This project is a high-precision Airsoft Rate of Fire meter.This is a proof-of-concept that shows that a few dollars’ worth of simple components can be used to accurately detect the firing of a BB. That event can then be fed into a counter over a specific period of time to result in an accurate Rate-of-Fire measurement.
The core of the design is a simple opto-interrupter based on an infrared LED and an infrared photodiode. The photodiode biases the input of an schmitt-trigger inverter (like from a 74HC14) to provide a digial signal that can be interfaced to pretty much anything.
WHY IS THIS USEFUL?
Not only is it fun an interesting to know your gun’s RoF, but it is interesting to see how your different guns measure up to one another in terms of RoF.
Also, this tool (in combination with a chronometer) could make possible the gathering of concrete measurements on exactly what impact different springs, motors, gears, and batteries have on a gun’s muzzle velocity and rate of fire.
I’d like to gather the info needed to create a database of information cross referencing gun type, internals, and battery type with Rof and FPS. For example, you could find out exactly what you’d need to, say, increase your FPS to 350 but maintain your current RoF.
An additional possibility is to chart the RoF change as the battery is run down to find out which battery brands measure up the best in the real world.
PARTS AND SCHEMATIC (Click for larger version)
This is not a complete schematic of the design I used for the RoF meter; it lacks the precision timer. However, the schematic clearly shows the parts needed to make a detector suitable for placing into a tube or a mock supressor to trigger something when a BB is fired.
In the schematic, the DETECT signal lights an LED and triggers the counter chip below it. But the element triggered could be:
- a counter and timer (to measure RoF, like I did here)
- a counter and timer to measure muzzle velocity
- a homemade tracer unit
- a counter and LED display to make a “shots fired” display (like the “rounds remaining” counter on the pulse rifles in “Aliens”)
The possibilities are quite numerous.
HOW IT WORKS
The theory is the same as electic eye doorbells. You make a beam of light and shine it on a detector. When the detector can no longer see the beam (by something passing between the emitter and detector), a signal is generated.
In this case, we set up the emitter (infrared LED) and the detector (infrared photodiode) at the muzzle on opposite sides of the barrel opening. In this way, the infrared beam crosses in front of the barrel and is broken when a BB passes through it. In this prototype, the emitter and detector are built into a brass tube that is temporarily attached to the muzzle of the gun. In a more finished version, the emitter and detector could be built into a mock supressor, or even into a flashider for an integrated look. The tube helps with placement of the unit, and also helps shield the detector from ambient light. Visible light (especially sunlight) contains high amounts of infrared that can saturate the detector.
As for the part that actually measures the Rate of Fire, those are the two counter chips. For each BB that passes out of the muzzle, the counter chips increment by one.
A high-precision timer unit (accurate to within a few nanoseconds – more than enough) waits until it sees the first BB, then turns on the counters and waits 3 seconds. Once 3 seconds is up, the counter chips are disabled and subsequent shots are ignored.
The user then reads off the total BBs detected in that 3 second fullauto burst, and multiplies by 20 to get the RPM (rounds per minute).
WHY MEASURE A 3-SECOND BURST?
Good question – why use a 3-second burst? Why not just measure 1 second, then multiply by 60?
The reason is a tradeoff between accuracy and expediency. Guns fire at pretty much a constant speed (assuming battery drain, etc is not an issue), but the shorter your sample time, the less accurate your sample is likely to be. If you measure two 1-second bursts, and 1 is 13 shots and the other is 14 shots, that’s a margin of error of +/- 60 rpm! Readings over 3 seconds are more likely to be consistant over a variety of circumstances.
A future version of the meter could measure 1-second bursts and average them as long as you kept firing. The longer you fire, the more accurate the reading. But so far results are good with the 3-second system.
Actually, the results are just as good as the 2-second system, but I changed to 3-second since I find multiplication by 20 easier to do in my head than multiplying by 30 (to get rounds per minute). 
EXAMPLE: Test Results from my SG-1
The output may be a little hard to figure out for those of you not familiar with binary. The output is BCD (binary coded decimal) and I have added notations to the lights to make it easier to read.
The second picture shows the reading after a 3-second burst from my stock SG-1. 46 rounds detected in that 3-second window. Multiply by 20 = 920 rounds per minute. Whew!
UPDATES
Selectable Timer Added (Dec 2002)
I have in fact added the capability of measuring either 1, 2, or 3-second bursts. This is handy if you’re using locaps, or are firing a gas gun (many of which don’t have the magazine capacity to sustain a 3 second burst).
AEG RoF Fluctuation (Jan 2003)
An AEG will have different rates of fire for the same battery if you measure it at different points during the battery’s charge. For example, my freshly peak-charged 1300mAh large battery gives my stock SG-1 a RoF of 1260 rounds per minute. If I measure again after firing a few mags, the RoF drops to 900 rounds per minute (the same measurement as shown above). So if you want to aim for consistancy, you should remember to always measure your RoF under identical conditions.
Sadly, the power supply for an AEG is practically never identical, so measuring RoF precisely is a bit of a waste of time for cataloguing purposes. It’s a little like measureing your weight (which changes slightly over the course of the day) right down to the tenth of a gram. That’s not to say the RoF meter isn’t useful, just that you should be aware that the numbers will change under normal use of the battery and gun.