Reed receiver

The decoder of the reed receiver is based on the 'resonant reed' unit. This comprises a number of vibrating metal reeds, each one having a tuned vibration frequency like a tuning fork. These reeds are manufactured from a single tapered sheet of iron or steel, giving a comb of reeds of varying length. This resembles the comb used to sound musical notes in a music box. Like a music box, the length of each reed affects its resonant frequency. The reeds are powered magnetically, by a single solenoid coil and an iron core wrapped between the ends of the reeds.[1]

A reed's resonant frequency is a mid-range audible frequency of perhaps 300 Hz.[1] The solenoid is driven by the output of the radio control receiver,[note 1] which is an audio tone or tones. If the receiver output contains the appropriate tone for the resonant frequency of a reed, that reed would be made to vibrate. As the reed vibrates, it touches a contact screw above its free end. These contacts form the output of the decoder. Decoder outputs are generally fed to small relays. These allow a high current load to be controlled, such as the model's propulsion motor. Using a relay also adds a damping time constant to the output, so that the intermittent contact with the reed contact (which is vibrating at the transmitter audible tone frequency) becomes a continuous output signal.

Each reed forms an independent channel and they may be activated individually or in combination, depending on the signal from the transmitter.

Reed system channels are an on/off output, not a proportional (i.e. analogue) signal.[note 2] These could be used to drive an escapement, or rapidly switching a channel on and off could be used as pulse-width modulation to provide a proportional signal to drive a servo.

To avoid potential problems with harmonic frequencies simultaneously activating multiple reeds, the reed frequencies were kept within an octave of each other.[note 3] The number of distinct frequencies usable within this range depends on the selectivity or Q factor of each reed. Typical radio control reed units used six reeds, sometimes four or eight on simpler or more sophisticated systems.[1]

The sensitivity of each reed is controlled by mechanically adjusting the contact screw above each reed.[1] This adjustment is critical and temperamental, so a system where reed resonance is pronounced and separate from the other reeds is easiest to adjust. If adjacent reeds also vibrate (at a lesser amplitude) for the same tone, the contact adjustment must not be too sensitive, or else it could be false-triggered by an adjacent channel. This problem becomes worse, the more closely the channels are spaced.

Twelve reed systems were known, but were only required for large ship models, typically warships, with many channels for triggering "working features" such as turrets and cannon firing. In practice these were unreliable and so these models used a sequential drum sequencer instead. One channel, probably from a reed, would be used to step the sequencer through each step of a pre-planned demonstration sequence.

A suitable transmitter need only generate a number of audio tones. Most had a single oscillator, that generated different tones as control buttons were pressed one-by-one. As the control actuators on the model were usually escapements at this time, this limitation was relatively minor.[note 4] To keep the channels fully independent and simultaneously triggerable, would have required a separate oscillator for each channel, not merely a single tunable oscillator. In the valve era before transistors, that would have been unusually expensive. Many period transmitters merely used a number of push-button switches on their case, although some combined these into joystick or wheel controls.[3]

• Vibration galvanometer