The problem this month is to count pushups.  A real first world problem.  Paul found an iPhone app that required you to touch the screen with your nose on each pushup.  Not!

• Ultrasonic sensor pointed horizontally across the area where the pushups are done
• Accelerometer
• Easy Button connected to + key of calculator
• IR sensor pointed either horizontally or vertically
• Phone App or fitness band
• Alexa/Siri to count grunts somehow
• pedometer
• OpenCV + Pi + Camera
• Other image processing
• Capacitive touch sensor
• Capacitive volume like Theremin
• Big lever touching back that actuates a switch
• Muscle sensor
• Force Sensing Resistor
• EEG monitor to scan brain waves

The first suggestion was from our guest speaker, a fifth grader.  I had not considered any solutions that used a horizontal beam.  I only thought of looking up or down at the torso.

A three axis accelerometer should work fine.  I wondered to myself if we would need more processing power than an Arduino.  Scanning brain waves from an EEG would definitely need more compute power than an Arduino.

Let me know in the comments if you try any of these solutions.