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 Case Report #3: DIY Hop Up Tracer Unit (HUTU)

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PostSubject: Case Report #3: DIY Hop Up Tracer Unit (HUTU)   Sat Aug 02, 2008 8:14 am

Case Report #3: DIY HUTU (Hop Up Tracer Unit)
(Created on 07/27/2008 by Jawz)

Due to the urgency of this subject, I have published an incomplete Case Report about a DIY HUTU where the connection to a power source will be discussed at a later time. OP Red Invasion is nearing, and my intention is to help as many people as possible to get their guns ready for the night mission.

Therefore, this report will solely deal with the installation of the LEDs. Please keep in mind that LEDs must be connected to a resistor or you risk burning them up (speaking from experience). Laughing

Consider this report "a work in progress" where the content and information can change. Any updates or changes will be published at the end of this paragraph:

07/27/2008: No edits at this time

A. Purpose
I have received many requests for detailed information about the construction and installation of a self-made hop up tracer unit (HUTU). Especially in view of the upcoming huge 1st-Sword event Operation Red Invasion on August 28-30, 2008 (, tracer units are a mandatory part of the night game requirements that will enable all operators to fully submerge themselves into an unforgettable atmosphere!

1st-Sword leader “Tank” has already been installing HUTUs for past night games, but I have taken this concept – which was published on different websites and on YouTube videos before – and enhanced it in an attempt to match the performance of the 2nd generation TM Tracer Units and to improve accessibility and breakdown of the HUTU and gearbox.

2nd Generation TM TU:

Disclaimer: Installing the HUTU requires intimate knowledge how to take down your gun and gearbox! Installation of the HUTU will require invasive and irreversible procedures that can cause damage to your gun! The author only provides information for educational purposes and is therefore not responsible for any damages derived from this knowledge. If you are not familiar with the internals of your gun or do not feel comfortable to perform any of the described procedures, it is highly recommended that you seek help from a professional!

B. Concept
Some people would ask why constructing a HUTU and not a replica of the TM? After all, TM is the original creator to implement tracer units into airsoft guns, and their units work very well!

Indeed, many concepts have been suggested and attempts made to develop a cheap knock-off by using inexpensive LEDs. But all the designs failed – usually due to lack of their ability to output enough energy that would illuminate tracer BBs sufficiently.

A quick calculation will show this dilemma: Let's assume a gun shoots at 400 FPS (feet per second) and at an ROF (rate of fire) of 20 BPS (bullets or BBs per second). Let's also assume that we would use a one foot long silencer-type tracer unit (TU) at the end of the barrel plastered with LEDs, which are already lit up and ready to make tracer BBs glow.

400 FPS means that a BBs will travel through the silencer TU in 1/400 sec., ie. in 0.0025 sec. This is a tremendously short period of time for anything to get illuminated. If we chose a HUTU, each BB would receive light for 1/20 sec., ie. 0.05 sec. at our assumed 20 BPS! That means the HUTU illuminates the tracer BBs 20 times longer than a silencer-type TU! Moreover, the LED light usually shines further down the feeding tube (especially with metail hop up chambers) making this design even more effective. So it's no wonder that the HUTU actually works! The silencer TU replica (based on LEDs) may have a chance if more electronics were implemented to increase light output – for example as a flash by overcharging LEDs, but this design would drive costs up making the original TM TU a better choice.

B.1. Design and Preparation
The idea of putting an LED into the hop up chamber is pretty simple assuming that you know how to break down your gun, use a soldering iron, and a drill: Drill a hole, plug in an LED, connect it to a battery, and then light up the LED.

Unfortunately, if you want to do it “right,” you need to take into account your ability to break down your gun in the future for maintenance or upgrading purposes. Therefore, you have to develop a modular design!

