DoctorD

Member Since: April 16, 2009

Country: United States

  • You are right. But the fun part is actually trying it. :-)

  • The trick to allow LEDs to sense light as well as emit it, using only digital I/O pins, came from Mitsubishi Electric Research Laboratories (MERL), and was originally published in this paper in 2003. The accompanying video is here.

    This is not the most sensitive method of detecting light with an LED; you'll probably want a transimpedance amplifier for that. However, it is certainly the cheapest.

    Fun experiment to try at home: wire two LEDs in parallel (i.e. cathode to cathode, anode to anode). Place one LED in bright light and hold the other one close to your eye, shielded from the light. What do you see?

  • I like the microphone idea. It would be inexpensive and sensitive, as long as the microphone would take the abuse. The trick would be in mounting the microphone appropriately so that it picks up the bag punches but is less sensitive to ambient noise.

    An ideal solution might be an absolute pressure sensor inside the speed bag. That would pick up the air compression from each punch but be insensitive to movement and most environmental noise. The problem is that it would need to be coupled to the air inside, which would be troublesome to implement. You would also need a pressure sensor with sufficient bandwidth to pick up individual punches. These exist, but are not the tiny surface mount kind.

    Mounting the accelerometer differently would also help. Instead of mounting it to a fixed part, trying to pick up the shocks when the bag is punched, why not attach it to a small finger that moves a small amount as the bag moves a large amount. Think of it as a tiny lever or an impedance matching transformer. The accelerometer is far too sensitive to mount it directly on the bag, but a little finger that goes from the accelerometer to the top of the bag, allowing it to move slightly with each punch, would allow you to see the gross motion of the bag without clipping the acceleromoter, and without having to deal with the complicated signals received when the accelerometer is mounted to a fixed part.

  • Something as simple as this would probably work: https://www.sparkfun.com/products/10293

    Try taping one onto the speed bag and looking at the leads with an oscilloscope. No power supply or other circuitry needed for a simple test.