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Guide to Airsoft Tracers

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A Guide to Constructing and Installing a Current Regulated Airsoft Tracer

The purpose of this guide is to walk the reader through the construction and operation of using a current regulator to drive UV LED’s for use in airsoft tracers. I will also discuss the use of different color LED’s and the common resistor-LED combination.


Now, there are a couple guides that can be found in odd places on the internet, but the two that I found either cover using the resistor method, which is not very reliable or flexible, or use a Blu-Ray laser diode and driver; components not a lot of people have easy access to. I probably spent a good week or two trying to figure out ways to drive LED’s that was inexpensive and small. The circuit in Figure 1 is what I found worked the best.
I will also go over my experimenting with different color LED’s. These LED’s were lower in wattage that the UV’s I ended up using, but were still ‘bright’ enough that they would have at least some effect on the BB’s if they were to have an effect at all.
A third option, one I won’t go into too much detail about, would be to use an LM317 as a current regulator, but because there is a necessary 1.25v drop, using a 7.4v with two 3.7v LED’s begins to under power the LED’s when the battery gets around there. Not necessarily bad for the circuit, but the LED’s won’t be as bright as they need to be.


The Circuit

Fig. 1. Current regulator using a NPN 2N3904, a NMOS IRF610 MOSFET, and a couple resistors.

The components can be found on Digikey as follows;
• 2N3904 NPN


• 3mm UV LED’s

• 100k-Ohm Resistor

• 10-Ohm Resistor (This will operate the LED’s at 60mA)

Fig. 2. Components

Fig. 3. Circuit constructed. The 100k-Ohm resistor is shrink tubed on the top.

Fig. 4. Circuit from a different angle.

Fig. 5. Mounted LED.

Fig. 6. Mounted LED.

Fig. 7. Power wire from motor leads.

Fig. 8. UV LED's in Hop-Unit.

This unit is installed with the power leads (The +/- from the battery in Fig. 1) connected in parallel to the motor leads so that when the gun is firing the LED’s turn on.. However, you can connect it to a switch or button to have it either running all the time or only when needed. I used JST connectors from Deal Extreme so that I could unplug the unit when not in use. The LED’s are simply mounted in holes drilled in the hop unit and the circuit is fitted on the left side of the unit. On a side note, different guns are going to have different difficulties in mounting. Forward wired M4’s will have more wires running around then an AK with a full stock.

The way this circuit works is as follows;

1. The NPN is in cut-off (Turned off) which leaves the gate voltage on the NMOS equal to the battery voltage. For MOSFETS, the gate draws next to nothing for current.

2. With no current through the 100K resistor, there is no current drop, and so it looks like a short.

3. With the NMOS on, current begins to flow through the LED’s and the bottom resistor, but as the current flows through the resistor it creates a voltage drop IE voltage across the base-emitter of the NPN.

4. If/When the current is high enough to create a sufficient voltage drop, the NPN it will turn on, grounding out the NMOS gate to 0v.

5. This turns the NMOS off, reduces the current through the resistor until the voltage drop is no longer high enough to turn the NPN on.

6. With the NPN off again the NMOS turns back on and current begins to flow again.

So essentially, the circuit bounces back and forth until it hits equilibrium between current and the voltage drop across the NPN. For my circuits using the IRF610 and 2N3904, this voltage drop has been about 0.6v.
However, you can use any NPN transistor or NMOS MOSFET you want, but you need to be sure that the MOSFET can handle the current for the LED’s and that the gates and bases are rated for the battery’s voltage. If you decide to use an IRF610 like I did, the highest you can have the gate voltage is 20v before you start to damage it. So if you run an 11.1v LiPo and your MOSFET is rated to only have a gate voltage of 10v you’ll likely burn something out. Fortunately, most MOSFET’s have a pretty high gate voltage and the UV LED’s are fairly low in current.
In the current circuit I am using two 100mA, 3.7v UV-LED’s and running them at around 60mA as they were running up to 4.2v at 100mA. They also appear to work just was well at the lower current.
The reason I am using two LED’s is because it is actually more efficient than using only one. This is because there are three things that drop voltage in the circuit; The LED’s, the MOSFET, and the bottom resistor. Since the LED’s are dropping 3.7v each and the bottom resistor is dropping 0.6v, that means the rest drops across the NMOS as heat.
So by using two LED’s, you drop more across the LED’s which translates into less across the NMOS. The more the NMOS has to drop, the more it heats up. I’ll go over this in greater detail in the next section.
The reason I generally only use two and not three or more is because the battery does need to have enough voltage to supply everything. Three drops of 3.7v, plus the 0.6v ends up higher than most batteries. If you use an 11.1v (Full charge at 12.6v) or higher, you can use three. But, when you use a lower battery there isn’t enough to turn everything on and the LED’s end up very dim, draw less current, and don’t turn on the NPN (Doesn’t hurt anything because there isn’t enough current, it just won’t perform well).
Another trick you can use if you generally use somewhere around a 9.6v is to install a resistor to drop the addition voltage between the LED’s and the Drain of the NMOS. This is helpful in high current applications as you can get a resistor that is rated for high heat and save the wear on the MOSFET. Just figure how much extra voltage you should drop (Note that the NMOS should still drop at least around 0.5v or more), and divide by the current to get the resistance value.

