|You might think that the earliest Radio Control systems used valve technology.
Well believe it or not, even valves have a predecessor - and this was long before the cat grew his whiskers. A type of communication by radio waves called a "Coherer" first appeared in 1890, invented by French scientist Édouard Branly. Developed further by Marconi, it became the standard for radio reception around 1900-1910 and appeared in Radio Controlled toys as late as the 1940s.
The Coherer made use of a powerful spark transmitter - remember how during a storm, a flash of lightening would cause a crackle on your domestic medium-wave radio?
This is the energy detected by the Coherer.
The Coherer receiver consists of a tube of iron filings, loosely packed between two electrode terminals within a glass tube.
The tube and its terminals would be connected in series with a relay or sounder or bulb, such that if the electrodes conducted through the filings, it could complete the circuit just like a switch. An antenna was connected to each terminal.
Just like erasing an 'Etch-a-Sketch' picture, when agitated, the filings would settle in the bottom of the glass tube away from the electrodes, and so no current could flow through the Coherer. When the sudden and powerful electromagnetic jolt of a huge spark was picked up by the two antennas, the iron filings would align between the electrodes, providing a conductive path and actuating the bell, buzzer, bulb or relay.
Here is a real antique, a radio-controlled model boat based on a Spark-gap transmitter and a Coherer receiver:
Now, always up for a challenge, Shaun set about building a Coherer set from scratch, even making his own iron filings. For a transmitter, he employed a piezo gas-lighter with added antennas. The results are frankly amazing, not only does it work, it works really well!
Eventually, developments in electronics produced reliable valve transmitters and receivers with much better range and a somewhat less violent use of the spectrum than the old spark transmitters.
Here is my first R/C receiver, built by my late Dad, and fitted into an Aerokits Sea Commander launch in the early 60s. The actuator was made from an alarm clock, with a paxolin pawl and a hand-wound armature. The transmitter (long lost, unfortunately) was ground based, with a control button mounted in an 'Anadin' tin on a four-foot lead:
Commercial Radio Control equipment was incredibly expensive at the time, and so most R/C gear was home made. Here, however is an early commercial Telecommander receiver. Note the protruding tuning slugs:
and a similar design from ED (Electronic Developments):
Most of the sets of this era were carrier-wave only, with no modulation as such, giving a single control such as rudder. Soon the carrier was made continuous, with a modulating audio tone used to trip the escapement. Receiving a constant carrier made the receiver far less susceptible to interference. A single channel set used a single fixed modulating tone, but homebrewers soon ventured beyond a single channel - here is a home-made 10 channel reed set:
The valves needed a high voltage supply, and a 90 volt battery was an expensive item from the Ever_Ready domestic battery range.
|Early batteries of the type used in valve transmitters and receivers. These were very expensive and would have had every last milliamp/hour drained before they were reluctantly discarded...|