First of all, every gun type is different and hence, the most challenging part is the planning process how to route your wires, where to place your switch, where to install your LEDs, and where to put your quick connectors. A supposedly trivial part of the design is to find the right spot to accommodate your LEDs without interfering with the regular function of the gun. This sounds easier than it is sometimes, and due to space constraints, I can tell you right away that M14 owners are out of luck to install a HUTU! They have to either resort to the TM tracer unit or a modified magazine that hosts the LEDs and the battery. Unfortunately, the performance of the latter is pretty unsatisfactory compared to the HUTU.

After experimenting with many 3 and 5 mm LEDs in white, blue, and UV colors, I have come to the conclusion that the key to illuminating tracer BBs are super bright blue or UV LEDs. The dye of the tracer BBs seem to react best in the presence of UV light, though. UV light is defined as <= 400 nm wavelength. Unfortunately, small and space-saving (ie. 3 mm in size) UV LEDs are not very bright. The highest “luminous intensity” I found in that category was 3,000 mcd (millicandela). Although this intensity pales in comparison to the available 13,000+ mcd blue LEDs, UV LEDs perform admirably well, which is proof that the dye of the tracer BBs do indeed react to UV light best.

But whichever LED color you prefer, you need at least two of them.

Finally, you also have to plan what battery source you want to utilize for the LEDs: either the AEG's battery or a secondary battery source.

B.2. Quick Intro into LEDs: What is an LED?
“A light-emitting diode (LED) is a semiconductor diode that emits light when an electrical current is applied in the forward direction of the device, as in the simple LED circuit. The effect is a form of electroluminescence where incoherent and narrow-spectrum light is emitted from the p-n junction.” (

“The light emitting diode converts an elecrical current directly into light. Therefore, the light emitting diode (LED) is more efficient than many other light sources. [...] The forward voltage across a diode must exceed a threshold level before a current can cross the junction. For silicon, which emits a tiny amount of near-infrared, the threshold is 0.6 volt. For gallium arsenide, which emits considerable near-infrared, the threshold is 1.3 volts.” (“Getting Started in Electronics” by Forrest M. Mims III; 3rd Edition; pg. 66)

You need to know some data about your specific LED before you should hook it up: The forward voltage (Vf) and the forward current (If). Of course, it's always good to know the wavelength and light intensity, but those are factors that are not need to calculate the value for the resistor. This information is available from its datasheet.

In order to choose the correct resistor value, you apply the following formula:

Resistance (R) = Voltage of battery – forward voltage of LED (Vf)/forward current of LED (If)

Example: If you have an 8.4 V battery, your LED requires a Vf of 3.2 V, and the If is 20 mA (= 0.02 A), the calculation would look like this:

R = (9.6 V– 3.2 V)/0.02 A
R = 320 ohm

... and the next available resistor value would be 330 ohm. This value would present a safe voltage for the LED. From personal experience, I have observed that LEDs are able to take a significant higher amount of voltage (“supercharging the LED”), although I'm pretty sure that this would reduce the lifespan of it. In a 24-hour marathon, I have used 4 UV LEDs with a 150 ohm resistor in conjunction with an 8.4 V NiCd battery. Result: The LEDs performed marvellously. So the choice is yours whether you opt to be on the safe side or if you want to squeeze out as much brightness as you can.

Before you grab your soldering iron, take a close look at the LED (picture courtesy of wikipedia):

You can see that LEDs are polarized, ie. if you connect the LED in reverse, it will not light up. The best way to identify the polarity is to look for the flat side on the LED. This side by definition points towards the negative lead. Of course, you could identify the polarity by the lengths of the LED legs, but what would you do if the legs were already cut off and soldered to a board?

B.3. Required Parts
Depending on the “fanciness” of your planned HUTU, there are more or less parts that you may require. The following parts list will allow for a modular HUTU design, which will facilitate the disassembly and reassembly of your AEG and simplify access to the gearbox for maintenance or upgrading purposes.
  • 2x superbright blue (13,000+ mcd) or UV (3,000+ mcd) LEDs: please make sure you match or even exceed those values! At the time of this writing, RadioShack for example does not have suitable LEDs for this project!
  • 1x Mini switch
  • 1x Resistor: value depends on your battery voltage and LED
  • Heatshrink
  • 1 pair of JST connectors (quick connectors)
  • Superglue

For a secondary power source, you would need to add a battery holder and your battery of choice, which has to give you more than 5 V of power to energize the LEDs.

