This method is not invented by me, but I modified it with stereo channel of the mic input. The 2 separate channels make construction easier and more accurate.
Items need:
1) stereo plug x 1
2) 5mm IR photodiode receiver and sender, 2 pairs
3) connecting wires
4) 2cm diameter tube of length 25cm (Diode separation 20cm)
5) PC/notebook with soundcard/ motherboard with stereo mic in
6) Software - Audacity version 1.2.6
Facts:
1) Stereo mic, with 2 channels input enable entry and exit measurement be done separate and reduce noise/ interference.
2) IR photodiode is cheap. Do not use phototransistor (more expensive). Do not use those with metal cap (with internal pulse IC for IR TV remote control )5mm diode is used as the area cover will be bigger. If 3mm diode is chosen, you may have difficulty to catch the BB accurately. Make shoulde you know which diode is receiver as sometimes the purchased receiver diode is darker in colour, while sometimes is sold as clear diode (the one I got).
3) Length of travel tube governs the accuracy of the chronometer. The longer the separation (L) of the entry and exit diode placement, the less critical on the accuracy of time measurement. However, the longer the tube, the higher the chance the BB is shot in an angle , resulting in longer path (negligible) and chance the single exit diode not being crossed. (Then U need more than 1 exit diode , in parallel)
4) Time (t) measurement is done manually by cropping the signal spike between the entry and exit diode, from graph plotted on Audacity.
5) Time measured is translate to velocity (L/t) in fps or m/s, and hence the Muzzle energy can be calculated by 1/2 mv^2 Joule in Excel.
6) Error Tolerance is expected to be 1-5%. The higher the soundcard/ mic sampling frequency (44k to 100k from setting in the Audacity software), the higher the accuracy.
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