Hat rpizero tx

STM32 Constant-Current LED Driver Board

A compact STM32-controlled LED driver with an adjustable linear current sink, high-side current sensing, USB-C, and expansion headers. The circuit can drive an external LED from a 5 V supply, with the current reference provided by the MCU's PA4 DAC output.

This document describes the checked-in KiCad schematic and PCB. Values are design values, not measured performance guarantees. Output current limits, modulation bandwidth, and thermal performance still require characterization.

Board specifications

The project filename does not establish Raspberry Pi HAT compatibility. The current PCB has no 40-pin Raspberry Pi header.

Power

USB1 VBUS and POWER2 pin 1 connect directly to the same +5 V net. U4 generates +3.3 V for the MCU and logic headers; U2 and U3 use +5 V. USB-C CC1 and CC2 each have a 5.1 kΩ pull-down resistor (R9 and R10).

The design has no separate power-source selection or isolation between USB VBUS and POWER2. Use one supply source at a time unless the external power arrangement explicitly handles the shared rail. The files do not establish an approved input-voltage range or total current budget.

LED current driver

The LED current path is:

+5 V → R8 (0.110 Ω) → LED1 pin 1 → external LED
     → LED1 pin 2 → Q1 collector/emitter → R7 → GND

U2 compares the R5 wiper voltage with Q1's emitter voltage and drives Q1's base. R5 is a 10 kΩ trimmer between DAC1/PA4 and ground. R7 is a 100 Ω trimmer used as a variable emitter resistor; its third terminal is unconnected. R6 is a 10 kΩ resistor between Q1 base and emitter.

While the feedback loop is regulating, the approximate relationships are:

V_reference = α × V_DAC1                  (0 ≤ α ≤ 1, set by R5)
I_emitter ≈ V_reference / R7_effective
I_LED ≈ I_emitter                        (neglecting base current and R6 current)
V_shunt = I_LED × 0.110 Ω
V_I_LED ≈ G_INA199A1 × V_shunt            (within the amplifier's operating limits)

These equations describe circuit operation, not a safe current range. R7 can approach zero resistance, so its marked 100 Ω value does not provide a fixed minimum current-limiting resistance. The usable current depends on trimmer settings, DAC voltage, LED forward voltage, transistor dissipation, component ratings, and available voltage headroom. The board does not include a switching boost stage.

U3 measures the voltage across R8. Its output is available at SW2 pin 4 (I_LED); the netlist does not connect this output directly to an MCU ADC input. Check the amplifier output range before connecting it to a 3.3 V input.

Connector pinouts

Pin numbers below follow the schematic symbols and PCB pad numbers. Use the footprint's pin-1 marker to determine physical orientation.

The pin headers use 2.54 mm pitch. POWER2 and LED1 use two-position terminal-block footprints with 2.54 mm pitch.

USB1 routes D− to MCU PA11 and D+ to PA12. USB functionality depends on firmware. The serial header exposes logic signals; no RS-232 or RS-485 transceiver is present.

Boot, reset, and programming

• SW1 selects the BOOT0 connection between +3.3 V and ground through R18 (10 kΩ). R1 adds a permanent 10 kΩ pull-down on BOOT0.
• SW3 pulls the MCU NRST pin to ground. R2 (10 kΩ) pulls that pin up to +3.3 V, and C9 (100 nF) connects it to ground.
• SW2 provides SWCLK and SWDIO through 75 Ω series resistors. It provides neither a ground pin nor a +3.3 V reference pin; obtain those from another header when connecting a debugger.
• Firmware must configure the DAC and peripherals before their intended behavior can be specified. No default LED current, USB protocol, serial baud rate, or modulation waveform is defined by these hardware files.

Design review notes

The following observations are present in both the exported schematic netlist and PCB pad-net assignments:

These are connectivity observations, not a complete electrical-rule, routing, or manufacturability review.

Initial setup and characterization

1. Review the connector orientation, LED polarity, and design notes above.
2. With power disconnected, set R5 toward its ground end and set R7 to a high effective resistance. Verify the settings with a meter.
3. Connect the external LED to LED1 and apply a current-limited nominal 5 V supply through one power input.
4. Verify +5 V and +3.3 V, then program the MCU using SWD with a shared ground and suitable voltage reference from another header.
5. Start with a low DAC1 reference. Measure LED current externally while adjusting the reference and trimmers; monitor transistor and trimmer temperature.
6. Record the usable current range, thermal limits, and current-sense calibration before operating at the intended load or modulation rate.

Project files and manufacturing

Open the .kicad_pro file in KiCad 9 or a compatible later version. Resolve any library paths needed on the local machine. Regenerate fabrication and assembly exports from the reviewed PCB before ordering: existing exports are snapshots and are not established here as matching the latest layout. The configured stackup is a CAD setting; confirm the actual fabrication stackup with the manufacturer.

Specifications still to establish