Straight Key

Part of Telegraph

The straight key is the simplest Morse code keying device — a spring-loaded lever that closes an electrical circuit when pressed, producing the dots and dashes of telegraph communication.

Why This Matters

The straight key is to telegraph communication what the pen is to writing: the fundamental tool, unchanged in essential design since 1844, that connects human thought to the communication medium. Every advanced keying device — the semiautomatic bug, the electronic paddle keyer — is an elaboration of the straight key’s function. Understanding the straight key means understanding what all keys must do: produce electrical pulses timed precisely enough for the receiving operator to distinguish dots from dashes, and characters from spaces.

The straight key is also the most appropriate keying tool for many situations: emergency use (no electronics required), field deployment (robust, no batteries needed beyond the telegraph circuit itself), and training (the straight key’s demands on timing develop skills that transfer to all other keying methods). Professional telegraph operators who learned on straight keys typically have superior timing awareness compared to those who learned only with automatic keyers.

Building a straight key from basic materials is entirely within the capability of any community with metalworking skills. The straight key requires only a frame, a lever arm, a pivot, a spring, and two contact points. Its simplicity is its virtue.

Key Design and Components

The key frame: a flat base that holds all components, with a vertical support for the pivot at one end and the front contact assembly at the other. The frame should be heavy enough to stay put on the desk without clamping — at least 400–600 grams for a desk key. Cast iron is traditional; formed sheet steel or machined aluminum also work. The base should be wider than the lever arm and the operator’s hand to prevent tipping during vigorous operation.

The lever arm: extends from the pivot (back) to the knob (front). Traditional lever length: 80–100 mm from pivot to knob center. Shorter levers give less mechanical advantage and require more precise timing control; longer levers give more mechanical advantage but increase inertia. The lever should be stiff — it should not flex under keying force. Steel or brass, 4–6 mm thick in the vertical plane, 10–15 mm wide.

The pivot: a transverse pin or screw through the rear of the lever, with the frame supporting both ends of the pin. The pivot must be smooth-running (low friction) and rigid (no lateral play). A bronze bushing in the lever, running on a steel pin, is the classic solution. A binding screw allows pivot tension adjustment to add or remove friction.

The return spring: a compression spring (in a cup) or a flat spring (leaf spring) pushing the lever up to its rest position. Adjustable spring tension allows the key to be set for different operators’ preferences. Traditional adjustment range: approximately 30–200 grams of return force as measured at the knob.

The contact points: upper contact on the lever (below the knob area), lower contact on an adjustable post on the frame. Traditionally silver-nickel alloy or silver-copper — materials that resist arc erosion and oxidation. The gap between contacts in the open (rest) position should be adjustable: 1–3 mm is typical (0.5 mm for fast sending, 2–3 mm for beginners who need more visible gap confirmation).

Building a Straight Key

A basic key can be built from available materials in a few hours with a workbench and basic tools.

Frame: cut a base plate of 3–4 mm steel, approximately 100 mm × 60 mm. Bend a vertical support at the rear end (or bolt a separate piece). Drill and tap for the pivot screw, the front contact post, the spring cup, and the lead-in binding posts.

Lever: cut from 4 mm × 15 mm flat steel bar, 120 mm long. Drill the pivot hole at one end, 10 mm from the end. File the knob end smooth and rounded (to be comfortable under the fingertips). At the contact-side position (roughly 80 mm from pivot), drill and tap for the upper contact screw.

Pivot: a 3 mm diameter steel bolt through the lever pivot hole, held in the frame by two nuts locked against each other. The lever should swing freely but without lateral slop — adjust the pivot tension with the inner nut while locking with the outer.

Contacts: brass or steel screws with their flat heads facing each other provide minimal contacts adequate for demonstration. For production use, fit small silver solder balls (melt a silver solder ball on the contact screw end) for long-lived contact surfaces. The lower contact screw threads into the front post with a lock nut — adjust the gap, then lock.

Spring: a small compression spring in a cup on the rear of the frame, bearing upward against the lever from below. Alternatively, a strip of phosphor bronze, clamped at one end to the frame and bearing up against the lever at a point behind the contact region. Adjust spring tension by tightening or loosening the clamp position.

Binding posts: two small brass screws on the frame — one connects to the upper contact (through the lever, via a flexible wire to allow lever movement), and one connects to the lower contact post. These are the circuit connections: one wire to the line or equipment, one wire to battery or return.

Setting Up and Testing

After assembly: close the key by pressing the knob. The contacts should meet cleanly with a faint click and firm electrical contact (measure with a multimeter — contact resistance should be less than 1 ohm). Release: the lever should return to the rest position smartly, with contacts clearly separated.

Adjust the contact gap: with the lever at rest, use feeler gauges or a thin strip of cardboard of known thickness to set the gap at 1.0–1.5 mm for initial setup. Adjust to taste — experienced operators often prefer a larger gap for more physical feedback.

Adjust the spring tension: with the key connected to a tone oscillator, close and release the key repeatedly, listening for double contacts (a weak secondary contact following the main contact on either make or break). Double contacts indicate either spring tension too low (allowing mechanical bounce) or contact gap too small. Increase spring tension slightly or widen the gap slightly.

Adjust the contact angle: the lever should be approximately level at rest, with the contact surfaces meeting flatly (not edge to edge). If the contacts are at an angle, file the contact surfaces to parallel before proceeding.

Test for damping: if the lever bounces on quick key release (producing extra contacts), add a small felt or leather pad on the underside of the lever at the front end, bearing lightly against the frame. This damps the bounce without adding significant return force. A well-adjusted key produces clean, well-defined contacts at any speed up to its design maximum.

The Key as Practice Tool

For training new operators, the straight key and a code practice oscillator are the complete learning kit. The oscillator converts key closures into an audible tone — the operator hears their own code and learns to correct errors immediately through auditory feedback.

A simple code practice oscillator: a single transistor (NPN), emitter to ground, collector through a speaker and resistor to 9V battery, base driven by the key through a 10 kohm resistor. When the key closes, the transistor conducts, current flows through the speaker, and the magnetic field collapses and restores at the natural resonant frequency of the speaker cone — producing a tone. Add a small capacitor (0.1–1 μF) from collector to base to make the tone resonant and more pleasing.

Practice with the oscillator in a quiet room, listening critically to the produced code. Record sessions and compare to reference code. Quantify improvement in words per minute and accuracy rate. The improvement curve is steep early and levels off gradually — expect significant progress in the first month, slow steady improvement thereafter.

A skilled straight-key operator, trained and maintained, is a communication asset that remains relevant for as long as Morse code is used anywhere in the network.