Courtesy of All About Circuits
Who doesn't love lasers? Did you know you can build a communication system for around $10?
Before we get started, it’s important to be careful with lasers: even weak lasers can cause permanent damage to the human eye in just a few seconds. More powerful lasers can cause blindness instantly. Never shine a laser at anybody's face, that includes animals. If you’re working with a powerful laser, I recommend making a laser lab and using laser goggles for the frequency of your laser. With that covered, let's move on to the main event: a fun, safe project using LASERS!
Communication systems almost always break down into two parts: a transmitter and receiver. Laser communication systems are no different: the laser is the transmitter and a photo-resistor is used for the receiver. So this project is actually two circuits. I will be keeping both circuits very basic for this project. This means this project could be completed in just one evening, and it's inexpensive.
In my design, I wanted to send audio from my tablet to a set of computer speakers. This means my input is max 2 VPP from 10 Hz to 22 kHz. I would like my output to also be 2 VPP from 10 Hz to 22 kHz, but will suffer losses at higher frequencies. Sound quality should be better than AM radio. The transmitter and receiver should be powered by their own 9 V batteries.
Because we are sending a continuous analog signal, we will need our laser bias such that the laser is on at all times. I still recommend this if you are sending a digital signal. Some lasers don’t like to be pulsed at high frequencies. I biased my laser at 4.5 VDC and modulated it from 4 V to 5 V. To this, I measured the current draw of the laser at 5 V, for my laser it was drawing 29 mA. This is more than what many op-amps can provide so I powered my laser with a transistor set up as a voltage follower. To drive the voltage follower, I used an op-amp to mix my input signal with 4.5 V and attenuated the signal from 2 VPP to 1 VPP.
To test my transmitter, I used my function generator as an input and measured the voltage across the output with my oscilloscope. If you don't have an oscilloscope, you can use a multimeter to measure the voltage across the laser and just vary the input from -0.5 V to +0.5 V. Once your laser is modulating correctly, you can set up and align your laser.
This is how I aligned my laser for a desktop setup. However you do your setup, it’s important to align the laser and photoresistor so you can get accurate measurements of your photoresistor with the laser incident on it.
I started by drilling a hole the same size as my laser into a block of wood. Next, I fixed the laser in the block of wood (mine was held in place by pressure, but you can also use glue). I then glued the block of wood with my laser to a long piece of 1.7 cm X 3.5 cm wood. Next, I powered on the laser and placed a second block of wood on the other side. Next, I marked where the block of wood was located and the location of the laser dot on the block of wood, giving me a separation of 40.4 cm. I then drilled a hole for the photoresistor and glued it in, being careful to keep the legs of the photoresistor separate.
After that, glue the second block of wood where you marked it. Turn the laser back on and your laser dot should be on the photoresistor. This process isn’t practical for a large setup, but for just setting it up on the desk it works well.
The picture above is my setup. I added paper shields because the dot made by the laser was so bright. On the left, you can see the leads of the photoresistor poking through the paper, and on the right, you can see the leads for the laser.
To get started, we’ll want to get measurements of the photoresistor’s resistance. I started by reading values in some arbitrary locations.
I then set up my laser and photoresistor in my alignment setup. I then took some more measurements. Because these will vary with the distance, laser aperture, laser wavelength, laser power, and other conditions, I strongly recommend generating your own results.
Graphing the values helps to see that it's not perfectly linear, but reasonably close over this small range of values.
To design my receiver, I made a voltage divider using the photoresistor. I then connected the voltage divider to the input of an inverting amplifier. The inverting amplifier output was passed through a high-pass RC filter with a cutoff of 10 Hz, this removed the DC component without any major effects on the system’s audio quality. When selecting the second resistor for my voltage divider, I looked at my photoresistor measurements and choose the value closest to the 4.5 V measurement. This gave me a value for R7 of 680 ohms, I could then use that to solve for would likely be my max and min voltages. That gave me a voltage that would voltage ranging from 3.84 V to 4.37 V, a swing of 0.82 V. For the gain stage, I would need a gain of 2.4 to regain my original 2 V input. Due to the photoresistor having a poor rise and fall time, I gave myself some extra gain and built a stage with a gain of 3.6. I then began testing and quickly decided to up the gain further to 5.4, a much higher gain than I was expecting.
Any design of a variable gain can be used for a receiver: you could use an op-amp, a transistor gain stage, or some other method of detection.
Now we can test our system. I used my function generator to provide a sine wave as the input and then measured the receiver’s output. As expected, I found a strong frequency dependence with -3 dB corners of about 20 Hz and 550 Hz.
Now that I’ve measured the system’s performance, it’s time for a sound quality test! I connected my tablet to the input and the output to a set of computer speakers.
The video demonstration is me playing the song "Son of Man" from Disney's Tarzan and breaking the laser beam to show that it’s working. The sound quality is not perfect, and you can hear the loss of the higher frequencies. As for the specs, the pass band is not close to 22kHz, but the output has reasonable sound quality, similar to AM radio. You won't be hosting any parties with this system, but you could easily have a private conversation over the system.
So that's how you can build a very basic laser-based communication system for under $10. If you want 2-way communication, build a second copy of the system so that each side has both a receiver and transmitter. If you build a system with multiple channels going in each direction, you may want to use linear polarizers to prevent "cross talk" and allow you to pack more receivers in a small space.