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Choosing parts

On this page: The method in six steps · Cells · LEDs · Optics · Drivers · Microcontrollers · Chargers and protection · Sensors, switches and small parts · Housing and materials · Worked example · Cost · Search tools and AI assistants

This guide shows how to choose the parts of a flashlight so that every choice can be explained and checked: first a method that works for any part, then what matters for each part family, then a complete worked example (a 1200 lm 18650 everyday-carry light).

Numbers come from datasheets and measurements, with links. Prices and stock are from October 2026 and change quickly: check them again before you order. Part names are examples, not endorsements; the project has no commercial relationship with any maker or shop.

Contents: the method · cells · LEDs · optics · drivers · microcontrollers · chargers and protection · sensors, switches and small parts · housing · worked example · cost · search tools and AI assistants

The method in six steps

Step Question Output
1. Limits What must this part do, in numbers? Must-have limits from the requirements
2. Candidates Which parts could do it? A list of 3-8 candidates with datasheets
3. Screen Which candidates break a hard limit? 2-4 survivors
4. Score Which survivor is best for this design? A decision matrix
5. Measure Does the winner do what the datasheet says, in this design? Prototype measurements
6. Record Why was it chosen? A design-log record

1. Turn the requirements into part limits

Start from the requirement sheet (REQ-1, templates/requirements.md) and work out what each part must handle. Example: “1200 lm at 30 s from one 18650 cell” means an LED that emits clearly more than 1200 lm at a junction temperature of 85 °C (the optic loses some light), at no more than 80 % of its maximum current (LED-2); a driver that delivers that current over the cell’s voltage range; and a cell that supplies the resulting current at no more than 70 % of its rating (BAT-1).

Write the limits down as numbers: voltage range, current, power, size, temperature range, interface, price per unit at your quantity, and how the board will be assembled (DFM-3).

2. Find candidates

Look in several places and always end at the maker’s datasheet:

3. Screen with hard limits

Remove every candidate that fails a must-have limit, before you compare anything else:

4. Score the survivors

Use a weighted decision matrix (templates/decision-matrix.md): 4-7 criteria, weights that add up to 100 %, a 1-5 score with a written meaning for each score, the weighted total, and a sensitivity check (change the biggest weight by ±10 points: does the winner change?). If the winner changes, the choice is close; say so in the design log and decide with a measurement.

5. Measure

Datasheets give typical values under the maker’s conditions. Measure what matters in your design: LED current and forward voltage when hot, driver efficiency at your most-used levels, standby current, the temperature of the part in the housing. Order samples of the top two candidates when the matrix was close.

6. Record

Write a design-log record (REQ-3, templates/design-log.md): question, options, criteria, data with links and the date you read them, decision, reasons, risks, and how it will be verified.

Cells

What matters: capacity (mAh) and energy (Wh) for runtime; the maximum continuous current for turbo (BAT-1); size and terminal type (button top or flat top, protected or unprotected); the charge temperature window (CHG-3); the standard charge current (CHG-2); whether users can buy and replace it (BAT-2).

Cell Format Capacity Max continuous current Notes Source
Vapcell H10 14500 1000 mAh 10 A independent review lygte-info
Vapcell M11 V2 18350 1100 mAh 10 A retailer data retailer
Molicel P28A 18650 2800 mAh typ (2700 min) 35 A charge 0-60 °C, standard charge 2.8 A, cut-off 2.5 V datasheet
Samsung 30Q 18650 3000 mAh 15 A charge 0-50 °C, standard charge 1.5 A, cut-off 2.5 V datasheet
Murata VTC6 18650 3000 mAh 15 A retailer data retailer
Molicel P42A 21700 4200 mAh 45 A   datasheet
Samsung 40T 21700 4000 mAh min 35 A (45 A with an 80 °C cut-off)   datasheet

Rules of thumb. In the same size, high-current cells have less capacity than high-capacity cells; pick by what the light does most. An EDC light spends most of its life in low and middle modes, so capacity usually matters more than a current rating far above the turbo current. Small lights with a pouch cell (as the FlashKa up to v0.6, two pouch cells in parallel, about 800 mAh) need the cell’s own datasheet for current and charge limits, a protection circuit (BAT-3), and a plan for replacement (BAT-2).

Traps. Fake or relabelled cells (capacity claims far above the known cells of that size); protected cells that are longer than the tube was designed for; flat-top cells that do not reach the contact; datasheet cut-off voltages taken as normal use (set the low-voltage protection higher, BAT-5).

