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Infared Transmitter / Receiver This page describes my Analog Integrated Circuits final project, which I completed last spring as part of a design team. Our project focused on the design and construction of an IR transmitter/receiver pair. It proved to be a very difficult task to get the parts working together. To begin our work, we looked at possible designs for oscillators, and ways to mix signals. Our goal for the project was to design an IR transmitter and receiver capable of communicating an amplitude-modulated audio signal a distance of several feet. The transmitter uses a carrier frequency of 455 kHz to modulate an audio signal in the human speech bandwidth of 0 - 4 kHz. The receiver demodulates and amplifies the signal to a power capable of driving a typical 8 ohm loudspeaker. Both the receiver and transmitter will be powered with a 9V DC source. Note: 455 kHz is not a radio frequency (RF) as defined by the FCC, it is an industry standard for sub-RF intermediate frequencies. We designed for this frequency, but due to a shortage of inductors we were forced to build the circuits to run at a lower frequency of 100 kHz. Below is a picture of the breadboard transmitter and reciever circuits. We built and tested our project on breadboards first before moving to the more permanent pcb boards. In order to ensure accurate transmission of the RF signal, the audio input must be mixed with the higher frequency carrier signal. For our project, we chose the Sony oscillator for its stability, and a differential mixer coupled with the oscillator to combine the two signals. We used an infared LED to transmit the signals. One of the tougher parts of this project was providing the correct bias for transistors Q6 and Q8. This was because the current across the transmitter LED had to be precise to ensure accurate transmission and at the same time not overload the LED. This is the design we settled on for the transmitter. From left to right, you can see the Sony oscillator, differential mixer, and the transmission stage. The frequency range of human speech is traditionally understood to extend up to 4 kHz, so the bandwidth of our system needs to accommodate this entire range. The bandwidth is set with another resistor and capacitor in the RLC circuit in the mixer stage, while the capacitor and inductor set the resonance frequency equal to the carrier frequency of the oscillator. The result is a bandpass filter, centered at 455 kHz, with -3dB points ideally at 458 kHz and 452 kHz for a 6 kHz bandwidth. Since light incident on the phototransistor in the receiver creates a very low current, gain stages are needed before demodulation. We used modified degenerated common emitter amplifiers to generate a gain constant of around 40 for each stage. Each of the three stages are also inverting, and the gain increases the signal presented to the demodulator to about 1V. The demodulator we chose consists of a PNP transistor and a low-pass filter to remove the carrier frequency and recover input audio bandwidth. The first-order low pass filter has a cutoff frequency of 6 kHz. There is also an LC tuned circuit in the reciever to match the resonance of the mixer. The final piece of the reciever is an audio power amplifier, the LM386. Below is the schematic of the reciever used. From left to right is the demodulation stage, the LC tuning circuit, gain stages, and audio amplification. During the design phase of the project, we did extensive simulation of our circuits to ensure functionality. This is a simulation of the sony oscillator at a frequency of 455 kHz. The start-up time for our oscillator was about 100 uS. The next simulation shows the mixer output, created by mixing the 455 kHz carrier signal with a 1kHz sine wave input signal. After simulation of the mixer and oscillator, the entire transmitter circuit was analyzed. The LED used to transmit has a maximum current of 70 mA at 2.1V, so we set the DC collector current to 40 mA, with a range of +/- 15 mA. This simulation shows the current through the LED as a range from 26mA - 62mA. Under lab test conditions the Sony oscillator performed as expected, with a peak-to-peak voltage swing of about 1 V with a 9 V supply. This is the result: The mixer we built also performs as expected. We tested the mixer with the output of the oscillator and an input of a 1 kHz sine wave at 1V peak-peak. The output on the oscilloscope followed our simulation results, although the peak-to-peak voltage observed in the lab was about 100mV less. To test the communication channel we placed the transmitter and the receiver directly facing each other a few inches apart. We then used the oscilloscope to measure the output of the phototransistor after the tuning stage, which can be seen in the figure below. The peak-peak voltage is about 150mV, which is much more than our assumptions in the design of this circuit. This allows the transmitter and receiver to be placed much farther apart, significantly reducing the amplitude of the output of the phototransistor. This image is a comparison of the input signal with the output of the receiver. The lower graph is the 1 kHz, 1V peak-to-peak sine wave input to the mixer on the transmitter side, and the upper curve is the demodulated output from the receiver. It can be seen that although the output is noisy, it is definitely a 1 kHz tone. The input and the output are 180 degrees out of phase due to the use of the three inverting gain stages before demodulation. The following stage will be an inverting power amplifier, so the output to the speaker will be in phase. The gain of the receiver output is extremely variable, changing with the tuning of the receiver oscillator, amplitude of the input signal, and distance and angle from the transmitter. The use of infrared light as a communication channel was an interesting and challenging addition to the task of building a transmitter/receiver communications network. In general, the lab testing followed from what was expected from simulation, however, there were a few difficulties along the way. For example, we found that the tuning of the oscillator in the receiver circuit was critical to obtain accurate transmission of the signal. In testing we connected a variable capacitor in parallel with the resonant circuit, and small changes to this capacitor had a great effect on the overall gain and signal quality of the resulting output. By making these small adjustments to the receiver tuning circuit we were able to place the transmitter and receiver more than three feet apart and output a clean, high amplitude sine wave. In addition, we discovered that the quality of the received signal is also highly dependent on the angle of the emitter and detector. Shifting the emitter just a few degrees in any direction off center greatly decreased the amplitude of the signal received. Testing on the breadboards added significant noise to our system, as expected, but this noise cleaned up nicely when the circuits were placed on PCB boards. Note: The IR transmitter/reciever pair works much better with the lights off :P Questions about this project? Email me