Grow light

Grow-light

USB-C PD grow light in two assemblies: an FR-4 controller (MAIN_BOARD/) and a one-layer aluminum LED panel (LED_BOARD/), joined by an 8-wire 2×4 IDC harness. Schematic connectivity and the as-drawn BOM come from KiCad (MAIN_BOARD/production/, LED_BOARD/production/). This file records why parts and numbers are what they are.

Architecture

USB-C is the only power inlet. An HUSB238 PD sink negotiates the contract over I2C. An STM8S003F3 requests 9 V or 12 V (never 20 V) and PWM-dims four TPS61165 boost channels from PC6. Each channel drives one isolated LED string: four 4000 K whites plus one 660 nm red in series.

Boost, not buck: string forward voltage is ~15.8 V, so the converter must sit above that. TPS61165 VIN is 3–18 V recommended (20 V abs max). A 20 V PDO leaves no boost headroom and violates that VIN rating.

On/off and dimming are MCU functions. There is no panel switch or pot.

LED current and power

TPS61165 FB is 200 mV. String current is set only by the sense resistor:

I_LED = V_FB / R_sense
      = 0.200 V / 0.82 Ω
      ≈ 0.244 A

A 1 Ω 1206 Basic (C17928) is a same-footprint swap: 0.200 A. Dissipation in the sense resistor is I²R ≈ 49 mW at 0.244 A, so 1206 is thermal margin, not a requirement.

Nominal string voltage and power (generic 3 V whites / ~3 V red):

V_string ≈ 4 × 3.2 V + 3.0 V = 15.8 V
P_string ≈ 15.8 V × 0.244 A  ≈ 3.85 W
P_LEDs   ≈ 4 × 3.85 W        ≈ 15.4 W

XP-E2 / Oslon reds are ~2.0–2.1 V, which drops V_string a volt or so; the boost still works. Input current is about 1.7 A at 9 V or 1.3 A at 12 V plus converter and LDO overhead. Product class is still ~20 W at the USB-C port.

Open-string OVP on TPS61165 is ~38 V. Do not hot-plug the LED harness with drivers enabled.

Controller choices

HUSB238 R2 49.9 kΩ / R3 5.1 kΩ are the typical-app VSET/ISET pair. 5.1 kΩ ISET is about 1.5 A. 49.9 kΩ is not a datasheet VSET tap; firmware must request 9/12 V so the default contract cannot land on 20 V.

TPS61165 VIN ceramics are 25 V. That is correct for 9/12 V and is not a 20 V design.

JLCPCB assembly (controller)

Economic PCBA charges $3 per unique true Extended line. Preferred parts often show as “Extended” in the cart and do not get that fee.

No $3 (Preferred): D1–D4 B5819WS, D8 BZX84C10, D9 SMAJ15A, R2 49.9 kΩ, U2 STM8.

$3 each (true Extended): U1 HUSB238, TPS61165DRVR (one fee for four), L1–L4 (one fee), J2 USB-C, F1 PTC, U3 USBLC6, R4–R7 820 mΩ.

Leave J1, J3, J4 unselected and solder the TH headers yourself (avoids the hand-solder surcharge). The only cart swap that drops a $3 line without touching copper is R4–R7 → C17928 1 Ω.

LED panel

Intent is a 1-layer aluminum MCPCB: SMT on the copper face, no vias, no through-hole parts (leads can short to the aluminum). The saved KiCad board is already electrically single-layer (all F.Cu, no B.Cu copper, no vias) but still declares two copper layers. Set copper layers to 1 before Gerbers and order aluminum / 1 layer (typically 1.6 mm, 1 oz, white mask).

The checked-in LED_BOARD/production/grow-light-LEDS.zip is deliberately plotted as a front-copper-only fabrication package. Do not regenerate it with KiCad's automatic layer selection while the source board still declares two copper layers, or an empty B_Cu file will be added and the upload may be treated as a two-layer board.

Do not JLC-assemble this panel. Fab bare aluminum; populate by hand (preheat / hot plate — the core sinks iron heat).

Heatsink and camera mount

Selected heatsink: Advanced Thermal Solutions ATS-CPX070070025-136-C1-R0. It is a 70 × 70 × 25 mm blue-anodized, coarse-pitch X-cut aluminum heatsink. ATS specifies 2.17 °C/W at 100 LFM (~0.5 m/s) airflow; it does not publish a natural-convection value. The LED panel is expected to put about 10–12 W of heat into the MCPCB (15.4 W electrical LED input less emitted light). Thus this heatsink is suitable with intentional airflow, but is not a qualified passive full-brightness solution. For a passive revision, target a heatsink rated ≤2.0 °C/W natural convection (≤1.5 °C/W preferred for a warm grow enclosure).

The ATS part has a four-corner 63.0 × 63.0 mm mounting pattern. Direct MCPCB mounting uses four 3.2 mm NPTH clearance holes on that grid (centres 3.5 mm from the edges of a 70 mm square), M3 screws, and a thin layer of thermal grease/TIM against the flat heatsink base. These are mechanical holes, not component leads: keep front copper clear and insulate the LED-side screw heads from copper/exposed core with shoulder washers. The current board has no mounting holes; add them through the KiCad workflow before relying on this mounting method.

The 1/4-20 UNC camera interface belongs on a separate metal crossbar or saddle bolted to the ATS factory holes, not in the heatsink or MCPCB. Capture a steel 1/4-20 coupling nut in that crossbar for adequate thread engagement; include an anti-rotation feature and a separate safety tether for overhead use.

This layout: 16× 3030 2-pad whites (asymmetric A/K) and 4× 3535-3P reds (A/K + isolated thermal; pad 3 is unnetted). A 3030 outline is not a 3030 footprint.

Whites

Drive ~0.244 A needs a 3 V, ~350–400 mA 3030. Not mid-power 65 mA parts.

Cree JK3030 3 V P class is EOL (PCN-06128, last-time buy 2026-09-04). Remaining Digi-Key/Mouser reels are leftover. Cree’s named substitute is JB3030SWT-J-H40EA0000-NZ000001 (JB3030S 3 V J class): 65 mA test / 480 mA max, 757 / Samsung-style pads, not a 1:1 clone of JK3030 P.

Do not buy 6 V Cree …EB… 3030s (Vf ~6.1 V; four of those plus a red blow the string budget and the 244 mA drive). Do not use Samsung LM301H / EVO (EOL, and typically max ~200 mA).

Reds

Samsung LH351H 660 nm is EOL (same lighting-LED shutdown: last ship 2025-12-31).

Next revision — one footprint

To actually share lands, put the whole panel on Oslon Square (ceramic 3-pad 3.0 mm): red GH CSSRM6.24 + white GW CSSRM4.HW. Same stencil, same pad. Those whites are ~700 mA class and ~$1.50; at 0.244 A they run cool but you pay 1 W prices. Do not mix Oslon Square pads with Cree/Everlight 2-pad 3030 whites.

Firmware notes

• Remap STM8 OPT2 AFR0 so TIM1_CH1 appears on PC6. Default pinout does not.
• Hold CTRL low through reset and PD negotiation. Read the contract before enabling PWM.
• ~20 kHz PWM is a reasonable starting point; confirm startup does not enter TPS61165 EasyScale.
• Four channels at once, on the intended heatsink, after thermal equilibrium.