USB laser switch

Laser switch circuit

A Waveshare RP2040-Zero running CircuitPython switches a small 5 V line laser on and off. The laser is powered from the board’s USB 5 V. An NPN transistor switches the laser’s ground connection, and the board drives the transistor from pin GP2.

Parts

Ref	Part	Notes
U1	Waveshare RP2040-Zero	Powered and controlled over USB-C
Q1	2N3904 or PN2222A NPN transistor, TO-92	Good to about 100 mA of laser current; see “Laser current” below. The carrier board expects E-B-C pin order
R1	1 kΩ, 1/4 W axial resistor	Base resistor, GP2 to Q1 base. Use 470 Ω if the laser draws 50–100 mA
R2	10 kΩ, 1/4 W axial resistor	Pull-down, Q1 base to GND. Keeps the laser off during boot and reset
L1	5 V line laser module	Must have its own driver board (see below)

Schematic

U1 5V ────────────────────── L1 +  (red)
                             L1 −  (black)
                                │
                                C
U1 GP2 ──[ R1 1kΩ ]──┬───── B  Q1 (NPN)
                     │          E
                  [R2 10kΩ]     │
                     │          │
U1 GND ──────────────┴──────────┘

Connections

From	To
U1 5V	L1 +
L1 −	Q1 collector
U1 GP2	R1
R1 (other end)	Q1 base, R2
R2 (other end)	GND
Q1 emitter	GND

Design notes

Low-side switching. The laser’s + wire stays on 5 V and the transistor connects its − wire to ground. With an NPN transistor the base only needs to be about 0.7 V above ground, which the 3.3 V GPIO provides easily.
Base current. With R1 = 1 kΩ: (3.3 V − 0.7 V) / 1 kΩ ≈ 2.6 mA, enough to fully switch on about 50 mA of laser current. R1 = 470 Ω gives about 5.5 mA, good to about 100 mA. Both are within what an RP2040 pin can supply.
Laser current. Measure or look up what the laser draws at 5 V.
- Up to about 50 mA: R1 = 1 kΩ.
- 50–100 mA: R1 = 470 Ω.
- More than 100 mA: replace Q1 with a logic-level N-channel MOSFET wired the same way: drain to L1 −, source to GND, gate through about 100 Ω to GP2, and 10 kΩ from gate to GND. Through-hole logic-level MOSFETs come in TO-220 (for example the IRLB8721), which needs a different carrier board and a bigger case.
Laser driver. Most small line-laser modules have a constant-current driver built in and run straight from 5 V. A bare laser diode, with no small circuit board on it, must not be connected this way. It needs a current-limiting driver.
Never drive the laser directly from GP2. An RP2040 pin can only supply a few milliamps at 3.3 V.
USB power budget. The laser’s current comes out of the USB port together with the board’s own (about 25 mA). A normal USB port supplies 500 mA, so this is not a concern for a small laser.
Pinout. Check the transistor’s datasheet for which leg is emitter, base and collector. 2N3904 and PN2222A in TO-92 are E-B-C (flat face toward you, legs down, left to right), which is what the carrier board expects. Some parts sold as “2N2222” in TO-92 are C-B-E.
Board and case. See mockup.md for the single-sided carrier board and the printed case.

Firmware interface

code/code.py reads commands from the USB serial console (115200 baud), one per line: on, off, toggle, status, help. The front NeoPixel shows the state: dim green means the laser is off, red means it is on.