Everyone loves watching television. Sitting on the couch and watching your favourite TV programme is a great relaxation, isn’t it? The control is in your hand and everything works according to your wish. Sitting at your place and changing the channel, increasing or decreasing the volume or brightness of the television is really a great comfort. But did you ever think how this small device in your hand works when you press a button?
In early days, remote controls were based on ultrasonics. The controlling circuit included a hand held transmitter that transmits and a TV-based receiver circuit. Electronic filter and stepper motors were used to allow or select certain frequencies and perform various functions depending on the key pressed.
Since the early 1980s, remote controls have generally used light in the infrared range (IR), with a low frequency.
There are two parts.
1. Transmitter which would like to send some information (Remote)
2. Receiver which would like to see that information (The TV)
The transmitter, which emits photons in the 750 nanometres frequency range (IR), can be turned on or off by an electronic control circuit. The receiver, which is a photo diode or a photo-transistor detects this IR signal and converts it into an electrical impulse.
A communication method is required for the physical link between the Transmitter and Receiver. The method used in remotes is called as PCM or Pulse Code Modulation.
Here’s a sample 3 bit sequence.
000 – Turn Off TV
001 – Select Next Channel
010 – Select Prev Channel
011 – Select Volume Up
100 – Select Volume Down
101 – Select Brightness Up
110 – Select Brightness Down
111 – Turn On TV
Now, if you want to turn on the TV, you press a button on the remote, then the electronic circuitry turn the IR LED on in the ‘111’ (ON ON ON) pattern. If you press the switch to increase volume, the LED is turned OFF, ON, ON (011) each with a predefined delay.
At the receiver (television) this pattern is detected as a sequence of ONs and OFFs.