LEDs

What matters: light output at your current and at a hot junction (85 °C), luminance (for throw, LED-1), forward voltage (it decides the driver topology, DRV-1), maximum current (LED-2), colour temperature and CRI (LED-4), package and the available copper boards (LED-3).

LED Size / emitting area Max current Datasheet output (condition) Character Source
Luminus SST-20-W 2.0 mm² 3 A (CRI 95 version 2 A) min 530-585 lm at 1.5 A, 85 °C; Vf 3.0 V at 1.5 A small, efficient, throw and general use datasheet
Luminus SST-40-W 4.0 mm² 6 A min 594-673 lm at 1.5 A; 986-1269 lm at 3 A; 1446-1861 lm at 5 A; 1640-2110 lm at 6 A (85 °C) high output from one die datasheet
Luminus SFT-40-W   8 A min 610-680 lm at 1.5 A, 85 °C; Vf 2.8 V at 1.5 A “narrow beam angle, long beam distance” datasheet
Nichia 519A 3.5 × 3.5 mm package 2.2 A typ 730 lm at 1.4 A, 25 °C pulsed (5000 K, Ra 70); Vf 2.94 V Ra 70 / 80 / 90 (R9 ≥ 80), good colour datasheet
Samsung LH351D 3.5 × 3.5 mm package 3 A see datasheet CRI 70 / 80 / 90 maker datasheet
Cree XHP50.3 HI 5 × 5 mm package 3 A (6 V configuration) min 970-1120 lm at 6 V, 1.4 A, 85 °C; Vf 5.6 V flood or wide beam, 6 V or 12 V datasheet
Cree XP-L HI 3.45 mm package 3 A see datasheet throw maker datasheet
ams OSRAM KW CSLNM1.TG 0.96 × 0.96 mm² emitting area 3 A 280-450 lm at 1 A very high luminance, long throw maker datasheet

How to estimate output. Read the flux at your current from the datasheet curve, correct it to 85 °C if the datasheet gives 25 °C values, and subtract the optic and lens losses (measure, or use the optic maker’s efficiency). Compare LEDs only at the same conditions: a 25 °C pulsed value (Nichia’s table above) is not comparable with an 85 °C value (Luminus, Cree).

Traps. Choosing by the headline lumens at maximum current (the light cannot hold it, THM-2); a 6 V or 12 V LED with a 3 V driver; high-CRI versions with a lower maximum current; LEDs that only come on boards with a dielectric layer.

Optics

For a reflector or TIR optic filled by the source, peak intensity is at most about luminance × aperture area × optical efficiency. tools/lux_calc.py throw --lumens --die-mm2 --optic-mm computes this upper bound; real lights reach roughly 50-80 % of it (rule of thumb). A small, bright die in a wide optic throws furthest (LED-1).

Drivers

What matters: the topology (DRV-1), current regulation and its accuracy (DRV-2), efficiency at the levels used most and the quiescent and shutdown current (DRV-3, BAT-6), the lowest stable level (LED-7), size and heat.

Part Type Key data Typical use Source
AMC7135 linear current sink 2.7-6 V, 340-380 mA each, 120 mV dropout, SOT-89 several in parallel for budget regulated drivers maker datasheet
TLV62585 buck 2.5-5.5 V in, 3 A, 1.5 MHz, 35 µA quiescent, 100 % duty, SOT-563 or 2 × 2 mm QFN one Li-ion cell, white LED up to about 3 A (FlashKa) TI
TPS61088 boost 2.7-12 V, 10 A switch, Vout 4.5-12.6 V 6 V or 12 V LEDs from one or two cells maker datasheet
TPS61288 boost 15 A switch, 500 kHz higher-power boost maker datasheet
MP3429 boost 21.5 A current limit high-power lights (named in driver efficiency tests) BLF thread
TPS63020 buck-boost 1.8-5.5 V in, 2 A out at 3.3 V (4 A switch), 25 µA quiescent constant output over the whole discharge, moderate power TI
TPS63802 buck-boost 1.3-5.5 V in, 2 A out, 11 µA quiescent small lights with constant output maker datasheet

How to decide. Plot the cell voltage over the discharge (4.2 V to the low-voltage cut-off, under load) against the LED’s forward voltage at the planned current. If the LED voltage sits inside the cell’s range (one Li-ion cell and a white LED), a buck regulates for most of the discharge and runs at 100 % duty near the end, and a linear regulator is simpler but wastes (Vcell - Vf) × I. If the LED voltage is above the cell’s range (6 V LED, one cell), use a boost. For constant output to the very end at moderate power, use a buck-boost. A FET direct drive is the cheapest way to the highest peak, but its output falls with the cell voltage and the lowest modes depend on PWM.

