This simple Arduino setup and sketch will judge just how UP a sit-up needs to be to count. It will play one tone when you recline enough, and another when you sit up enough for a full crunch to count!

Ever get into an unnecessarily heated debate about someone else's crunches not counting? Me neither. But this gadget runs off a 9V battery and is light enough to hold to your chest or be placed in a chest pocket, and do the sit-up judging for you.

Supplies

Arduino Uno
Tilt sensor (aka inclinometer)
Piezzo buzzer (passive buzzer)
9V battery & barrel connector
M-F jumper wire

Step 1: Step 1: Wiring Up

The Arduino Uno can be powered by a 9V battery using a barrel power connector. Stick a cheap tilt sensor in the GND and Pin13 terminals, and wire the piezzo buzzer to the other GND and Pin6. The buzzer will only work if the negative (-ve) side is grounded.

Step 2: Step 2: Code

In the Arduino IDE, load this code and upload it to the Uno:

int inPin = 13;
int reading;
const int SpeakerPin = 6;
int lastsensorValue;

void setup() {
  pinMode (inPin, INPUT);
}

void loop () {
  delay(100);
  reading = digitalRead(inPin);
  if (reading == 1 && lastsensorValue == 0) {
    tone(SpeakerPin, 1500, 300);
    delay(500);
    lastsensorValue = 1;
  }
  if (reading == 0 && lastsensorValue == 1) {
    tone(SpeakerPin, 200, 300);
    delay(500);
    lastsensorValue = 0;
  }
  else {
    delay(1);
  }
}

Step 3: Step 3: Test & Enjoy

The buzzer will play one tone when you recline enough, and another tone when you sit up enough for a full crunch to count. Connecting the tilt sensor directly into the Uno headers leaves little room for the sensor to tilt away from the Uno board.