What is on it

The two black towers

The towers are one part, a GK105A slotted optical switch: 12.9 × 6.3 mm and 11 mm tall, with a slot 5 mm wide and 8 mm deep between them. The right-hand tower holds an infrared LED. The left-hand one holds a phototransistor, a light sensor that passes current while light reaches it. They face each other through small windows across the slot.

Pick each part. Below the slot sensor:

• U5, an LM393 comparator. It compares the light sensor's voltage with a fixed one and answers on SIGNAL. It holds two comparators; this board uses one.
• R12 and R13, 10 kΩ each, make that fixed voltage: half of VCC.
• R10, 1 kΩ, feeds the infrared LED. R11, 10 kΩ, is the light sensor's load, and it also sets how fast the block can count.
• R7, 10 kΩ, pulls SIGNAL up to VCC. R9, 1 kΩ, and the red LED run from VCC onto SIGNAL, so the LED lights while SIGNAL is LOW.
• C3, 100 nF, steadies the supply at the comparator.

Never off

The infrared LED is on whenever the block has power, blocked or not: about 2.2 mA from 3.3 V and 3.9 mA from 5 V. With the comparator, the divider and, while the slot is clear, the red LED and the pull-ups, the whole block takes about 3 to 5 mA from 3.3 V and 5 to 8 mA from 5 V. That is worked out from the parts' typical voltages, not measured, and it matters only on a battery.

The part is marked GK105A; the BOM orders it from Everlight, whose own sheet for it could not be found. This book quotes SIVAGO's GK105A sheet, a part of the same name and package, and says so where a number depends on it.