Telegraph Key

Part of Telegraph

The telegraph key and sounder are the physical interface of the telegraph system — the key converts finger movements into electrical pulses, and the sounder converts received pulses back into audible clicks. Building these devices from scrap metal is one of the most practical communication projects.

Why Build Your Own

In a rebuilding scenario, telegraph keys and sounders are among the first electrical devices you will need. They are mechanically simple, require no exotic materials, and enable long-distance communication the moment you have wire strung between two points. A skilled metalworker can build a complete key-and-sounder set in a single day from scrap metal, wood, and wire.

The telegraph key is also the input device for radio CW (continuous wave) communication, so building a good key now serves double duty as you progress to radio.

The Straight Key

The straight key is the standard telegraph sending device. It is a spring-loaded lever that closes an electrical circuit when pressed down.

Anatomy

Part	Function	Material
Base	Stable mounting platform	Hardwood, 120x60x20mm
Lever arm	The moving part you press	Brass or steel strip, 100x15x3mm
Knob	Finger contact point	Turned wood, hard rubber, or metal dome
Pivot	Fulcrum for lever movement	Bolt and nut, or pin through bracket
Contact points	Where the circuit closes	Silver, copper, or brass discs, 6-10mm
Spring	Returns lever to open position	Steel spring wire or flat spring
Adjusting screws	Set gap and spring tension	Machine screws with locknuts

Building Step by Step

1. The Base

1. Cut a hardwood block (oak, maple) approximately 120x60x20mm
2. Sand smooth and finish with oil or varnish
3. Attach rubber feet (leather pads work too) to prevent sliding during use
4. The base must be heavy enough that the key does not move during fast sending

2. The Lever Arm

1. Cut a brass or steel strip 100mm long, 15mm wide, 3mm thick
2. Drill a pivot hole 25mm from the back end
3. Drill a hole at the front for the contact point
4. File all edges smooth — you will be touching this with your fingers for hours

3. The Pivot

1. Mount two small L-brackets on the base, 25mm from the back
2. Align the brackets so the lever arm fits between them
3. Insert a bolt or pin through both brackets and the lever arm hole
4. The lever should swing freely up and down with minimal lateral play

4. Contact Points

1. Mount the lower contact on the base directly below the lever’s front contact hole
2. Use flat-headed brass or copper screws for both contacts
3. Silver contacts resist oxidation best — salvage from old electrical switches if possible
4. Mount the lower contact on an adjustable screw so you can set the gap

Contact Gap Adjustment

Set the contact gap to 1-2mm for comfortable sending. Too wide a gap makes sending tiring (excessive finger travel). Too narrow a gap causes accidental closures. Start at 2mm and decrease as your technique improves.

5. The Return Spring

1. Attach a flat spring or coiled spring to the lever arm behind the pivot
2. The spring should lift the lever when you release the knob
3. Adjust spring tension with a screw — it should feel firm but not stiff
4. The spring must return the lever cleanly and quickly without bouncing

6. The Knob

1. Turn a wooden or hard rubber knob, approximately 20mm diameter, 15mm tall
2. Slightly concave on top for comfortable finger placement
3. Attach to the lever arm at the front end with a screw
4. The knob height should position your wrist naturally — adjust by shimming the base if needed

Wiring

1. Attach one wire terminal to the base contact (lower)
2. Attach the other wire terminal to the lever arm (connected to the upper contact through the metal lever)
3. Label terminals clearly
4. The key connects in series with the telegraph line, battery, and sounder

The Telegraph Sounder

The sounder converts received electrical pulses into audible clicks. An electromagnet pulls a metal armature down (click) when current flows, and a spring pushes it back up (clack) when current stops.

Components

Part	Function	Material
Base	Mounting platform	Hardwood block
Electromagnet	Converts current to magnetic force	Iron core + wound copper wire
Armature	Moving metal piece	Soft iron strip
Sounding bar	Amplifies the click sound	Steel or brass strip attached to armature
Anvil screws	Upper and lower stroke limits	Adjustable screws
Return spring	Pulls armature up when current stops	Flat spring or spring wire

Building the Electromagnet

1. Find or make two soft iron cores — bolts, large nails, or iron rods, 6-10mm diameter, 40-60mm long
2. Mount them vertically on an iron or steel base (yoke), spaced 15-20mm apart
3. Wind each core with 200-500 turns of 0.3-0.5mm insulated copper wire
4. Wind both coils in the same direction (or connect so their magnetic fields add, not cancel)
5. Connect the two coils in series
6. Total resistance should be 20-150 ohms depending on wire gauge and turns

Coil Direction

If the two coils are wound in opposite directions or connected backward, their magnetic fields cancel and the sounder produces no click. Test by connecting a battery briefly — the armature should snap down firmly. If it does not respond, reverse the connections on one coil.

