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Construction InformationThe Infra Red Receiver ModuleThe receiver module selected was TSOP2438 which has a peak reception frequency of 38kHz. This was chosen to match the frequency most standard remote controls operate at which would allow for easy testing. pin 1 = Output The device was connected to a 5V supply and an oscilloscope used to measure the output pin. The output pin is high when no signal is being received. Pointing a remote control at the sensor and pressing any button caused a pulse stream to be seen on the scope, the output dropping to zero and returning to it natural high state afterwards. As far as docking is concerned, I will just be interested in detecting a signal and not decoding it so we just need to detect a falling edge (perhaps with suitable error suppression). The Infra Red Transmitter
Initial testing showed that the signal could be received from 8 meters or more away which looks quite encouraging. This was with 3 LEDs in the remote. Ideally, I only want to use one (a focussed beam), but I can double-up if necessary. The next task was to build an IR transmitter. The IR receiver expects a modulated 38kHz pulsetrain and it will remove the 38kHz carrier to leave the modulated signal. A 'standard' remote control uses a modulating signal with a period of somewhere between 1.2 - 1.8 ms (550Hz - 850Hz) qith a roughly equal mark-space ratio so this seemed a good starting point. The transmitter was built from a 556 Timer chip. This was wired as two astables. The first astable runs at 38kHz to generate the carrier frequency. The second astable generates the modulation frequency (initially 650Hz) and its output is fed to the reset pin of the first astable which achieves the modulation required. The output was fed through an IR LED that was current limited to 20mA. Results were, to say the least, disappointing. I got this to work and send a signal to the receiver but the detection range was 1cm, 2cm at a push. This was not the 8-10m I am looking for :-( IR LEDs are often powered with much higher currents than a normal LED and because the output is pulsed, it does give the junction time to cool down. After consulting the datasheet, I pushed the current up to 100mA. I could now get a range of 1 metre, but not much more. I then swapped the IR LED for a more powerful XXXXX model and now was pushing 2 metres. The 556 outputs are struggling at this current so I feed the output through a transistor instead. It was now possible to achieve 3 metres but even if I increased the current further, the range did not improve appreciably. Going back through the specs of the IR LED, I saw that it is possible to drive the device with over 1 amp, but for very short periods (100uS). So the period of the modulated signal was adjusted to give a modulation frequency of 300Hz, with a 100uS pulse and a 3.2ms space. I could then drive the diode at 1 amp during the 100uS and it gave 3.2ms for the junction to cool, and still be operating within specification. In order to achieve this disproportionate mark/space ratio, the resistor feeding the modulator capacitor was bypassed with a diode to give greater control of the waveform generation. My main concern is whether the IR Receiver will be able to demodulate such a signal, especially as it uses a PLL so it might drift over time and lock out the demodulated signal. Test results showed that the transmitter was now fit for prime time. I could detect the signal at a distance of 10 metres, more than adequate for my needs. Time will tell whether the IR Receiver can detect it reliably but I feel it is time to build the transmitter on a circuit board and move away from breadboard as I'm disappearing under a mass of wires. Stage1 Stage 2
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