Traps. An inductor whose saturation current is below the peak current; a buck that drops out of regulation earlier than expected because of resistance in springs and traces (BAT-8); an enable pin left on in standby (35 µA for the TLV62585 is more than the whole standby budget of an 800 mAh light); low-level instability.

Microcontrollers

What matters: supply voltage (can it run straight from a Li-ion cell, or does it need a regulator, which adds standby current?), sleep current (BAT-6), PWM resolution and frequency (LED-6, LED-7), ADC for voltage and temperature, programming interface (DRV-9), memory, wireless needs, price and stock, and firmware support.

MCU Core Flash / RAM Supply Sleep current Notes Price (LCSC, Oct 2026) LuxLibre
ATtiny1616 AVR, up to 20 MHz (at 4.5-5.5 V) 16 KB / 2 KB 1.8-5.5 V 0.1 µA typ power-down 10-bit ADC, UPDI, the MCU of many enthusiast drivers $2.92 (1 pc), out of stock port, builds
CH32V003 RISC-V RV32EC, 48 MHz 16 KB / 2 KB 2.7-5.5 V 8.76 µA standby (3.3 V, LSI off) 10-bit ADC, one-wire SWIO programming $0.25 (5 pcs), $0.13 (4000 pcs) port, builds
PY32F002A Cortex-M0+, 24 MHz 20 KB / 3 KB 1.7-5.5 V about 5 µA stop 12-bit ADC, SWD $0.41 (5 pcs), $0.20 (5000 pcs) not yet
ESP32-C3-MINI-1 RISC-V, 160 MHz, module 4 MB / 400 KB 3.0-3.6 V 5 µA deep sleep Wi-Fi and Bluetooth LE 5, USB serial/JTAG $1.29 (1 pc) port, experimental
nRF52832 Cortex-M4F, 64 MHz 512 KB / 64 KB 1.7-3.6 V 0.3 µA System OFF Bluetooth LE $4.03 (1 pc) not yet
STM32C011F6 Cortex-M0+, 48 MHz 32 KB / 6 KB 2.0-3.6 V see datasheet 12-bit ADC, SWD $1.49 (1 pc) not yet

Sources: ATtiny1616, CH32V003, PY32F002A, ESP32-C3-MINI-1; the others from their makers’ datasheets.

Rules of thumb. LuxLibre needs about 11 KB of flash and 0.5 KB of RAM today, so 16 KB parts fit with room for board features. An MCU that runs straight from the cell (ATtiny1616, CH32V003, PY32F002A) saves a regulator and its standby current. Choose a wireless MCU only when the requirements ask for an app, and budget its standby current honestly. The CH32V003 port keeps its low-speed oscillator on for the wake-up timer, so its real standby current is a little above the datasheet’s LSI-off figure: measure it.

Chargers and protection

What matters: charge current (CHG-2), heat (a linear charger burns (VBUS - Vcell) × I: at 5 V, 3.7 V and 1 A that is 1.3 W inside the light), a temperature input (CHG-3), input voltage rating and over-voltage protection (CHG-4), status outputs for the indicator, and power path (running the light while charging).

Part Type Key data
TP4056 linear 1 A, SOP-8
TP4057 linear 0.5 A, SOT-23-6, 6.5 V absolute maximum input
MCP73831 linear 0.5 A, SOT-23-5
IP2312 switching 3 A, 6.5 V absolute maximum input, ESOP-8
BQ25601 switching 3 A, NTC input
BQ24075 linear with power path 1.5 A, 28 V absolute maximum input

Protection for built-in cells (BAT-3): DW01A controller with an FS8205A dual MOSFET (27 mΩ at 4.5 V gate drive; typical DW01A thresholds 4.30 V over-charge, 2.40 V over-discharge), or an integrated part such as the XB5333A (4.30 V / 2.4 V, 3 A over-current, 58 mΩ, SOT-23-5). High-current lights need protection rated for the turbo current, or protected cells. Check all values in the datasheet of the exact part you buy.

Every USB-C receptacle that charges needs one 5.1 kΩ resistor per CC pin (CHG-1). Six-pin “power only” USB-C receptacles are easier to solder but carry no data lines, so they cannot be used for a USB console or firmware updates over USB.

