DIY Airsoft Chronometer/ Chronograph, PC based (Part 4 - Modification)

Just did 2 experiment:
1) Use 2 pairs of IR diode at exit site
easier to catch the exit signal , especially for faster BB > 500fps

2) Use shorter travel ~10cm
Accuracy / consistency of velocity can be kept within 2-3% deviation. Shorter tube results in easier to record the exit signal. The compromise is the time interval becomes shorter and needs more careful selection of the interval.
Audacity is set at 100k Hz sample rate with 16/24 bit (do not use floating as the signal will be lost during higher magnification of time)
Mic gain in Audacity needs to be set at ~90%

DIY Airsoft Chronometer/ Chronograph, PC based (Part 2- Construction)

Circuit diagram. Reverse polarity of the diode will not burnt the diode as the reverse voltage is not high enough to damage it. Make sure your mic input support stereo input. Most P4 or above motherboard has stereo mic.
IR diode is quite resistant to ambient light. U may cover the receiver diode if U found too much noise. Make sure all connection is solder well as small vibration may produce noise in the wave capture in Audacity. Get the orientation of which one is the entry reciever diode end, by testing each channel in the Audacity tracing.

IR sender diode connect in parallel with a battery source. The specified nominal voltage of my 5mm IR diode is 1.7V at 20-200mA. So I use 3V lithium battery with a voltage drop resistor. So 10-100 Ohm 0.5W resistor is ok. Note the polarity of the diode. The smaller metal piece inside the diode is to be connected to the battery positive.

For the IR recevier diode, one of the pin of the entry and exit diode is connected to the stereo plug middle and tip segment respectively (provide 2 channel information . The ground (base of the stereo plug) is connected to the other pin of each diode.

I check the IR sender by using my Sony DV camera. One should see the IR sender diode is lit on.

The IR receiver diode can be checked by using any TV/HiFi/DVD remote controller. One can see the IR spike on the Audacity software.

For small velocity gun, the tracing is easy to be capture. For higher velocity gun, a single exit diode may not be able to capture the signal in just 1 shot, as the BB may pass the exit diode on sideway. U can enhance this limitation by adding more parallel IR receiver diode in the exit end - a curtain of IR that will not miss the BB.

Audacity Software setting .... (to be cont)

DIY Airsoft Chronometer, PC based (Part 3-Result)

This Entry and exit signal with BB firing from my L96 stock. The mic gain setting on Audacity is around 0.9 . One can see the entry signal with some recoil due to the spring (relative movement of the travel tube as I put it in direct contact with my L39 on firing. Measured speed is 143-149 m/s or 490 fps. Muzzle energy just below 2Joule.

Velocity and Muzzle energy calculated from Excel. My M92 blowback - 0.6-0.7J only. Can only shot through 1 side of a coke can.
The 2 channel signal from Entry diode and Exit diode. Note this is captured from my CO2 pistol , in which the gush of CO2 signal is captured as 2 small spike before the BB pass the 1st diode.
Initial Audacity capture interface, showing testing with 3 shots of BB.

Audacity Interface and setting .........

DIY Airsoft Chronometer, PC based (Part 1)

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.