Minecraft wiki entry for Telegraph, associated text: 1 How they work
2 Telegraph components
2.1 Sending devices
2.1.1 Lever
2.1.2 Button
2.1.3 Pressure plate
2.2 Receivers
2.2.1 Redstone torch
2.2.2 Redstone lamp
2.2.3 Redstone repeater
2.2.4 Pistons
2.2.5 Note block
2.3 Message displays
2.3.1 Display loops and erasing devices
2.3.2 Erasers
2.3.3 NOR gate
3 Types of telegraphs
3.1 One-way telegraph
3.2 Classic telegraph
3.3 Multi-directional telegraph
4 Simple multi-directional Morse telegraph
4.1 Single direction
4.2 Multi-directional
5 Alternative designs
5.1 Frequency matching
5.2 Analog telegraph
6 Redstone clock telegraph
6.1 Steps
6.2 Pros and cons
6.3 Tips
7 Bugs