What Is Airsoft Mag Tracer Unit

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Project Design Walkthrough

In my Guide to Starting a New Projection , I mentioned working on some projects for a friend, and for this continuation commodity, I think information technology'south worth mentioning that nosotros're both Airsoft players. It's a great way to spend a solar day out in the forest with some friends and go some exercise, and what amend project is at that place for united states of america to work on together than a tracer unit of measurement for dark games. A tracer unit is a mock suppressor for a replica gun that shines UV light on a glow-in-the-dark BB every bit it exits the barrel. This is the perfect time to brand one because there's a large Airsoft event spanning multiple days coming up, which he is going to. I'd beloved to join him, merely instead, I'll be going to AltiumLive .

During the day, information technology'south very easy to see a white BB fly through the air toward your target, while at night, it's more of a challenge to see what you're hitting. Tracer units are surprisingly expensive and typically take under a single watt of LED power. For a similar price, we tin build our own tracer unit and buy about 140 watts of loftier power SMT LEDs. I'k a fan of the phrase "if it's worth doing, it's worth overdoing", and 140 watts seems like plenty of overdoing, so that's the plan. The claiming will be packaging this many LEDs and powering them!

Unlike many project articles yous may encounter, this will be a series to take yous through my full projection pattern process as information technology happens. I'm writing this article every bit I'k planning and designing, and information technology volition cover the project blueprint process to the point of circuit lath layout, and will be submitted before breadboard prototyping or schematic capture and PCB layout begins. This way, you will exist able to see the total project design procedure, including changes that are driven by the schematic or PCB layout, rather than the cooking testify's typical 'here's ane I prepared earlier' arroyo. This is my get-go Upverter™ project. I've been an Altium Designer® user for over a decade, and then I'm excited to run into what the more modern Upverter has to offer in design experience.

If you're new to designing your own electronics, this article volition guide you lot through the procedure of an experienced engineer and maker equally they tackle a projection. As mentioned in the Guide to Starting a New Project , whether you are building a basic device, such equally this tracer unit, or a complex project, the project design process all boils downwardly to the same gear up of steps.

Specifications

My specifications document is fairly simple: Plough the LEDs on when a BB enters the tracer unit, and plough the LEDs off when the BB exits the unit.

I was planning to build this purely with logic devices, however, the number of ICs was rapidly growing and the board infinite/packaging was going to be an issue. Instead, I'thou going to apply a microcontroller to detect the BB's passage and control the LEDs. Not to mention, as a fun added bonus of using a microcontroller, I can also use the tracer unit of measurement as a chronograph, to measure out every BB's velocity as information technology passes through.

To discover the BB, I'g going to use an infrared LED and detector, equally the BB travels past it will pause the axle telling the microcontroller to turn the lights on. The light gate will be at the exit of the tracer unit, and used to plow the lights off. I'll also desire to add some code to the firmware to automatically turn the LEDs off after a menses of time if an go out result was not detected. If there is potent infrared low-cal, such as daylight, shining into the tracer unit, it may not detect the exit event, and with such a huge ability describe for the LEDs, I desire them switched off as quickly equally possible to conserve battery life and prevent the whole unit from overheating. There volition be very little cooling for these LEDs, so I'one thousand relying on the very short duty cycle to forestall overheating.

Role Selection

Before fifty-fifty looking at a schematic or a PCB layout, I need to effigy out which critical components the pattern volition utilize. For this design information technology will be the battery, LEDs, LED driver, microcontroller and photogate. Since this is a tracer unit, it'south going to take a lot of LEDs, which ways beyond the nuts of merely detecting the BB travelling through the tracer unit of measurement, the power requirements for the LEDs will be driving many decisions.

Battery

For the battery, I am going to select a Turnigy 700mAh iii cell LiPo bombardment with a 60C belch rating, equally it's the most compact bombardment I tin detect that meets my discharge requirements. I've been using HobbyKing for many years, and their batteries are high quality yet cheap, and they take absolutely no circuit protection every bit y'all'd find on batteries targeted at consumer devices, which is great for stuff like this! Don't think of the lack of protection as a bad affair, because in the radio controlled hobby industry, the electronics which the batteries plug into provide the necessary protection, leaving the batteries able to discharge staggering amounts of power. I volition demand to build a low voltage detection circuit into the design to stop the LEDs from running if the battery starts to be besides depleted.

