Advanced Projects in Laser Communication

The last section presented a number of basic free-air laser light communications projects. You learned how to modulate a He-Ne laser beam using a transformer, transistor, and even a piece of Mylar foil stretched in a needlepoint hoop. You also learned various ways to electronically modulate laser diodes and recover the transmitted audio signal. This section presents advanced projects in free-air laser-beam communication. Covered are methods of remotely controlling devices and equipment via light and how to link two computers by a laser beam.

TONE CONTROL

Everyone is familiar with Touch-Tone dialing: pick up the phone and push the buttons. You hear a series of almost meaningless tones, but to the equipment in the telephone central office, those tones are decoded and used to dial the exact phone you want out of the millions in the world. You can use the same technique as a remote control for actuating any of a number of devices, such as motors, alarms, lights, doors, you name it. The tone signals are sent from transmitter to receiver via a laser light beam. With the right setup, you can remotely control devices up to several miles away, and without worry of interference or FCC regulations.

The Touch-Tone (or more simply “tone control”) system supports up to 16 channels. Each channel is actuated by a pair of tones. Tone selection depends on the buttons pressed on the keypad. Dividing a common 16-key keypad into a matrix of 4 by 4, as illustrated in ill. 14-1, shows how the tones are distributed. E.g., pressing the number 5 key actuates the 770 Hz and 1336 Hz tones. Pressing the number 9 key actuates both the 852 Hz and 1477 Hz tones.

ill. 14-1. Keypad tones for tone dialing matrix.

The dual tones help prevent accidental triggering, but they also present a somewhat difficult decoding dilemma. The first tone dialing circuits used tuned components that were expensive and difficult to maintain. As tone dialing caught on, custom-made ICs were developed that dispensed with the tuned circuits. Until recently, these ICs have been rather costly and required some sophisticated interface electronics. Now, Touch-Tone decoding ICs cost under $25 and operate with only two or three common components.

Most telephones use a matrix of 12 keys, not 16, so the last column of buttons isn’t used. The circuits in the phone may or may not be able to reproduce the tones for the last column, but most off-the-shelf Touch-Tone dialing ICs are capable of the full 16-channel operation.

Making Your Own Controller

Dialing other phones isn't the goal of this project, but using a phone dialer as a remote controller is. You have several alternatives for making the controller.

* Salvage the innards of a discarded phone (must have tone dialing).

* Use a portable, hand-held tone dialing adapter.

* Build the controller from scratch using a keypad and dialing chip.

The last approach allows you full access to all 16 combinations of tones. The other two approaches limit you to the 12 keys on a standard telephone—digits 0 through 9 as well as the # and * symbols.

Salvaging the keypad and circuits from a phone requires some detective work on your part, unless you happen to receive a schematic (not likely), but the advantage is you get the entire controller as one module. The biggest disadvantage is that the dialing circuits may require odd operating voltages.

The portable tone-dialing adapter, such as the one in ill. 14-2, is an easier approach, with the added benefit of an easy-to-carry (fits in your pocket) module that runs on battery power. The adapter is meant for use with rotary or pulse dial phones when you need to access services that respond to Touch Tones (long-distance services, computer ordering, etc.). You place the adapter against the mouthpiece of the phone and press the buttons.

ill. 14-3 shows how easy it's to modify the dialer for use as a tone source for the PFM laser diode modulator. Alternatively, you can connect the output of the dialer to an audio amp and process it through the transformer or transistor He-Ne laser modulator described in the previous section. The connections to the dialer’s speaker can remain in place, thereby providing you with audible feedback that the controller is working and sending out tones.

Table 14-1. Pocket Dialer Parts List

• 1 Pocket Touch-Tone dialer
• 1 1/8-inch miniature plug
• Misc. Wire

ill. 14-2. A battery-operated portable pocket tone dialer (shown with hookup leads attached).

ill. 14-3. Wiring diagram for adding external hookup leads to the pocket dialer.

The Tone Receiver

A number of electronics outlets, such as Radio Shack, sell an all-in-one tone receiver chip. This chip deciphers the dual tones and provides a binary weighted decoded output (8, 4, 2, 1). Add to the basic circuit a 4-of-16 de-multiplexer, as shown in ill. 14-4, and you can control up to 16 different devices. When used with a telephone keypad or dialing adapter, you can decode the first 12 digits. Use the universal laser light detector to capture the beam and amplify it for the receiver. See the parts list for the tone receiver in TABLE 14-2.

INFRARED PUSH BUTTON REMOTE CONTROL

The Motorola MC14457 and MC14458 chips form the heart of a useful remote control receiver-transmitter pair. The chips are available through Motorola distributors as well as several mail-order outlets and retail stores (including, at the time of this writing, Digikey and Circuit Specialists; see Section A for addresses). Price is under $15 for the pair.