B.4. Recommended Tools
  • - Soldering iron incl. accessories, such as wet sponge, solder with rosin core, desoldering braid, etc.
  • Hot glue gun
  • Dremel or alike, including nylon disc, polishing wheel, acrylic polishing paste
  • Drill with 7/64” drill bit
  • Heat gun or simply a lighter
  • Sharp knife such as a scalpel or Exacto knife
  • Old towel or tissue to wipe your hands and protect your working area
  • Optional, but is of huge help: Bright headlight

B.5. Installation Example on an M4 CQB Using the AEG Battery
The following installation was done on an M4 guns with a crane stocks and contain pictures from different installation sessions. The installation principle remains with all guns, but a different approach to wire routing is of course necessary with different guns, and that's why proper planning part is so important.

B.5.1. Take Down Your Gun
  • I have included links in the “References” section with guides how to take down your specific gun. M14 guides are also included also I already mentioned that there won't be enough space for a HUTU.
  • Put your gun down on a flat area and lay down your wiring on top of the gun. Try to make a mental picture of your final result.

B.5.2. Remove the Upper Receiver and Inspect Your Hop Up System

B.5.3. Remove Any Interfering Parts
    To install the LEDs, you need to unscrew the cable holder and its corresponding rectangular column (highlighted in green).

B.5.4. Mark the Drilling Holes and Use a 7/64” Drill Bit for the Holes
  • In this case 4 LEDs were used.
  • Watch out not to push too hard when you are drilling. Otherwise, you may damage the walls of feeding tube when you suddenly pierce through the plastic.

  • Use a sharp knife or scalpel to remove drilling flashes around the holes.
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PostSubject: Re: Case Report #3: DIY Hop Up Tracer Unit (HUTU)   Sat Aug 02, 2008 8:15 am

B.5.5. Prepare the LEDs
  • First of all, take a closer look at the LED: One leg of the LED is shorter than the other. The shorter leg coincides with a flat surface on the side of the LED. That leg is the negative lead. The other leg is therefore positive. Know your polarity or the bulbs will not glow!
  • In order to keep the LEDs compact and reduce space requirements, cut off the dome of the LEDs with a Dremel using a nylon disc.

  • The surfaces of the “chopped” LEDs need to be polished to restore their transparency. You can see the roughened and milky surface of the LED here:

  • And shiny after polishing:

B.5.6. Try in the LEDs

  • It is important that you place all the LEDs facing in the same direction! Here is a picture of what I mean:

    Do you see the flat ends on one side of the LEDs in the picture above? They all are aligned pointing into the same direction, and you will see later why.

  • After inserting the LEDs, look down the feeding tube and make sure that no LED is protruding into the tube. That is the main reason for cutting the dome of the LEDs! At this point, you can use a drop of superglue to keep the LEDs in place.

B.5.7. Bend the LED Leads
  • This part will challenge your artistic skills and electrical understanding: You will need to bend the wires that have the same polarities together so that they form two rows: one row will have the positive leads, the other row the negative. At this point, it would be helpful if the superglue was allowed some time to cure.

  • You can see in the following picture that the negative leads of the right LEDs were all bent to the left side and vice versa. Go ahead and twist the leads as shown below.

  • If you look down the feeding tube, you will now see two rows of leads:

  • Insulate the “bottom” of the gun with some silicone grease so that you can remove the feeding tube in the future if you want to access your gearbox. Theoretically you should insulate right before you embed the LEDs into the hot glue. If by any chance you should contaminate the leads, this would not affect the solder joints.

  • Solder the correspondings leads.