Now that the bombardment has been divers, I could apply its dimensions to set the minimum length of the tracer unit of measurement to about 80mm. The minimum length of the tracer unit of measurement will as well affect the layout of components and the amount of space available on excursion boards within the unit of measurement. Knowing the bombardment size is critical to beingness able to design the nearly meaty unit possible, which not only is better for real world usage, only too will determine the PCB manufacturing cost.

LEDs

Because I'm trying to light upwardly a phosphorescent cloth, I outset demand to effigy out the optimal wavelength of light to utilise. Phosphorescent materials work by absorbing a lower wavelength of light and re-emitting it equally a higher wavelength. I tin can exist fairly sure that typical Airsoft tracer BBs are using SrAl 2 O 4 , strontium aluminate , for the phosphorescent material every bit the cheaper ZnS probably wouldn't get lit brightly plenty to satisfy players. Wikipedia tells usa that "excitation wavelengths for strontium aluminate range from 200 to 450 nm", then the shorter UVB or UVC wavelengths are probably going to exist the best options, if suitable LEDs tin can be found.

For the UV LEDs, I headed to Digi-Central to start filtering down the Infrared, UV, Visible Emitters category. Initially I'm looking for Active, In Stock, Ultraviolet ones that are on Cut Tape to encounter what options come up up. Sorting by price at a quantity of 100 units, I can encounter there are a lot of low-ability options there which just won't give the free energy density this project requires, so I'll further filter by forward current for anything over 500mA. Now I have lots of 1A Forward DC Electric current (I f(Max) ) options, which is fantastic. The cheapest options have a 120°-130° viewing angle, which I suspect is going to exist likewise wide, so I'll head to Fusion360 to mock up the design to see what might work. I want the LEDs to shine on the BB for every bit long equally possible, so perhaps a wide angle LED will work well, but if I can cram a lot of LEDS in a row, I could possibly go a college energy density by using a tight beam.

Mock up design for a 120 to 130 degree viewing angle LED in contact with a 6mm BB Wide angle LEDs accept to be placed close to the ball to achieve loftier energy density.

The box at the elevation is a 3.5mm wide LED, and the circle at the bottom is a 6mm BB. With a 120° viewing angle, I feel at that place volition be adept coverage of the LED as information technology comes past, but the peak energy on the LED perhaps isn't going to be as high every bit it could be. To get the phosphorescent cloth glowing every bit brightly as possible in the shortest amount of time, I need to give information technology a lot of free energy. My gut feeling is that travelling past more than tightly focused LEDs volition 'accuse' the phosphorescent fabric better than more than fourth dimension in the emissions of a wide angle LED.

50degree LED Narrow angle LEDs offering high energy density while maintaining distance from the BB.

If you look at the same drawing with a 50 degree viewing angle LED, you lot will notice that I tin bring the viewing angle lines down to be tangent with the BB and still maintain skillful clearance (4.1mm) from the path the BB volition travel. This should reduce turbulence in the tracer unit, assuasive for a more accurate shot.

I now have a short listing of possible UV LEDs, with the Everlight ELUA3535OG5-P0010U23240500-VD1M existence the cheapest. It is a 405nm LED which isn't possibly as optimal every bit a 385nm or shorter wavelength, but the price is 37% lower than the same LED in a mildly shorter wavelength. I feel every bit though 37% more LEDs of the marginally longer wavelength is going to provide more effulgence than nosotros could get with fewer LEDs of the shorter wavelength. However, I do non have actual data to back this up. Inolux accept several LED options that are just a little more expensive, but the Everlight datasheet is more comprehensive and has much clearer land blueprint and pin details, which are very of import to me designing the board. So we'll become with the cheapest option, as non only are they the cheapest per unit, they are also the cheapest per watt. Equally this function is only available from Digi-Key, I made a quick check for UV LEDs on Mouser and European suppliers, but didn't find a cheaper option that would suit my requirements.

LED Driver

The chosen LED's datasheet has a nice voltage/current graph. Using the 405nm option, information technology looks as though 3.55v to 3.6v will give about 900mA to1000mA of forrad electric current. This lines up very nicely with our bombardment, as a LiPo battery offers a nominal voltage of 3.7v per cell. However, a fully charged battery will exist at four.2v, making information technology clear that we're going to demand a constant current commuter of some sort so as non to burn the LEDs out.

vf-vs-ma College wavelengths let for larger forward current for a given forward voltage.

There are two possible approaches to take here: either use an op amp on a current sense resistor to enable/disable the gate of a MOSFET to limit the pinnacle current, or get with a total blown LED driver IC that can boost the voltage and allow more LEDs to be run in series.