Sensors, switches and small parts

Housing and materials

Worked example: a 1200 lm 18650 light

An everyday-carry light with one 18650 cell, at least 1200 lm at 30 s, a sustained output that is honest about heat, and USB-C charging. The numbers below come from the datasheets in this guide, tools/lux_calc.py and the LuxLibre simulator. Scores are illustrative: your weights may differ.

Step 1: requirements (summary)

ID Requirement Target
R1 Output at 30 s (FL 1) ≥ 1200 lm
R2 Surface temperature ≤ 48 °C continuous, ≤ 51 °C peak (THM-1)
R3 Cell one 18650, user-replaceable (BAT-2)
R4 Size about 25 mm × 120 mm, aluminium
R5 Charging USB-C in the light, full in under 3 h
R6 Water and impact IPX7, 1 m drop
R7 Colour 4000-5000 K, CRI ≥ 70 (90 preferred)
R8 Standby ≤ 3 % per month (BAT-6)

Step 2: check the physics first

The heat budget does not depend on the parts. A 25 × 120 mm anodised cylinder has about 104 cm² of surface:

$ python3 tools/lux_calc.py thermal --diameter-mm 25 --length-mm 120 --mass-g 80 --heat-w 14

It can lose about 2.6 W continuously at a 23 K rise (THM-2). In turbo the light makes roughly 12.5-14 W of heat: about 17.5 W input with a 90 % switching driver, or about 19 W with a linear regulator on a full cell (4.0 V × 4.8 A), minus about 5 W that leaves as light. An 80 g light then reaches the 48 °C surface limit after about 130-150 s in the lumped model (THM-3). So: 1200 lm at 30 s is realistic, 1200 lm for ten minutes is not, and the sustained output will be a few hundred lumens. Writing this down now prevents a specification the light cannot meet.

Step 3: LED

Limits: clearly more than 1200 lm emitted at 85 °C (the optic loses part of it), at most 80 % of the maximum current.

Candidate Data Screen
SST-40 (CRI 70) 1446-1861 lm at 5 A, 85 °C; max 6 A, so 4.8 A allowed passes
XHP50.3 HI (CRI 70-95) 970-1120 lm at 6 V, 1.4 A, 85 °C; max 3 A passes with more current; needs a 6 V driver
3 × Nichia 519A (Ra 90) in a triple optic 730 lm typ each at 1.4 A, 25 °C pulsed passes on paper; check the 85 °C curve; three LEDs on one board
SST-20 min 530-585 lm at 1.5 A, 85 °C; max 3 A fails: one LED cannot reach 1200 lm within 80 % of 3 A

Scoring (illustrative): the SST-40 wins on simplicity and cost with a 3 V driver; the triple 519A wins on colour; the XHP50.3 HI wins if a boost driver is planned anyway. This example takes the SST-40 at up to 4.8 A and notes the 519A triple as the high-CRI variant.

Step 4: cell

Turbo cell current depends on the driver (step 5). With a switching driver at 90 % efficiency, 4.8 A at about 3.3 V is 15.7 W at the LED and 17.5 W from the cell; with 50 mΩ of cell resistance that is 5.24 A at 3.34 V under load:

$ python3 tools/lux_calc.py cellcurrent --led-a 4.8 --vf0 2.8 --rs 0.10 --driver-eff 0.9 --cell-v 3.6 --r-int 0.05 --cell-max-a 15

With a linear regulator the cell current simply equals the LED current, 4.8 A. The table uses the larger value:

Candidate Capacity 70 % of rating Margin at 5.2 A Charge window
Samsung 30Q 3000 mAh 10.5 A good 0-50 °C
Molicel P28A 2800 mAh 24.5 A very large 0-60 °C
Murata VTC6 3000 mAh 10.5 A good retailer data only

Both datasheet cells pass. The 30Q gives more runtime, the P28A more margin; users will fit other cells anyway, so the firmware limit is set for a 15 A cell: BAT_MAX_MA = 10000.