  • At this point you may want to test the LEDs. But before you connect any power source to it, do not forget to use a resistor! For this test, I connected alligator clips to a resistor and hooked up the rest to a battery.

B.5.8. Verify the Seat of the LEDs for any Interference
  • Important: Take the upper receiver and hook it back up with the lower receiver. You need to verify now whether the leads are hitting any parts of the upper receiver. At this point, you can still adjust and bend the leads as needed. Both the negative and positive leads should point towards the channel that leads into the foregrip.

B.5.9. Add the Female Quick Connector (JST Connector)
  • Solder the female quick connector to the LEDs. The male JST connector is larger and would probably not fit through the channel that leads towards the foregrip (see point B.5.11)
  • Do not forget to slide the heatshrink over the wires before soldering!

B.5.10. Add Hot Glue
  • Embed the LEDs in hot glue. This will stabilize the HUTU and adds resistance to moisture. Notice how the screws remain accessible and clean!

B.5.11. Thread the JST connectors though the delta ring:
  • You can see here why I recommended to use the female JST connector: The male counterpart would be a hassle to fit through the delta ring channel – if it would go through it at all.

  • Connect the wire to the switch and power source. Do not forget to use a resistor! A report about power sources will be furnished at a later time.

B.5.12. Miscellaneous Pictures
  • This picture beautifully shows the advantage of a modular design on a G36:

  • You can see how easy it is to disconnect the HUTU and then access the gearbox.

  • LEDs inserted into a feeding tube of an AK-47:

  • Hole drilled into a metal hop up (UMP) and smoothened with a metal file:

  • A working HUTU in all of its beauty:

C. Discussion
Many attempts have been made to copy the TM Tracer Unit (TU), and there are some companies that will come out with their own solutions in the near future. Even private people are trying their luck as evident in this forum:

The HUTU turns out to be a very cost-efficient and functionally sound alternative to the much more expensive silencer-type TUs. It is true though, that the performance of the TM TU stays unmatched in comparison with this LED solution – whether you use 2 or 4 superbright LEDs connected to either a 7.4 V or 11.1 V LiPo using only a 150 ohm resistor.

If only the best of the best is good enough for you, then the TM TU is the only choice for you. The selection of TUs from different manufacturers may increase soon, but their performance remains to be seen.

I can give HUTUs both thumbs up without hesitation because on the field, HUTUs do their job as expected. The TM TU may be brighter, but from experience, your focus during the game will be on the objective, and it simply doesn't matter whether you hit your opponent with a bright or superbright GID (glow-in-the-dark) BB! Your adrenaline rush will easily make up for the lack of brightness!

One thing I started to appreciate about HUTUs is that they are always there wherever your gun is. You won't have a situation where you say “Oh man, I forgot my tracer unit on the table!”

So if you would like to save some serious money and rather spend the cash on other upgrades, go ahead and enjoy the fruits of your labor: The HUTU will take you to places “where no man has gone before.” Anyone else a Trekkie? :mrgreen:

Just keep a few things in mind:
  • Plan ahead!
  • Proceed slowly – there is no reason to rush!
  • Check and re-check the polarity!
  • Do not forget a resistor!
  • Make sure the LEDs fit in their designated space!
  • Use a weak battery for testing, not a LiPo!
  • Do not leave the soldering iron unattended!
  • Wear goggles!
  • Do not forget to turn off the HUTU after use or it will drain your battery to death!

D. References
A few selected take down guides (no emphasis on gearbox guides):




G36 family:

M14 (AGM):

M14 (G&G):

M14 (TM):

M16 family:

Classic Army M15A4:

MP5 family:



My sources for this project:
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PostSubject: Re: Case Report #3: DIY Hop Up Tracer Unit (HUTU)   Sat Aug 02, 2008 8:16 am

intentionally left blank!
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PostSubject: Re: Case Report #3: DIY Hop Up Tracer Unit (HUTU)   Sat Aug 02, 2008 8:17 am

intentionally left blank!

PS: If you have any comments or questions, please follow this link:
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