# Continuous string of 46.8 on LCD shield - not reading reed switch

**URL:** <https://forum.arduino.cc/t/continuous-string-of-46-8-on-lcd-shield-not-reading-reed-switch/216439>\
**Category:** Programming\
**Created:** [March 1, 2014, 3:06pm UTC](https://forum.arduino.cc/t/continuous-string-of-46-8-on-lcd-shield-not-reading-reed-switch/216439 "2014-03-01T15:06:52Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![system](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/system/32/1140315_2.png) [@system](https://forum.arduino.cc/u/system)\
**Post date:** [March 1, 2014, 3:06pm UTC](https://forum.arduino.cc/t/continuous-string-of-46-8-on-lcd-shield-not-reading-reed-switch/216439/1 "2014-03-01T15:06:52Z")

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Hello, I have some codes for an arduino speedometer and I'm using a reed switch and a magnet. The reed switch is fine. I just don't understand what's wrong with the code because I already grabbed this code from a website and it should work and I've adjusted it to produce something on my LCD (MPH : 0.00) . The problem is that the LCD usually says mph 46.8 or it changes between this and some other speeds. I don't think it is measuring the times the magnet is passing through the magnet either that or the interrupt isn't working? Forgot to mention I'm using an LCD shield.

```cpp
 #include <LiquidCrystal.h>
  #include <String.h>
  
  //storage variables 
  float radius = 13.5;// tire radius (in inches)- CHANGE THIS FOR YOUR OWN BIKE 
  
  int reed = A1;//pin connected to read switch 
  int reedVal; 
  long timer = 0;// time between one full rotation (in ms) 
  float mph = 0.00; 
  float circumference; 
  boolean backlight; 
  
  int maxReedCounter = 100;//min time (in ms) of one rotation (for debouncing) 
  int reedCounter; 
  LiquidCrystal lcd(8, 9, 4, 5, 6, 7);
    
  void setup(){ 
  reedCounter = maxReedCounter; 
  circumference = 2*3.14*radius; 
  pinMode(A1, INPUT); 
  lcd.begin(16, 2);              // start the library

  cli();//stop interrupts 
  
  //set timer1 interrupt at 1kHz 
  TCCR1A = 0;// set entire TCCR1A register to 0 
  TCCR1B = 0;// same for TCCR1B 
  TCNT1 = 0; 
  // set timer count for 1khz increments 
  OCR1A = 1999;// = (1/1000) / ((1/(16*10^6))*8) - 1 
  // turn on CTC mode 
  TCCR1B |= (1 << WGM12); 
  // Set CS11 bit for 8 prescaler 
  TCCR1B |= (1 << CS11); 
  // enable timer compare interrupt 
  TIMSK1 |= (1 << OCIE1A); 
  
  sei();//allow interrupts 
  //END TIMER SETUP 
  
  Serial.begin(9600); 
  } 
  
  ISR(TIMER1_COMPA_vect) {//Interrupt at freq of 1kHz to measure reed switch 
  reedVal = digitalRead(reed);//get val of A1 
  if (reedVal){//if reed switch is closed 
  if (reedCounter == 0){//min time between pulses has passed 
  mph = (56.8*float(circumference))/float(timer);//calculate miles per hour 
  timer = 0;//reset timer 
  reedCounter = maxReedCounter;//reset reedCounter 
  } 
  else{ 
  if (reedCounter > 0){//don't let reedCounter go negative 
  reedCounter -= 1;//decrement reedCounter 
  } 
  }
  } 
  if (timer > 2000){ 
  mph = 0;//if no new pulses from reed switch- tire is still, set mph to 0 
  } 
  else{ 
  timer += 1;//increment timer 
  } 
  } 
  
  void displayMPH(){
  lcd.setCursor(0,1);
  lcd.print("Speed: ");
  lcd.setCursor(7,1);
  lcd.print(mph);
  lcd.setCursor(11,1);
  lcd.print(" MPH"); 
  //lcd.print(mph); 
  //lcd.print(" MPH "); 
  } 
  
  void loop(){ 
  //print mph once a second 
  displayMPH(); 
  delay(1000); 
  }

```

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<div class="post-metadata">

**Author:** ![system](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/system/32/1140315_2.png) [@system](https://forum.arduino.cc/u/system)\
**Post date:** [May 5, 2021, 8:33pm UTC](https://forum.arduino.cc/t/continuous-string-of-46-8-on-lcd-shield-not-reading-reed-switch/216439/2 "2021-05-05T20:33:53Z")

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Closed on May 5, 2021, 8:33pm UTC