EE 230: Electronic Circuits and Systems

Fall 2012

FINAL DESIGN PROJECT Optical Signaling Circuit

Brittany Duffy

Electrical Engineering

baduffy@iastate.edu

Partner: Jingxuan “Charlie” Sun

Section D

Instructor: Dr. Pandey

  1  

The Goal

The objective of this final design project was to build a simple circuit that would light one light when one switch is closed, and a second light when the other switch is closed. The optical signal will be transmitted wirelessly from an inferred LED to a photo diode. With filters connected to the output of the photo diode receiver, the signal will activate one of the two lights to turn on, depending on the frequency of the optical signal.

General Circuit Configuration

   

  2  

Final Design Project The Oscillator

The oscillator was the component of the circuit that we started designing first. We decided to create two separate Wein bridge oscillators in order to generate two separate frequencies for the LED to transmit. We

used the formula 𝑓 = !!!"#

to create our two frequencies of 1k Hz and 9.2k Hz. Using this formula, the correct

resistor values, and the potentiometers in our lab kits, we were able to build two successful working oscillators. A switch was then placed to control which frequency would be transmitting. We tested our circuit with the inferred LED installed using the oscilloscope. We had great oscillations for both our 1k Hz oscillator and our 9.2k Hz oscillator.

1k Oscillator Circuit

9.2k Oscillator Circuit

1k Oscillator Waveform

9.2k Oscillator Waveform

1k Oscillator Circuit Diagram

9.2k Oscillator Circuit Diagram

  3  

The Photo Diode Receiver

The photo receiver circuit was the second part of the circuit that we created. We were sure to give the signal a significant gain so the signal would be detected by the low pass and high pass filter. We set up this amplifier as a simple inverting amplifier with a 10MOhm resistor across the inverting pin to the output pin. We powered the op amp by connecting the V- pin to the -10 volt rail and the V+ pin to the +10V rail. Not only were we trying to make a functional circuit, but also very neat and aesthetically pleasing. Using the rails as the power supplies for all of the op amps helped to reduce the amount of wires on the breadboard. The essential element in this circuit is the photo diode. The photo diode was the element of the circuit that was used to detect the inferred light pulses created by the oscillator. We were able to transmit the inferred signal about a distance of approximately 1 foot before the signal was too weak to light the LED. Before we began construction of the filters, we wanted to be sure our waveforms were still coming through the amplifier without too much noise or distortion. We then took two different leads from the output of this amplifier; one lead to the low pass filter and one lead to the high pass filter The brown lead in the photo is the input to the low pass filter and the orange lead is the input to the high pass filter.    

                                       

Photo Diode Amplifier Circuit

Photo Diode Amplifier 1k Hz Waveform Photo Diode Amplifier 9.2k Hz Waveform

Photo Diode Amplifier Circuit Diagram

  4  

Low Pass Filter

The low pass filter was designed to let our 1k Hz signal pass through while not allowing our 9.2k Hz signal to pass through. Like our other op amps in the circuit, we powered this one by connecting the V- pin to the -10 volt rail and the V+ pin to the +10V rail. For this filter we used a 4-pole Butterworth filter. This type of filter is optimized for flat frequency response in the pass band. We used a program that calculates the resistance values given the capacitor values, center frequency, 3dB Bandwidth, and voltage gain. Using this calculator, we constructed a low pass filter. We did have an issue at first where the low pass filter output LED would light when the photo diode became very close to the infrared LED transmitting a 9.2k Hz frequency. The amplitude of the 9.2k Hz frequency signal was just large enough for the diode to light. This is when we decided to change resistor values in our filter. This resolved our issue.

                           

Low Pass Filter Circuit

Low Pass Filter Circuit Diagram

Waveform After Low Pass Filter with 1k Hz Passing Through

  5  

High Pass Filter

Our high pass filter was designed very similar to !our low pass filter using the Butterworth. The V- and V+ pins were ! powered by the -10 Volt and + 10 Volt rails. Originally, we set up ! to design a filter to allow 10k Hz signal !to pass and block out the 1k Hz signal. This filter was not nearly as straightforward to build as the previous filter was because the filter did not block out the 1k Hz signal well enough when the switch was on the 1k Hz frequency oscillator. Therefore, the diode would stay on no matter which frequency was passing through it. After changing around resistor values, we could not get the diode to light at all. We found that the 10k Hz frequency signal amplitude was almost half a Volt too low for the light to come on. After that, we amplified the signal, but then the amplifier would also amplify the 1k Hz frequency signal so that the diode would stay on always. We were back to square one. After trying to use a comparator between the two high and low frequency signals and countless different resistors values for the high pass filter, we had no success. Then we thought of an idea to vary the high frequency oscillator according to the filter output instead. We adjusted the potentiometer to do so, and we found that 9.2k Hz was the perfect frequency to light the green light without affecting the low pass filter’s already superb performance.

 

Waveform After High Pass Filter with 9.2k Hz Passing Through

High Pass Filter Circuit

High Pass Filter Circuit Diagram

  6  

Reflection  

I personally had a lot of fun working on this final project. When we completed the circuit, it was great to activate the switch to see the lights on the other side turn on and off. This project really makes you have an appreciation for how TV remote controls function. After completing the circuit, I wish we had more time to really dial in and perfect our project. One flaw in our design is that our high pass filter would only light the LED within a certain range between the photo diode and the transmission LED. We could have fixed this issue by having a stronger current through the infrared LED and/or attaching more infrared LEDs to transmit twice the signal. This high pass filter also had the issue of outputting 9k Hz when a 1k Hz frequency was inputted. Thankfully, the amplitude was so low at that point that the LED would not light. I would have liked to alter this frequent issue.

If I were to do this project again, I would design precise band pass filters instead of low pass and high pass filters. The band pass filters would make sure only the correct frequencies were making it through, and it would be more precise. I would also make a full wave rectifier to make the signal DC. The full wave rectifier circuit would create a constant DC voltage output that we would use to power our LEDs in the circuit. The way our project was designed, the LEDs are basically blinking so fast that it looks like they are just a bright constant light. This is not a well-designed plan because according to the LEDs specifications, they have a limited number of times to blink before they stop working. The constant DC voltage (or a small rippled waveform) would prevent this from happening so that the LEDs would have a much longer life.

After this class and this project I feel like I am learning a lot about circuit design, and I am really

beginning to feel like an Electrical Engineer.