The emitter never rests
The emitter sits straight across VCC through 120 Ω, so it is lit whenever the block has power: about 17.5 mA from 3.3 V, about 31 mA from 5 V, both well under the TCRT5000's 60 mA maximum. The part with the least room is the resistor: at 5 V it turns about 117 mW into heat against a 125 mW rating.
One resistor sets it
The emitter is an LED, and like any LED its current is set by the resistor in series with it: what is left of VCC after the LED's own forward voltage, divided by 120 Ω. Vishay gives the forward voltage as 1.25 V typical at 60 mA, a little less at lower currents.
- From 3.3 V: (3.3 − 1.2) / 120 ≈ 17.5 mA.
- From 5 V: (5 − 1.25) / 120 ≈ 31 mA, about half the datasheet's absolute maximum forward current of 60 mA.
These are arithmetic, not measurements. The spread of real parts moves them a few milliamps: at 5 V anywhere from about 29 mA, with a forward voltage at the datasheet's 1.5 V maximum, to about 33 mA with the 5 % resistor at the low end of its tolerance.
Set VCC and the air temperature. Each bar is against its own limit.
The resistor has the least room
The 120 Ω is an 0805 from UNI-ROYAL's 0805W8 series, rated 125 mW. The power in it is its current squared times its resistance: about 37 mW at 3.3 V, comfortable, and about 117 mW at 5 V, 94 % of its rating. That is inside the rating, and it runs warm. What it costs is margin: a USB supply at the top of its allowed range, 5.25 V, takes it to about 133 mW, past the rating.
The emitter itself has plenty of room. At 5 V it dissipates about 39 mW, under its 100 mW limit at 25 °C. The datasheet lowers that limit as it gets warmer, to nothing at 100 °C; read as a straight line, 39 mW stays allowed up to about 71 °C, far hotter than a project on a desk or a robot on the floor.
17.5 mA is plenty
At 17.5 mA from 3.3 V the emitter runs at nearly twice the datasheet's own test current of 10 mA. In this book's model white paper 3 mm away pulls SIGNAL down to about 1.3 V and black tape leaves it near 3.2 V: a wide gap for a line follower or a counter. The next article picks VCC for your board.
When it does not work
Yes. At 5 V the 120 Ω resistor turns about 117 mW into heat, most of its 125 mW rating, so it runs warm; it is inside the rating. In a closed box, or if the warmth bothers you, an Uno can run the block from its 3.3V pin instead: the heat in the resistor drops to about a third, and the reading still tells white from black.
Not from the header: nothing on the board switches it, and VCC also feeds the pull-up SIGNAL needs. To save power, switch the whole block's VCC from your board, for example with a transistor or a spare power switch, and allow a millisecond or two before reading.
The block takes about 18 mA from 3V3 all the time, and about 32 mA from 5V, day and night, whether anything is in front of it or not. A 1000 mAh battery feeds that alone for two days at most. Switch its power, or sleep between readings with VCC cut.
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