Step 5: driver

The LED needs about 3.3 V at 4.8 A; the cell gives 4.2 V down to about 3.0 V under load. The LED voltage sits inside the cell’s range, so:

Option Regulation over the discharge Efficiency Cost and size Low modes
FET direct drive none: output follows the cell high in turbo lowest PWM only
Linear regulator (MOSFET + op-amp + sense resistor) while Vcell > Vf + dropout Vf / Vcell (about 80 %) low good with current control
Buck, 5-6 A class until Vcell nears Vf, then 100 % duty about 90 % medium; few small parts at this current good
Boost + 6 V LED (XHP50.3) to the end of the discharge about 90 % medium; larger inductor good

This example takes the linear current regulator: simple, cheap, regulated for most of the discharge, with stable low modes. Its price is heat: on a full cell it burns (4.0 V - 3.3 V) × 4.8 A ≈ 3.4 W in the MOSFET, which shortens turbo (step 2) but is handled by the thermal regulator. The decision matrix example in templates/decision-matrix.md works through the same question and shows that the result is sensitive to the weights, which is a reason to measure a prototype.

Step 6: MCU, charger and the rest

Step 7: board header and simulation

The choices become a LuxLibre board header (firmware/porting.md); an excerpt:

#define LX_BOARD_NAME "Example 18650 EDC"
#define LX_OUT_MIN_Q16 64            // level 1 = 1/1024 of full scale (lowest stable output)
#define LX_MAIN_MAX_MA 4800          // level 150: 80 % of the SST-40's 6 A maximum (LED-2); set to the measured value
#define LX_LED_VF0_MV 2800           // LED model: V = 2.8 V + 0.10 V/A x I (measure!)
#define LX_LED_RS_MOHM 100
#define LX_DRIVER_EFF_PCT 78         // linear regulator: Vf at turbo / 4.2 V, so the turbo cell current is not underestimated
#define LX_DEF_RAMP_CEIL 130         // about 1.9 A
#define LX_DEF_TURBO_LVL 150         // 4.8 A
#define LX_DEF_BAT_MAX_MA 10000      // 70 % of a 15 A cell (BAT-1)
#define LX_DEF_THM_LIMIT 50          // replace with 48 + measured sensor-to-surface difference

The simulator’s 18650 model (an 80 g class aluminium light) with a 4.8 A LED predicts, for turbo without a timer:

$ cd firmware && make sim && build/luxsim --preset 18650 --imax 4.8 --set TURBO_S=0 "2C W600000"
simulated 602.7 s
peak output       1561 lm
output at 30 s    1509 lm   (ANSI FL 1 measures from 30 s)
output at 120 s   1158 lm
peak sensor       50.9 degC
final           level 122, 385 lm, 50.5 degC, 4.04 V, 90 %

This is a model, not a measurement: it shows that R1 is reachable and that the sustained output will be around 400 lm. The model assumes a 90 % efficient driver; the linear regulator’s extra heat on a full cell makes the real step-down come somewhat earlier. The prototype’s measurements (testing.md) replace these numbers in the specification.

Step 8: record and review

One design-log record per step, then the review checklist (templates/design-review-checklist.md) before the boards are ordered.

The second example: FlashKa

The FlashKa keychain light went through the same steps with very different limits: a 20 × 16 mm cross-section, two small pouch cells, a Luminus SST-20 at up to 2.2 A from a TLV62585 buck with a current loop, an ESP32-C3 for the Bluetooth app, and a Class 2 laser behind a hardware interlock. Its folder hardware/flashka-keychain-v05/ lists the parts, the reasons and the known limitations.

Cost: the BOM is not the price

For a single prototype, the parts are a small share of the money. One FlashKa v0.5 delivered to Lithuania cost $529.63; the electronic parts for two boards were $17.16 of that. Shipping, VAT, customs duty and fees were 41 %, and assembly fees (setup per side, loading fees per unique part, stencils, X-ray) were $127.15. Five lights cost about $196 each because these fixed costs are shared. DFM-1 to DFM-5 list what to change in the design to bring this down; the full breakdown is in the FlashKa folder.

Search tools and AI assistants

Search engines and AI assistants are fast at finding candidates and summarising datasheets, and they also make mistakes with confidence. Use them for step 2 (candidates), never for the numbers in step 3 onwards without checking the datasheet yourself.

A useful request names the limits and asks for sources:

I am designing a single-cell 18650 flashlight. I need a buck LED driver IC:
input 2.9-4.2 V, LED current up to 5 A, LED forward voltage about 3.3 V, 100 % duty cycle
allowed, package at least 0.5 mm pitch, available at LCSC. List 5 candidates in a table with
input range, maximum current, quiescent current, package and a link to the maker's datasheet
for each. Mark any value you are not sure of. Do not include parts without a datasheet.

Then open every datasheet and copy the numbers yourself. Record in the design log which numbers came from where (REQ-3).

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