First Design

Fig. 9. 100 Lumen, 400mA White LED.

Originally, I used a single 400mA, 3.7v, White LED (Fig. 9) for my first tracer setup in an AK Beta. To power it I had an LM317 voltage regulator setup for 3.6v output. While it definitely works great and helped out in a night game, it has several flaws. One, the white LED gets very hot. Within ten seconds of turning on it would start to give off smoke! Along with the LED getting hot, the regulator would also get fairly warm at 400mA while dropping the extra voltage. Using a 7.4v LiPo it wasn’t much of a problem, but on the 11.1v I like to use in the gun the heat builds fast.
So there are several flaws in this first design. One, white LED’s are not efficient for illuminating tracer BB’s as UV light seems to be the only light that works as I’ll discuss later. Two, by using only one LED the regulator (And the current regulator from part A) have to drop a lot more voltage as heat. You find this by multiplying the voltage drop by the current through the circuit. Three, LED’s should be driven by current and not voltage. The reason for this is as the temperature in the LED changes, so does the current at a specific voltage. If you drive it by current, the LED will basically pick an operating voltage.
The other flaw with this design was that the LED’s have their connection tabs on the sides. While this doesn’t have much effect on plastic units, it can make mounting on metal units much harder as you have to avoid shorting to the unit.

Resistor Method

Fig. 10. Using a single resistor to drive an LED.

While using a resistor is probably the simplest method to drive an LED. It really is only effective for constant voltage sources. If you run your guns using an AC wall adaptor or have a battery that is always the same voltage, then using a single resistor will probably suffice, but because airsoft guns tend to use a wide range of voltages for their batteries and those batteries have a range of voltages on them the output from the LED’s will not be consistent.
So essentially, the LED will draw a specific current, or at least try to, which creates a voltage drop across the resistor the limits the voltage on the LED. By subtracting the forward voltage of the LED from the battery voltage and dividing by the LED current you can find which resistor you should use. But because the battery will lose charge as it is used and will sag under the load from the motor, the voltage across the LED’s will shrink and so will the brightness.
Also, and this is probably the biggest flaw with this design. If you accidentally make the mistake of using a higher voltage battery without changing the resistor you will over-voltage your LED’s and burn them out .

Other LED Colors

Since I am at my parent’s house for winter break I do not have any tracer BB’s on hand to get pictures. However, I did experiment before I left using different colored LED’s. I used; red, orange, yellow, teal (Was supposed to be green), blue, and UV. I found that the red, orange, and yellow LED’s barely illuminated the BB’s, the teal partially did, the blue did to a more satisfactory level, and the UV did an amazing job of nearly instantly charging the BB’s. Before, I had mentioned that the white LED’s were not as efficient as possible. This is because the BB’s only seemed to use the UV light to charge up and since white is the full spectrum, you’re using all that extra energy to create light that isn’t used. I suspect that the blue and teal LED’s worked was because they may have a slight UV output to them. Or perhaps the BB’s are still able to absorb the other light frequencies, but not as efficiently as the UV.


I’m sure there are many other ways to power an airsoft tracer unit. I also know I am not the first to attempt this project, however, a couple years ago I remember looking for a guide on how to put tracers into a gun and surprisingly found next to nothing available. Now that I’m going to school and understand how these circuits work I thought it would be a good project to work on; to find a good way to power an in-gun tracer unit. Edited by Automobilie

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Hey guys, as an update, here are a few links for Digikey if you want to order some JST connectors so you can remove the units/gearboxes from each other.


Female JST

Male JST

Female Pin

Male Pin


And as another point, for enough to do three HUTU's it's about $20.

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Realized I've made a couple mistakes with this circuit.


Firstly; This circuit needs a kickback diode across the motor terminals. The guns I set up before had mosfets which already had these in them. Basically, get a 1n4007 or VSB2200S and have in it parallel with the motor so that it won't conduct when in normal use. When the motor stops spinning the coils produce an inducted reverse voltage that can quickly kill the NPN transistor. The diode is meant to stop this.


Secondly; Not use if the LED's changed over the years, but you need to use a 30-ohm resistor instead of a 10-ohm. This runs the LED's at their rated 20 mA instead of 60. If you want a snappier response you can go with a 20-ohm resistor that should put it at 30mA which may shorten the life of the LED's, but if they're only run during short trigger shots and not continuously it may last.

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