MadSci Network: Engineering

Re: How does a universal remote control work?

Date: Sun Nov 11 14:41:31 2001
Posted By: Eric Maass, Director, semiconductors / communication products
Area of science: Engineering
ID: 1004708302.Eg

Let me first give an overview, a simple explanation about how universal remote controls work, then give you some more details, including some links that provide more insight 
and which include cutaway views of the circuit boards from remote controls.

Universal remote controls today send control information using infra red light. When you touch a key on the universal remote control, the circuitry determines what sequence of 
flashes of infrared light correspond to that key, and then the signal is sent as a sequence of voltages that turn an infred LED (light emitting diode) on and off.  The appliance that 
you are pointing at -- a TV or VCR or DVD or stereo or whatever -- has an infrared light detector that picks up the infrared light signals, converts the infrared light to electrical 
signals, and then uses some digital circuitry to determine what function needs to be done in response to that signal sequence.  

A universal remote control has to be flexible enough to handle both of the two international standards which are used by remote  controls to encode the commands:
			RC5 and 
			RECS 80 code. 

RC5 coding represents each command as a series of "bits", just like the bits used in computers to represent numbers and letters. RC5 has a uniform duration of each "bit" that is 
being transmitted, and uses what is caled a "biphase code". In the middle of the duration of a bit,  a "0" is encoded by a transition from a high to a low voltage, and a "1" by a 
transition from a low to a high voltage.

RECS 80 code uses pulse length modulation, where information about the bits to be encoded is contained in the length of each pulse. Each bit to be transmitted is encoded
by sending an initial high level of the duration T,  followed by either a low level of duration 2T representing a logical '0',  or following the initial level with a 3T to represent a 
logical '1'.

Then, the code of bits (either RC5 or RECS 80 coding) is modulated with a 20-30 KHz oscillating signal, and the resulting pulses of oscillating signal are sent to the LED to 
make the light flicker in bursts corresponding to the bits.  The 20-30 KHz oscillation is added to make sure the receiving appliance is not confused by other lights flashing on and 
off around the home.   The receiver can receive the light, filterout the signals that don't include the 20-30KHz oscillation, then demodulate the signal to capture the bits from the 
20-30 KHz modulated signal.

Here is a deeper explanation of how remote controls work:
How Stuff Works: Inside a TV Remote Control
...and here is more on the coding used:
Decoding IR Remote Controls

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