Cell Phone as a TV Remote Control

I have been reading on how people control their televisions with cell phones by playing specially prepared sound files through a pair of infrared LEDs. Each of these sound files provides a properly encoded on-off pulse sequence modulating a carrier at frequency close to the upper limit of most audio devices found in computers and hand helds. Because one of the infrared LEDs is connected in reverse, one LED lights up during the positive half cycles while the other lights up during the negative half cycles. In doing so, they effectively double the carrier frequency to that designated for remote controls making it possible for the intended receiver to decode the signal. The method is simple and elegant. And it sounds like a lot of fun to experiment with.

But as soon as I tried it out, I realized that it does not work on any devices. It worked on my desktop PC and my laptop, but it did not work on my Palm Centro cell phone. This makes reasonable sense since some portable devices just do not generate enough output at high frequencies to overcome the forward voltages the infrared LEDs require (typically 1.2V). In order to salvage my little experiment, I decided to construct a simple transistor-based infrared transmitter that plugs into my cell phone's audio jack. To be a bit more adventurous, I restricted myself to using just one infrared LED and one AA battery. The latter not only made biasing the transistors difficult but also limits the range of the transmitter to only a few meters. So this really is a proof-of-concept design. Below is the transmitter's schematic. It was designed with my collection of electronic parts in mind.

Figure 1: Infrared Transmitter Schematic

Input to this transmitter is assumed to be a pulse-width modulated sine wave oscillating at around 20 kHz. Q1 acts as a phase splitter providing an inverting and a non-inverting output signals which are then rectified by D3 and D4 and superimposed across R8. The circuit so far behaves just like a full wave rectifier and, for this reason, positive pulses at this point are appearing twice as often as in the original input signal. Finally, Q2 and Q3 perform voltage level shifting and impedance matching so as to deliver sufficient signal to the base of Q4 to actually drive the infrared LED D5. To make the transmitter as small as I technically can, I used the following PCB layout to position my components on a 5x15 perf board.

Figure 2: Printed Circuit Board

Everything is enclosed quite nicely inside a modified 2 AA battery holder that came with a small power switch. The circuit board is small enough to fit into one of the battery slots. A small hole on the front side of the battery holder exposes the infrared LED. The whole unit, when fully assembled, is shown below:

Figure 3: Finished Infrared Transmitter

Everything works as expected. So far, I have created four WAV files to control my Sony Trinitron TV which uses the SIRC protocol. Those interested in trying it out can download the WAV files, SPICE file, schematic and PCB layout below:

• Sony Volume Up WAV File
• Sony Volumn Down WAV File
• Sony Channel Up WAV File
• Sony Channel Down WAV File
• SPICE file
• Eagle Schematic
• Eagle PCB Layout

For more information on protocols used by remote controls and instructions on how to capture infrared signals and create WAV files used in this post, please see the References section for a list of useful online resources.

References

• Turn your phone into an universal remote control
• Sony SIRC Protocol
• Sony TV Command Codes