Laser Pointer Voice Transmitter, Mike Swiderski, K4HBI

Introduction:

First, I will apologize for the resolution of my inexpensive digital camera’s close-up image capabilities. Second, I apologize to the died in the wool EE’s who are expecting Maxwellian equations of design. Third, I apologize to the kit builders for lack of Digikey or Radio Shack part numbers. I just did it My Way.

I have been doing a demonstration for the last couple of years that is geared to teens and I call it "Everything Runs on Waves". The standard slinky, microphone and scope, and a 70cm fast scan TV station were my props. After the first demo, I got a burst in the synopses that might get the kids’ attention. Being a fiber optics applications/troubleshooting engineer with Bell Telephone Labs at the time, I got to thinking about some optical gee whiz stuff. What came up was taking a garden variety laser pointer and let the kids talk over it. Well not being a procrastinator nor like reinventing the wheel, I went out to the "Junque Room", notice not "junk box" and started slinging stuff together. My intent was not to intimidate the audience with commercial gear nor requiring an MIT EE degree to understand the operation. The results are a "receiver" and "transmitter" that should tickle the home brewer, has WOWED several hundred students, and is as follows.

Receiver:

The principle here is detection of the audio from an AM modulated light beam. That is as technical as we are going to get. First find a "transistor" radio, the odder the design the better the OOOHs are. I happened upon an old toilet paper dispenser AM radio that used to hang on the bathroom wall which provided listening pleasure along with the "Readers Digest". The stories to the kids when they ask "What is that," are up to you, but I sure have some fun with it. A picture of the receiver follows:

Figure 1. The Complete Receiver

While in the junk box, find a wall wart type power supply with suitable voltage and current for the radio, batteries can get expensive. You now should have a working radio.

Modifications are not that difficult to make it into an optical detector / AM radio. It involves a small "solar cell", a SPDT switch, a high value variable resistor, and a capacitor.

The first step is to separate the radio’s detector out put from the audio amplifier, big words easy job. Find the volume control on the radio circuit board. There will be three connections to the volume control (5 if it is the power switch also). Find the wiper of the volume control, usually the center connection. The wiper connects to the radio’s audio amplifier and we need to open the connection between the wiper and the amp. Look around, a lot of radios have a disc cap connected to the wiper as a coupling capacitor. Unsolder the cap’s lead that goes to the wiper and lift that out of the board. Solder two small wires, one to the wiper, we will call it #1, and the other wire #2 to the free end of the cap not in the circuit board (or to the input of the amp if no cap was present). Your work should match Figure 2.

Figure 2. Radio Board Modification

Now we have wire #1 with the radio’s output and wire #2 with the input to the radio’s audio amplifier. It is time to build the optical detector, again big words, but a piece of cake. The circuit is as shown in Figure 3.

Figure 3. Optical Detector and Radio or Optical Switch

Figure 3 shows two circuits combined. To the right of the 0.1 ufd disc capacitor is a Single Pole Double Throw switch that is used to choose between listening to the radio or the solar cell input. To the left of the 0.1 ufd disc capacitor is the optical receiver. The solar cell is nothing special, mine is about ½" square and either came out of a kids electronic project box or something. The way this works is that light hitting the solar cell creates voltage across the cell. This voltage is put across a 50 K ohm variable resistor whose wiper is adjusted just like the radio's volume control when in the optical detector mode. The 0.1 ufd disc capacitor’s value is not critical and the voltage level is so low almost any voltage capacitor will work. The capacitor couples the AC voltage change (variations in light) across into the audio amplifier.

Construction hints, obviously put the solar cell outside the radio case and find a scrap piece of metal to make a hood or shield over the cell. This is a very sensitive optical receiver and ambient lighting will make it HUMMMMMMMMM. A good demo is to cover and uncover the cell with your hand to show that the ceiling lights are modulated and you are hearing their frequency, usually 60 Hz. The switch and pot are mounted anywhere convenient, remember these are young folks and the more the wiring is protected the longer your demo will last. Also remember to connect the ground side of the cell and pot to the ground on the circuit board, usually available right near the volume control on the radio board. Figure 4 is a picture of my receiver turned upside down with the back off and showing the solar cell, shield and control mounting. Have fun and be creative in your building and your stories.

