Looking at some of my old schematics, I can see that I haven't been entirely consistent when naming components. Nowhere near consistent, actually.
Sometimes I have used "T" as prefix for transistors and sometimes "Q". And especially IC's that are prefixed either with "U" or "IC" – often I have used both in a single schematic. And also connectors that are named "CON" or "J" or "JP". Sloppy…
Well, no more! This page on Wikipedia has a nice table with designators for various component types (even though it does not comply with with IEEE- or ANSI-whatever standard).
So this is what I will use henceforth:
| Designator | Component type |
|---|---|
| R | Resistors |
| C | Capacitors |
| L | Inductors, coils |
| D | Diodes – all types including Zener, Schottky and LEDs |
| U | IC's of all kinds. Technically it is used for "Inseparable Assemblies". |
| J | Connectors. The more fixed part of a connector pair (often the part soldered to the PCB). |
| P | Plugs. The less fixed part of a connector pair (i.e. the one on the wire). |
| K | Relays |
| M | Motors |
| Q | Transistors – all types including MOSFETs, IGBTs, BJTs etc. |
| S | Switches |
| Y | Crystals and oscillators |
| X | Transducers/sensors (no, X is not for crystals anymore). |
| FB | Ferrite beads |
| BT | Batteries |
| JP | Jumpers. Not connectors but only actual jumpers that are linked or unlinked. |
| TP | Test point (pad or pin for measuring) |
| MP | Mechanical parts (standoffs, screws, mounting hardware etc.) |