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18-02-2011, 04:43 PM
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Infrared remote controls make our life so much better, don’t you think? When I was younger, I’d have to get up and walk across the bright green carpet, past the 8-track player and crank the huge knob to change the channel on the television. Yes, life has improved so much since those dark days, but all of that is about to change, at least for the unfortunate victim of the remote control jammer presented here. Now, before we dig right into the schematic, let me explain a little bit about how remote-controlled appliances work so we can understand how to exploit their weaknesses for our own evil needs. Most remote-controlled devices use an invisible infrared link, and they can easily be identified by the one or more infrared LEDs that stick out of the end of the remote control casing as shown in Figure.
Figure also shows a handful of
940-nanometer wavelength infrared LEDs from m parts bin that is the same type used in most remote control units. When you press a button on the remote control, a microprocessor generates a series of binary pulses, which the infrared receiver inside the target appliance attempts to decode into one of several functions. The frequency of these ones and zeros is sent at a rate of
between 38 and 40 kHz from the remote control to the receiver, depending on the make and model of the unit.
I won’t get into the protocol of the binary signal because that is not important here, just the fact that the base frequency is between 38 and 40 kHz. To confuse the remote control receiver, we will be sending a non-stop stream of zeros and ones at the same frequency that the remote control would normally send, but our stream will not contain any information, so the receiver will just sit there and do nothing. Because the receiver is listening to our blank pulse train, the real remote control cannot get a signal through, so this essentially blocks the remote control from working, allowing you to hijack whichever channel you like, or simply stop anyone else from using the target appliance.
The schematic for the remote control jammer is shown in Figure 4-2. The 555 timer is set up as an adjustable oscillator with a variable frequency between 30 and 50 kHz, so you can fine tune it for the most effective jamming possible. The oscillator output is fed into the base of transistor Q1, which is used to switch on and off the two or more infrared LEDs.
The reason the oscillator frequency is adjustable is because not all manufacturers use the same base frequency for their remote control protocol — why would they? In my opinion, standardization would make the world less confusing, and who would want such a thing? Anyhow, the frequency must be set exactly on your jammer, or the original remote control signal may still squeak through, since the microcontroller inside the receiver module is very good at weeding out noise and erroneous data. While you are building the circuit on your breadboard, it is a good idea to test the frequency of the unit by measuring the output on pin 3 of the 555 timer so that you know your oscillator is working properly, and can be set from 30 kHz to over 40 kHz by altering the variable resistor V1.
Depending on the intended use of your remote control jammer, variable resistor V1 can either be a board-mounted unit or a potentiometer bolted to the cabinet for on the fly adjustments. A board- mounted variable resistor can be set to the exact frequency using a frequency counter, or by trial and error in front of your target appliance, and then will never need any adjustments, which guarantees it will be ready to ruin your couch potato’s day by simply aiming the output at the TV and hitting the switch. Of course, if you plan to use the jammer in several locations with different remote-controlled appliances, you may need the ability to tweak the frequency for best results, so the externally accessible variable resistor will be better. Remember that the frequency has to be pretty close to the target frequency expected at the receiver, and the LEDs must point towards the unit that you plan to jam, or the original remote control might still function.