Figure 4 Back View of Receiver with Cover Off

Transmitter:

The transmitter is five simple parts; a laser pointer, a microphone, a power supply, an audio amplifier, and the modulator. Figure 5 is a picture of my wooden board unit, believe me the board construction is more impressive to onlookers and kids than any fancy "BUD" box ever could be. Notice the plywood isn’t even square, what do you want for a night’s work.

Figure 5. Board Laser Pointer Transmitter

The poor resolution of the photo can be helped with blocking off the circuits on the board and their function. Figure 6 may help clarify the issue.

Figure 6. Functional Blocks of the Transmitter Board

Power Supply: The power supply is a transformer and full wave rectifier/filter board cannibalized from an old VCR carcass. It puts out about 14 volts and looked to be beefy enough to supply the needed current. Nothing special here.

Audio Amplifier: The amp was proffered by an old abandoned AM only car radio hulk. Simply cut it out, identify the input, output, and power leads and make sure it works. I chose the car amp because they are usually a "few" watts, and being an old AM’er – you always need plenty of audio power for modulation.

Microphone: The kids are usually quite vocal until you shove a microphone in front of them. I wanted something simple looking and hi level output, so I chose an old lapel style crystal unit that was left from an old cassette tape recorder. Besides if broken it would be no great loss, and we are going for a communication grade quality to demo, not Saturday night at the Met. It is hardwired to the input of the amplifier.

Laser Pointer: I truthfully don’t remember where I got it, but the cat was tired of the red light and the batteries were weak. Mine had three button cell batteries that should produce about 4.5 VDC. So the first step is to measure the height of the batteries, and then find a piece of wooden or plastic dowel rod that is slightly smaller in diameter than the batteries. Cut the rod about ¼ inch shorter than the battery height dimension. Screw a brass screw in each end of the rod for electrical contacts. Adjust the end-to-end length to match the battery height by adjusting the screws. Find some red and black thin gauge very flexible wire at least 6 ft long, I found 8 ft would probably have been better for passing around in the crowd. Solder a wire to each screw, red to the + side and black to the – side. Slide this dowel up where the batteries went, observe polarity to be same as the original batteries. Drill and deburr a hole in the laser pointer cap just big enough for the wires to pass through. Thread the wires through the hole and screw the battery cap back on the pointer. You might want to twist or heat shrink the wires together. Test with proper voltage (same voltage as original batteries, 4.5 volts for mine) to make sure the unit still works. While you are testing make note of how much current the laser draws when operating, mine drew about 350 ma.

Laser Modulator: The heart of the unit and simple to boot. Figure 7 is the schematic of this unit.

Figure 7 Laser Modulator

The Figure shows a simple series circuit, two resistors, a transformer, and the laser pointer. The 27 ohm resistor is there to protect the pointer from over voltage. My pointer draws about 350 ma at 4.5 volts and the power supply maximum is about 14 volts. This resistor is designed to drop 9.5 volts (14-4.5) at 350 ma so the maximum the pointer will see is 4.5 volts under load, given R= E/I = 9.5/0.35 = 27 ohms.

The variable 100 ohm resistor is used to adjust the laser voltage. When this circuit is connected and powered, adjust the 100 ohm pot so that the laser is "almost off", that is, the light can be seen but it is weak and further adjustment would turn the light off.

The transformer is something I grabbed out of the junk box and it worked fine. The transformer I grabbed was a "Vert Block Osc" unit that looked small enough and probably would not be used ever again for its intended purpose in a tube TV. The markings indicate 1: 4.14 ratio. I would suggest trying a few from your box, but avoid transformers with a high ratio between the coils. I have not tried it, but I would guess a commonly available1k Z to 2k Z unit might work just as well.

Adjustment is relatively easy, aim the pointer at a sheet of paper, press the pointers on button while you speak into the microphone, adjust the transmitter audio amplifier volume control to about half and see if the intensity of the pointer increases. If not adjust the laser 100 ohm pot for a bit more brightness. Aim the beam at the center of the solar cell and speak. Fine tune the receive volume level and the transmitter volume level until soft and loud speech is undistorted.

I typically use at distances of four to five feet with the receiver placed in an area where no body is behind it, the aiming of the pointer by some of the folks gets a bit wild. Remember to push the pointer "ON" button just like a push-to-talk button. This setup has amused and amazed youngsters and adults, have fun and be safe. 73 Mike K4HBI