How Do You Identify an Unmarked Switch?

You identify an unmarked tact or micro switch by its physical specifications, not by a part number, because most board-mount switches carry no marking on the body, and the code silkscreened on the PCB next to it (such as S15 or SW1) is a board reference designator, not the part number. The identifying data is the body size, the terminal layout and geometry, the mounting type, the function, and the travel, all read off the part and its footprint. Once those are captured, the practical goal is to match a switch of the same footprint and function rather than to recover the exact original, which is often proprietary, discontinued, or made without a visible brand. In other words, identifying an unmarked switch is a measurement task that ends in a footprint-compatible replacement, not a search for a hidden serial number.

Most people start by typing the only text they can see, the “S15” on the board, into a search box, and get nothing, because that is a position label, not a part. The originals usually have no number at all, and the equipment is often old enough that the exact part is gone. That sounds like a dead end, but it is not: the engineers who answer “identify this switch” threads all ask for the same short list of measurements, because that list is enough to find a part that fits. What follows is that list, what changes between a tact switch and a micro switch, and the two or three details that quietly break a match.

Why there is usually no part number to look up

Tact and micro switches are commodity electromechanical parts, and unlike an IC, they are almost never marked with a readable part number on the body. The black 6 mm tact switch on a board and the snap-action micro switch on a limit arm look generic on purpose, because dozens of makers build to the same footprints. What you can see, a code on the silkscreen like SW1, S15, or K2, is a reference designator the board designer assigned to that position, and it tells you nothing about the part except where it sits in the schematic.

That is why “read the marking” advice written for SMD chips does not transfer. A magnifier or digital microscope helps you read a tiny logo or a date code if one exists, and a multimeter or LCR meter confirms how the contacts behave, but for a switch the identity lives in its dimensions and geometry. You are reverse-engineering a footprint, not decoding a number.

What to measure: the identification checklist

Capture these before you search or ask anyone, because they are exactly what a component engineer will ask for, and a clear photo plus these figures usually settles it:

• Body size: length, width, and height in millimetres. For tact switches this often lands on a standard class such as 4.5 x 4.5, 6 x 6, or 12 x 12 mm; the height includes the actuator at rest.
• Terminal or pin layout and pitch: how many leads, where they sit, and the spacing (for example, a 2-pin or 4-pin tact on a 2.54 mm grid, or a 3-terminal micro switch).
• Terminal geometry: gull-wing, J-bend, straight through-hole, right-angle, a 45-degree bent plate, a quick-connect tab (1/4” or 3/16”), or a solder lug. This is what decides whether the part drops onto the existing pads.
• Mounting type: through-hole, surface-mount, panel-mount, or edge or side-mount.
• Function: momentary (returns when released) or maintained (latches), and the contact form, NO, NC, or changeover. A multimeter on the contacts confirms this.
• Travel and actuator: how far the button or lever moves, and the actuator type and length (pin plunger, lever, roller, or a side-access button).

Photograph the part next to a ruler or against calipers, top and side, so the proportions are unambiguous. With this set in hand, you can filter a distributor’s catalogue by outline and mounting and compare pictures, which is how most identifications actually get made.

A worked example: an unmarked 12 x 12 mm, 2-pin, through-hole momentary tact switch is already enough to act on. Filter a catalogue to a 12 x 12 mm outline, through-hole mounting, and a 2-pin layout, compare the pictures, then confirm the actuator height and travel against your board. The same path narrows a micro switch by body size, terminal type, and actuator.

Tact switch or micro switch: what changes in the ID

The checklist is the same, but the deciding fields differ by switch family. For a tact switch (the light-touch button on a PCB), the size class, the through-hole-versus-SMD choice, the terminal style, and the actuator height carry most of the weight, and the click feel maps to an operating force that commonly runs from about 100 to 320 gf, with lighter and firmer options on either side. For a micro switch (the snap-action basic switch on a lever or limit arm), the terminal type matters most, solder lug versus 1/4” or 3/16” quick-connect, along with the actuator (plunger, straight lever, or roller lever) and the contact configuration, since a micro switch is usually SPDT with NO, NC, and COM terminals.

Naming gets blurry in these threads, which is worth knowing: “micro switch,” “basic switch,” and “snap-action switch” usually mean the same component, while a “limit switch” is that component in a rugged housing. If your part is a small light-touch button, you are identifying a tact switch; if it is a lever or plunger that clicks, you are identifying a snap-action micro switch, and the terminal and actuator fields are where the match is won or lost.

One scope note: this is about board-mount tact and micro switches, not the mechanical keyboard switches that share the word. Those you identify by the stem color and the brand engraved on the switch housing, which is a different method entirely.

The details that quietly break a match

A switch can be the right size and still be the wrong part, and a few details cause most of the failed matches:

Terminal geometry is the first. Two tact switches can share a 4.3 x 3 mm body and differ in whether the leads are flat or bent up at 45 degrees, and the bent version will not sit on flat pads. Actuator length and direction is the second, and it matters most on side or edge-mount switches, where the button has to reach the housing wall, so a part that is otherwise identical fails if the actuator is too short. Function is the third: a momentary part will not stand in for a maintained one, and a single-pole part will not replace a changeover. The last is ratings, which you cannot see: even a perfect mechanical match has to carry your voltage, current, and load type, so confirm the electrical rating on the candidate’s datasheet rather than assuming a lookalike is equivalent.

From identification to a footprint-compatible replacement

Once you have the measurements, identifying the switch and finding a replacement are the same step: you match a current catalogue part to the footprint, mounting, function, and rating you captured. For a one-off repair, that can be as rough as fitting a standard 6 mm or 4.5 mm tact switch with a long actuator and trimming it to length, which is the pragmatic fix shared on hobby forums. For a production fix or an EOL part across a fleet, you want a properly sourced match with a datasheet behind it, because the part has to install, reflow, and survive in volume, not just close the circuit once.

A manufacturer can shorten this when the original is unmarked. Swiclick, for example, builds tact switches across the KAN series and snap-action micro switches in the G series with its own tooling, so a part can be matched to the footprint and function you measured, or a variant tooled where the standard range does not fit. The match is footprint-compatible once the datasheets line up, not a blind drop-in, which is the honest version of “we can replace that.”

Getting an unmarked switch identified and sourced

When you hand the problem to a switch maker or a distributor, give them the same set: a clear photo top and side, the body dimensions, the pin layout and pitch, the terminal geometry, the mounting, the function, and the travel. For anything going into production, get a sample and verify the fit on your board before you commit, because a footprint that matches on paper still has to place and solder on your line. Anyone naming an exact replacement from a single blurry photo, with no dimensions and no ratings, is guessing from appearance.

Swiclick is one manufacturer that will work from a photo and a few measurements: send the part you are trying to identify, or the part number if it is a branded original, and an application engineer will match it to a KAN or G-series equivalent or tool a custom variant, with free samples in 4 days and datasheets available without a form. If your original is an Omron switch, the Omron-to-Swiclick cross-reference lists what crosses over and what does not; for a new design, start from the tact switch or micro switch range.

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Written by Eric Chen, Application Engineer, Swiclick.