Switching & protectionBench electronicsUnplug and unscrew

Optical interrupter

Also called opto flag, photo interrupter, TCST, endstop optical.

An LED and a phototransistor facing each other across a slot. Printers and scanners use them as home sensors. More repeatable than a mechanical switch, and already mounted in a little plastic flag.

If you are new to this category

A switch opens or closes a circuit. Some switches are for your finger. Some open when they get too hot and stay open. Those do not reset. A relay is a switch that a small current turns on so it can control a larger current. Use the rating printed on the part.

Click it and listen. Then use a meter on continuity, which beeps when the contacts touch. Do not guess that a small switch can run a motor.

The three legs of a snap switch

Common is the moving contact. NO means normally open: that leg connects only while you press. NC means normally closed: that leg is connected until you press. Click the button while the meter is on continuity and write down which beep matches which leg.

Specification. The right-hand column is what is typical. The label on the part in your hand wins if they disagree.
What to look for	Usual figure
Pins	Usually 4: LED anode/cathode, phototransistor collector/emitter. Sometimes 3 with a common.
LED	Infrared, about 1.2 V. A series resistor is mandatory. 5 V through 220–470 Ω is a typical start.
Output	Phototransistor to ground with a pull-up, or already that way on the donor board

Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.

What people actually use it for

Home switches
Encoder index
Drop-in endstop if you keep the cable

Leave this one when

The slot is full of toner or the plastic is cracked. Clean IR windows; do not scratch them.

Buying a new one is the better idea when

A TCST2103 or similar when you need a known pinout in a PCB footprint.

How to tell if this one still works

Diode-test the LED side. Then power LED and watch the transistor side change when a card enters the slot.

Tools you will want: Meter, 5 V and a resistor.

Words on this page

Short meanings, so the rest of the entry is readable if you are not already doing this every weekend.

series: Connected in a chain. Cells in series add their voltages. LEDs in series all go dark if one of them opens.

Where people actually find it

2 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.

Harvest at a glance
From	Call	How many	How often	Where it sits
Inkjet printer	Worth it	2–4	usually	Carriage home and paper path.
CD / DVD / Blu-ray drive	Bonus	1–2	usually	Tray position.

Worth pullingInkjet printer2–4 · usually
In this particular machine
Flag sensors.
Where to look
Carriage home and paper path.
How to get it out
Keep the little boards.
The condition to expect
Dusty but fine.
Bonus, if you are already in thereCD / DVD / Blu-ray drive1–2 · usually
In this particular machine
Flags the tray limits.
Where to look
Tray position.
How to get it out
Desolder with the connector.
The condition to expect
Good if not full of dust.