUTPUT);
// Zet de SIG pinnen als input of output
pinMode(receiveMuxSIG, INPUT); // SIG voor de receive MUX leest analoge waardes
}
void loop() {
// Doorloop de rijen (send MUX), nu van kanaal 8 t/m 15
for (int row = 0; row < 8; row++) {
// Stel de rij in via de send MUX (C8 t/m C15)
digitalWrite(OE_Pin1, LOW); //turn onn shift register 1
digitalWrite(SRCLK_Pin, LOW); // turn off clock signal shift register
digitalWrite(RCLK_Pin, LOW); // turn off clock signal shift register
shiftOut(SER_Pin, SRCLK_Pin, MSBFIRST, shift); // Send binairy value to shift register
delayMicroseconds(10); // Allow signal to stabilize
digitalWrite(RCLK_Pin, HIGH); // turn on clock signal shift register
digitalWrite(SRCLK_Pin, HIGH); // turn on clock signal shift registert
shift = shift * 2; //shifting the binairy value for controlling shift register
// Doorloop de kolommen (receive MUX)
for (int col = 8; col < 15; col++) {
// Stel de kolom in via de receive MUX
setMuxChannel(receiveMuxS0, receiveMuxS1, receiveMuxS2, receiveMuxS3, col);
// Lees de druksensorwaarde op de huidige rij en kolom
int sensorValue = analogRead(receiveMuxSIG);
if (sensorValue < 20){
sensorValue = 0;
}
pressureValues[row][col-8] = sensorValue; // Gebruik row - 8 voor array index
}
}
// solving error for the bit returning to 0
shift = B1; // When shift is 0 rewrite to 1
// Print de waardes naar de seriële monitor
printMatrix();
// Wacht even voordat de volgende uitlezing begint
delay(500);
}
// Functie om de MUX op een bepaalde channel te zetten
void setMuxChannel(int s0, int s1, int s2, int s3, int channel) {
digitalWrite(s0, bitRead(channel, 0));
digitalWrite(s1, bitRead(channel, 1));
digitalWrite(s2, bitRead(channel, 2));
digitalWrite(s3, bitRead(channel, 3));
}
// Functie om de druksensorwaardes in de matrix te printen
void printMatrix() {
for (int i = 0; i < 8; i++) {
for (int j = 0; j < 8; j++) {
Serial.print(pressureValues[i][j]);
Serial.print("\t"); // Tab gescheiden voor netjes printen
}
Serial.println(); // Nieuwe regel voor de volgende rij
Serial.println(); // Nieuwe regel voor de volgende rij
}
Serial.println(); // Extra lege regel tussen matrix uitlezingen
}
The issue only occurs when multiple points forming a square are pressed.
Single row or column presses work perfectly.
The readings at the "phantom" fourth corner are almost as high as if it's being pressed.
Questions:
- What could cause the fourth corner to give a high reading in this scenario?
- Is this a hardware issue in my circuit design, or could it be software-related?
- Has anyone else encountered a similar issue with matrix-based circuits, and how was it resolved?
I've attached my circuit schematic as well. Any help would be greatly appreciated!
Thank you in advance for your time and insights.