Being a lazy engineer, I'grand off to Texas Instruments Spider web Bench tool to design up a constant electric current LED supply. I simply input my requirements, and it came upwardly with a blueprint based on the TI TPS92691 multi-topology LED driver with a rail-to-rail current sense amplifier. It sure sounds fancy! Okay, I'g kidding somewhat, I wanted to use a constant current boost driver because I'grand going to be doing another UV LED projection for mail-curing parts that have been 3D printed from resin. Knowing I'grand going to be having similar LEDs and power requirements for that project allows me to design a power supply for this ane, and utilize the same schematic for the other project besides, which makes it both easier to gild parts, and reduces the total design time for both projects.

I have chosen to go with four 'rails' of LEDs within the tracer unit, and based on a series of decisions which are now more often than not irrelevant since deciding to use an LED driver, I decided to use 12 LEDs per rail. I was planning to run the LEDs direct from the battery using a MOSFET and an op amp for current limiting, which led to groups of three LEDs. A constant current LED driver, however, will supply a consistent brightness equally the battery is discharged and is a more elegant solution. The driver will run 2 sets of half-dozen series LEDs in parallel, which requires a 21.6v, 2A supply. This ends upwards being about 173W for the whole unit, which is definitely overkill and a flake of scope creep, but I'g okay with that.

Web Demote takes intendance of pretty much everything, notwithstanding, it does make some odd choices for capacitors and inductors. Given that I need this to be a relatively compact pattern, I used the integrated nib of materials editor to swap out parts for smaller, more than efficient options. I'chiliad not overly concerned with saving every last cent, equally I will only exist making a couple of these units, then a difference of a dollar or two in the BOM to get a more than efficient, more compact design is worth information technology.

TPS92691-sch TI Web Bench allows you to design power supply systems with complete ease.

My principal concern with using a driver like this is that I can't afford to accept a soft startup or actually any startup filibuster. As starting upwardly the LED commuter takes more time than simply switching the LEDs on and off, I'thousand going to take to put a N-Aqueduct MOSFET on the LEDs. This is going to make the commuter think the LEDs are broken or something's gone wrong; the datasheet specifically mentions this.

The TPS92691/-Q1 supports continuous LED condition check through the current monitor (IMON) output. This allows for LED brusque circuit or open circuit detection and protection.

The IMON pin is an counterpart output, and the driver mentions bicycle by cycle protection and sensing, so I'm hoping if the load goes away, it will give me full current on the next cycle if required. I don't commonly similar technical risks such as this in a design, nevertheless, I'g curious enough to effort information technology, and the project timeline isn't critical, so I could afford to spend time experimenting. When I get the circuit boards I'll notice out if I need to do a redesign!

softstart-sim Simulated driver startup shows that LEDs reach peak current after a delay.

Microcontroller

Next up, we're onto the microcontroller. To keep things like shooting fish in a barrel, I'k going to go with an NXP microcontroller with USB bootloading and MBED compatibility. The NXP LPC11U12FHN33/201 fits the bill nicely. It's inexpensive, has enough resources for this projection, and shows up as a USB mass storage device when bootloading. What more than could you ask for? If the board layout gets tight, I will switch to the LPC11U35FHI33/501 at almost twice the price, but in a 5x5mm footprint rather than a 7x7mm one. I could employ a microcontroller that fits in a WLCSP package (36 pins in a ii.4×2.5mm area) if I needed to go really tiny, just those are starting to achieve across the limits of my ability to hand populate a board reliably.

Photo Switch

There are multiple ways to implement a photo switch/photogate sensor, however, due to the short duration of the dark menses as the BB travels beyond the sensor, I need an implementation that is capable of depression latency response. Because of this requirement, I've decided to use an infrared photodiode connected to a transimpedance amplifier , which in turn will drive an S-R latch that is connected to a microcontroller pin. I may non need the S-R latch, only I haven't decided how I want to implement the code at this betoken (either as a loop or using interrupts and with or without MBED). I really demand to ensure the BB'due south transit across the sensor is not missed, so past using a latch I can ensure the issue is caught, and held, until the microcontroller has dealt with it.

In the Great britain, an automatic airsoft gun is limited to ane joule, which, with a common 0.2 gram BB gives around 340 anxiety per 2nd (104 m/south). A unmarried shot airsoft gun, such as a sniper rifle, is express to 2.iii joules which would give a 0.2g BB a 500 feet per second (152 m/s) velocity. Nonetheless, snipers typically use much heavier BBs to reduce deflection by wind and increment range, and so the projectile'south velocity could be effectually 340-400 feet per 2nd. At 340 anxiety per second, the BB will block a 3mm sensor surface area for about 29 microseconds, a relatively long fourth dimension in the world of microcontrollers, only nevertheless possible to miss if for case the code is saving chrono data to an EEPROM.