Building the Armature Assembly

1. Cut a soft iron strip (armature) 50-60mm long, 10-15mm wide, 2-3mm thick
2. Mount it on a pivot or hinge above the electromagnet cores, with one end directly over the core tops
3. Attach a sounding bar (stiff metal strip) to amplify the clicking sound
4. Add adjustable upper and lower stop screws (anvils) to limit travel to 1-2mm
5. Attach a light spring to pull the armature away from the magnets when current stops

Tuning the Sounder

1. Lower anvil screw: Set so the armature rests 1-2mm above the electromagnet cores (determines the attract gap)
2. Upper anvil screw: Set so the armature hits it cleanly when the spring returns it (produces the “clack”)
3. Spring tension: Just enough to pull the armature off the magnets reliably. Too much spring force requires more current; too little makes the sounder sluggish

The sounder should produce two distinct sounds: a sharp “click” when current flows (armature pulled down) and a “clack” when current stops (armature released). With practice, an operator reads the time between clicks, not the individual sounds.

The Buzzer Alternative

For practice or short-range communication, a buzzer produces a continuous tone instead of clicks. It is easier for beginners to hear and distinguish dits from dahs.

Building a Buzzer

1. Build an electromagnet as for the sounder
2. Connect the armature as a self-interrupting switch: the armature itself breaks the circuit when it is pulled down
3. This creates rapid make-and-break oscillation (buzzing) as long as the key is pressed
4. Mount on a resonant wooden box to amplify the sound

Complete Telegraph Station Setup

Two-Station System

1. Station A: Key + sounder + battery
2. Station B: Key + sounder
3. Wire: Single conductor between stations (use earth return for the second conductor)
4. Ground: Good earth ground at each station

Wiring Diagram

Connect in one series loop: Battery positive → Station A key → wire to Station B → Station B sounder → Station B key → wire back to Station A (or ground return) → Station A sounder → Battery negative.

When either key is pressed, both sounders click simultaneously. Both operators hear all signals.

Earth Return

You only need ONE wire between stations, not two. Connect the return circuit to a metal rod driven 1-2 meters into moist earth at each station. The earth itself conducts the return current. This halves the wire needed for the telegraph line.

Key Maintenance

1. Clean contact points regularly with fine sandpaper or a contact file
2. Apply a tiny amount of light oil to the pivot — do not over-oil
3. Tighten all screws and locknuts periodically
4. Replace worn contacts (look for pitting or erosion)
5. Adjust spring tension as springs age and weaken

Common Mistakes

1. Contacts too dirty: Oxidized or tarnished contacts increase resistance, weakening the signal. Clean contacts produce a clear, loud click on the sounder.
2. Gap set too wide: Excessive key travel (more than 2mm) causes fatigue and slow sending. Set the gap to 1-1.5mm for comfortable fast sending.
3. Wrong coil winding direction: Sounder coils wound in opposite directions cancel each other’s magnetic field. Test with a battery — if the armature does not respond, reverse one coil.
4. Inadequate base weight: A lightweight key slides around the table during use. Bolt it down, add weight to the base, or use a non-slip pad.
5. Spring too stiff on sounder: A strong spring requires more current to operate, reducing the effective range of the telegraph line. Use the minimum spring force that reliably returns the armature.

Summary

Telegraph Key -- At a Glance

• The straight key is a spring-loaded lever closing a circuit: pivot, contacts, spring, knob on a heavy wooden base
• Contact gap of 1-2mm and moderate spring tension produce comfortable, reliable sending
• The sounder uses an electromagnet to pull an iron armature down (click) and a spring to release it (clack)
• Wind sounder coils in the same direction, 200-500 turns each on soft iron cores
• A complete two-station telegraph needs: 2 keys, 2 sounders, 1 battery, 1 wire, and earth ground at each end
• Use earth return to halve the wire needed — drive a metal rod into moist soil at each station