Table 14-2. Tone Dialer Receiver Parts List

• IC1: SS1202 tone decoder IC
• IC2: 74154 IC
• R1: 10 megohm resistor
• R2-R5: 1 k-ohm resistor
• LED1-4: Light-emitting diode
• XTAL1: 3.57 MHz (color burst) crystal
• All resistors are 5 to 10 percent tolerance, ¼ watt. All capacitors are 10 to 20 tolerance.

ill. 14-4. A schematic diagram for the SS1202 tone-decoding chip and how to decode the outputs with 74154 IC. The LEDs provide a visual indication of the binary output of the tone-decoding chip.

ill. 14 Pin-out diagrams for the Motorola 14457 and 14458 remote control chips.

The 14457 is the transmitter and can be used with up to 32 push-button switches (we’ll be using 12). Pushing a switch commands the chip to send a binary serial code through the laser (preferably a diode laser). The 14458 chip receives the signal. Decoded output pins on the 14458 receiver chip can be connected directly to a controlled device, such as a relay or LED, a counter, or a computer port.

Ill. 14-5 shows the pin-out diagrams for the two chips. The 14457 comes in a small, 16-pin DIP package, and with all the other components added in, takes up a space of less than 2 inches square. The chip uses CMOS technology to conserve battery power, and when no key is pressed, the entire thing shuts down. Battery power is used only when a key is depressed.

Building the Transmitter

The basic hookup diagram for the 14457 transmitter circuit's shown in ill. 14-6; the parts list is provided in TABLE 14-3. Note the oscillator and tank circuit connected to pins 11 and 12. The oscillator required by the 14457 (and a matching one for the 14458 receiver) is a hard-to-find ceramic resonator, not a standard crystal.

ill. 14-6. Hookup diagram for the timing and output portion of the 14457 transmitter IC. XTAL1 is a 300-650 kHz ceramic res as discussed in the text.

Table 14-3. Remote Control Transmitter Parts List

Transmitter:

• IC Motorola MC 14457 remote control transmitter IC
• R1 330 ohm resistor
• R2 22 ohm resistor
• R3 10 megohm resistor
• R4 680 ohm resistor
• C1 47 uF electrolytic capacitor
• C2,C3 0.001 uF disc capacitor
• 1 2N2222 transistor
• D1 IN400i diode
• D2, D3 1N914 diode
• XTAL1 300 to 600 kHz ceramic resonator
• Misc. Laser diode
• Keypad
• Q1-Q4 2N2222 transistor
• Misc. Matrix keypad or switches

All resistors are 5 to 10 percent tolerance, ¼ watt. All capacitors are 10 to 20 percent tolerance, rated 35 volts or more.

The ceramic resonator works just like a crystal but comes in frequencies under 1 Mhz. The exact value of the resonator isn’t critical, I found, as long as it's within a range of about 300 to 650 kHz and the resonators for both chips are identical. I successfully used a 525 kHz resonator in the prototype circuit. A common ceramic resonator is 455 kHz, used as an intermediate frequency in many receivers. The inverted output of the 14457 (OUT’) is applied to the modulating input of the laser (see the previous section).

So much for the output stage of the transmitter; what about the input stage? FIG URE 14-7 shows how to connect a series of 12 push-button switches to the column and row inputs of the 14457. You can use separate switches, but a cheaper way is with a surplus telephone keypad.

Just about any wiring technique can be used to construct the 14457 transmitter, but because you’ll probably want to make the unit handheld, stay away from wire- wrapping. The stems of wire-wrapping posts and sockets are too long and will fatten the controller considerably. Use a set of four “AA” batteries or a 9-volt transistor battery to power the transmitter. You might need additional batteries for the amplifier and modulator, depending on the type used. And, you must also provide power to the laser itself.

Building the Receiver

Short-haul communications links don't require amplification on the input stage of the receiver. But if you plan on using the receiver some distance from the transmitter

ill. 14-7. Keypad connection for adding 12 push buttons to the 14457 transmitter. You may easily add additional rows of buttons by connecting them to Q3 and Q4 in the manner shown for Q1 and Q2.

(500 meters or more), an extra amplification stage is recommended. You may use the universal amplifier found in Section 13.

The basic wiring diagram for the 14458 receiver chip is shown in ill. 14-8 (parts list in TABLE 144). Once again, the ceramic resonator is used as a timing reference for the IC. One inverter from a 4069 is used to provide an active element and driver for the oscillator circuit.