When selecting the photodiode, it's important to look at the spectral response graphs. Many of the available options listing an infrared wavelength, but will as well detect some UV and a considerable amount of visible light upon closer inspection. Equally the tracer unit volition incorporate 170 watts of UV LEDs, and just a basic IR LED, light leakage from the UV LEDs could cause a missed detection on the exit sensor as could brilliant sunlight. The tracer unit will have 3D printed light baffles/shields around the IR gates. However, with orders of magnitude more light coming from the UV LEDs, the chance of some low-cal getting by the baffles is quite loftier.

Optimizing the role selection for the infrared spectrum just, and the highest possible current when exposed to light, I chose the Everlight PD15-22B/TR8 Silicon Pin photodiode. This photodiode has a peak sensitivity wavelength of 940nm, which is important to know prior to selecting an IR LED, so it can be chosen to match this wavelength.

Heading back to the emitters category on Digi-Central, I'm now looking for an Active, In Stock, 940-950nm surface mount IR LED with a ten-twenty degree viewing bending. I'g too trying to avoid gull fly and similar packages in general on this project, as they don't handle shock and vibration as well as something directly mounted to a circuit board. When setting upwards the BOM for the LED commuter in Web Demote, I switched from aluminium SMT capacitors to multilayer ceramic ones for the same reason. Whilst airsoft guns do not take anywhere almost the same recoil every bit a real gun, they do have some vibration and shock as they are used. Sorting by radiant intensity, my top option is an OSRAM SFH 4641-Z LED that should work quite nicely. Information technology'southward maybe a fiddling too power hungry, and 950nm rather than 940nm, just it nonetheless seems to be the best tradeoff given the options available.

Analog Devices take a nice tool to calculate the values for a transimpedance amplifier for a photodiode. It fifty-fifty has the backdrop for the photodiode I have selected in it's database, which is nice. Using this tool, I was able to design a two stage amplifier, which has significantly lower racket than a single phase amplifier, using SOT-23-5 sized op amps to save space. The tool very much preferred machine selecting dual and quad op amps, but those were just also big. My terminate design with the tool uses an Analog Devices AD8615U1 for the get-go phase, and an AD8061U2 2d stage. The resistor values will withal need some tuning one time I see what the actual current the photodiode generates within the device is, however, the tool does provide a very good starting indicate.

diodeamp Two stage amplifier designed with the assist of the photodiode tool from Analog Devices.

When searching for a surface mount S-R latch, there aren't a lot of options bachelor. In the smallest pin pitch/package, information technology comes down to the Texas Instruments CD4043BPWR or the same device in a NAND configuration. That's all Digi-Primal has for me, so that'due south what I will go with. Luckily, it has 4 independent latches built in, and so I only need i of these real-estate hungry devices.

The High Level BOM

Now that the high level components for the design take been selected, I have the following parts:

48x Everlight ELUA3535OG5-P0010U23240500-VD1M UV LEDs.
4x Texas Instruments TPS92691 LED drivers.
1x NXP LPC11U12FHN33/201 microcontroller.
2x Everlight PD15-22B/TR8 photodiode.
2x OSRAM SFH 4641-Z IR LEDs.
1x Texas Instruments CD4043BPWR quad S-R latch.
2x Analog Devices AD8615U1.
2x Analog Devices AD8061U2.

Next Time

The process of selecting parts to see requirements is disquisitional to a project'due south success. Even on a very simple projection such as this, at that place are many applied science decisions to be fabricated earlier even starting on a schematic. If this was a project I was taking to a customer or direction, I could provide them with a very broad idea of costs at this point, knowing that I'd have some miscellaneous IC's such as depression electric current linear voltage regulators, perhaps a clock crystal for the microcontroller, connectors for the battery and USB, every bit well every bit all the passives, and ESD and EMI filtering components. From by feel, I take a ballpark figure for how much those extras are going to cost me, and specifically for this design at the low quantity I volition be producing, it'south probably going to be $v-$7 and a set of PCBs from asia will be about $xxx-$forty delivered. The master cost in this project is the stupid, over-the-top number of high wattage UV LEDs, only thats half the fun of maker/hobby projects!

Continue to Office two and Office 3 of this project series.

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