You can’t just connect a load like a bulb to the output pins of a receiver and expect it to operate when you move the transmitter stick, because the receiver is not itself a switch. It puts out a signal voltage pulse through one wire of each channel’s connectors. The length of these pulses is varied by movement of the stick or switch on the transmitter and ranges typically between 1 and 2 thousandths of a second (1-2mS) as you move the stick. The other two wires of the channel’s output are at 0v and +5v all of the time. In order to make sense of this variable signal pulse, the device which is plugged into the receiver needs a circuit to detect it, decode it and change it to drive an output which can be used by the load. In a servo this output drives the servo motor. In a radio-controlled switch it operates either an electromechanical relay or a semiconductor, which in turn, switches an external circuit, e.g. lights, winch motor, sound unit.
The original remote switch for model boats was a sensitive mechanical type called a microswitch, mounted on top of a standard servo. The button of the microswitch, or a lever connected to it, was depressed by the rotating arm or disc of the servo, so making the electrical circuit connected to the switch. Figure 19
. The contacts are the same as a conventional single-pole changeover switch, Figure 20
. Some modellers still cling to this method, largely I think because they don’t understand the workings of a servo-less switch. A simple single-relay switch has exactly the same contacts as a microswitch and is physically much smaller than the servo + switch assembly. It plugs into the receiver exactly as a servo does, and requires no further setting up such as bending levers or repositioning the servo arm to make/break contact at the appropriate point, Figure 21
Radio control switches can behave like a bell-push, where the circuit is live only for as long as you hold the transmitter stick or switch in the ON position, Figure 22
. The alternative is that they operate like an Anglepoise lamp switch, where you have to push and let go of the switch to turn it on, and then repeat that action to turn it off, Figure 23
. The latter is called a latching switch while the former is, unsurprisingly, a non-latching switch. Each has its place within a model; for example you would use a latching switch for any load which you want to keep on for a while, such as lights, whilst a non-latching switch would be more suitable for a horn or whistle.
Some r/c switches can operate up to eight separate circuits from just one channel but beware! Those simple 2-way switched channels on multi-channel radios which are often marked with legends like 'Retracts' or 'Flap' are capable only of operating a single function switch (i.e. On or Off). Practically this limits them to one lighting circuit, a single sound card or a continually-rotating motor like a radar sweep arm. For two or more circuits per channel, then you need a spare radio channel which is operated either by a stick, a rotary knob or a 3-way switch (Centre = Off).
The main selection criteria for a switch is the current which the load to be switched will draw. Typically one Grain of Wheat bulb will draw 60 to 80mA; an LED will draw 20mA; a 12v smoke unit will draw 1 to 2A and a geared 12v winch motor between 1A and 5A depending on the motor size. Calculate the total current for each circuit and then buy the appropriate switch. With relay switches, the voltage doesn’t seem to matter as long as it doesn’t exceed about 30v whereas switches without relays can be limited to well below that value. Again, do some research before you decide. Popular makes of r/c switches include Electronize, Hunter Systems, Technobots, ACTion and Robbe-Futaba.