Still in the kit — active and available
ROHS3 compliant, no lead-free transition worries.
This is a fixed 5 V shunt reference from Texas Instruments in the LM4040 family. Shunt topology means it behaves like a precision Zener — you set the bias current with a resistor from the supply, and the reference sinks whatever current it needs to hold 5 V across its terminals. Output is rated for up to 15 mA, cathode current is 95 µA typical, so the bias resistor calculation is straightforward. The ±0.5% initial tolerance is the grade for general-purpose precision — good enough for an 8-bit ADC reference or a comparator threshold, not the ±0.05% you'd need for a 16-bit SAR. Noise is 80 µVrms over 10 Hz to 10 kHz, which is clean enough for most control loops.
Temperature range and package — field-swap friendly
The SOT-23-3 package is small but hand-solderable with a fine tip — no hot-air station required. Pinout is standard for a three-terminal shunt: cathode, anode, and no-connect. Orientation is obvious from the package marking, so you can swap it on site without a microscope.