Workshop

Bench worksheet · original sensor LED route

Measure the LED circuit

Record two meter readings, then derive current and resistor power without placing an ammeter across the circuit. This worksheet stores drafts only in this browser and sends no measurements anywhere.

01

Set up the meter safely

Probe the resistor, not the supply

LED measurement schematicNode A at Pico GP2 physical pin 4 connects through a 1 kiloohm resistor to node B at the LED anode. The LED cathode connects to node C at ground physical pin 3.A · GP2 · physical 41 kΩ resistorB · LED anodeLEDC · GND · physical 3

A · GP2 physical 4 → 1 kΩ resistor → B · LED anode → LED cathode → C · GND physical 3

A and B are the opposite ends of the resistor; B is also the LED anode connection. This is an electrical schematic, not a physical breadboard layout.
  1. 1

    Measure the isolated resistor first

    Unplug USB. Set the meter to resistance (Ω), then measure R before installing it. If it is already installed, lift one resistor end so it is isolated. Resistance and continuity modes must never be used on a powered circuit.

  2. 2

    Inspect the intact circuit

    Reconnect the resistor and inspect the original Pico / LED 297 / resistor 4294 route against the sensor LED build guide. Do not measure voltage until the intended circuit is intact.

  3. 3

    Measure voltage across the resistor

    Use high-impedance V/DC mode. Put the black lead in COM and the red lead in the V socket—not a current jack. With red on A and black on B, read the voltage across the resistor, not across GPIO and GND.

  4. 4

    Move clips only while unpowered

    When possible, place clips with USB unplugged, then connect USB for the voltage reading so a probe cannot bridge adjacent pins. Remove USB again before moving any wire or clip.

Do not place an ammeter across the circuit.This worksheet derives current from the measured resistor voltage and resistance.
02

Enter what you observed

Create the measurement record

Your first edit will save this draft in this browser.

No completed measurement record

This printout contains instructions and nominal reference assumptions only. No valid user observation was calculated.

Estimated reference only

Nominal assumptions

3.3 V output2.0 V LED drop1 kΩ resistor1.3 mA estimated current1.69 mW estimated resistor power

These assumptions stay separate from your observations and never replace them. A published LED forward voltage is commonly specified at 20 mA, not at this estimated low current, so 2.0 V is only a reference assumption here.