All that’s required now is to connect the devices to be controlled to the output lines, shown in ill. 14-9. Note the various sets of outputs and the VC’ function pin. The VC’ line goes HIGH when all but the number keys are pressed. The pin is used in some advanced decoding schemes as a function bit. The chart in TABLE 14-5 shows what happens when the 12 keys are pressed (the chip can accommodate another 20 push buttons; see the manufacturers data sheet for more information).

For most routine applications, you need only to connect the controlled device to pins 19 through 22 (labeled C1, C2, C4, and C8). These are binary weighted, and by connecting a 4028 one-of-ten decoder IC to the receiver, you can individually control up to 10 control devices or functions.

The receiver is wired to accept a single key-press on the transmitter as a complete command. The receiver can also be made to wait until two keys are pressed (this is used because in television and VCR applications, you are able to dial in multi-digit channels). The two-digit data outputs are used when the chip is this two-digit mode. To change from one- to two-digit mode, disconnect the power leads to pins 9 and 6.

ill. 14-8. The timing portion of the 14458 receiver IC. XTAL must be the same value as the resonator used with the 14457 transmitter. Connect a phototransistor or amplifier to pin 2 of the chip.

Table 14-4. Remote Control Receiver Parts List

Receiver:

• IC1 Motorola MC 14458 remote-control receiver IC
• 1C2 4069 CMOS hex inverter gate
• R1 10 megohm resistor
• C1, C2 10 pF disc capacitor
• C3 0.47 F disc capacitor
• XTAL 300 to 600 khz ceramic resonator

All resistors are 5 to 10 percent tolerance, ¼ watt. All capacitors are 10 to 20 percent tolerance, rated 35 volts or more.

Output Enhancements:

• IC2 4028 CMOS decoder IC or IC2 4001 NOR gate IC
• 1C3 4029 CMOS counter IC

ill. 14-9. A 4028 CMOS IC can be used to decode the binary output of the 14458 transmitter chip.

Ill. 14-10 shows how you can use the UP and DOWN functions with a counter to provide variable step control. Note the DOWN ENABLE and UP ENABLE lines. When pressing the UP or DOWN buttons on the transmitter, the UP and DOWN codes are sent continually rather than just one per push, as with the other buttons.

The enable outputs from the 4028 can only handle 10 functions, so the UP and DOWN functions are integrated with the number functions. That is, when the DOWN button is pressed, the output at the 4028 chip is the same as if you pressed the number 6 button (binary code 0110). However, the VC pin is toggled HIGH, which can be used in further decoding. Similarly, when the UP button is pressed, the output of the 4028 chip is the same as if you pressed the number 7 button (binary code 0111).

Table 14-5. Key Decoding

ill. 14.10. Decoding for the UP and DOWN functions. The BCD output of the 4029 can be interfaced to a display or computer. The 4029 chip can be made to count by 16s or 10s by connecting pin 9 to ground or V+ as shown.

To enable you to count the number of UP and DOWN pulses, connect the UP ENABLE input of the circuit shown in the figure to the number 7 output of the 4028 and the DOWN ENABLE input to the number 6 output. The VC pin acts to gate the circuit so that the counter doesn’t count when numbers 7 and 6 are pressed.

DATA TRANSMISSION

The UART (Universal Asynchronous Receive/Transmit) chip converts parallel to serial and serial to parallel. It’s much more involved than a shift register that simply converts parallel data to pure serial form, or vice versa.

The UART allows you to send data to devices like printers, plotters, and modems and yet be assured that all the information you are sending is getting there intact. Built into the chip are provisions for sending and receiving at the same time, for adding parity bits and stop bits to the serial data train, and more. A pinout diagram for the IC is shown in ill. 14-11. Note that a number of other UARTs will work as well and that these chips might even have the same pinouts. The functions of the pins are listed in any UART spec sheet.

ill. 14-11. Pinout diagram.

For all their sophistication, however, UART chips are surprisingly inexpensive— under $10 or $13. They require accurate timing, however, which means the addition of a crystal and a baud-rate generator (the generator can be replaced by other circuits, but in the long run, the generator is a better choice). With all the components added, a UART system costs about $45.

You can arrange the UARTs in a number of ways. E.g., a computer such as the IBM PC has its own UART built into it. You can connect it to a laser modulator and send serial data through the light beam. You can either receive the data and process it through the serial port on the remote computer, or convert it to parallel form with a receiver UART.

ill. 14-12. Schematic diagram for UART transmitter.

Table 14-6. Transmitter UART Parts List

• IC1 AY3-1015D UART IC
• IC2 AY5-81 16 baud-rate generator
• R1 33 kohm resistor
• Q1 2N2222 transistor
• S1 SPST switch (momentary, normally closed)
• XTL1 3.57 MHz (colorburst) crystal
• All resistors are 5 to 10 percent tolerance, ¼ watt.

The Commodore 64 computer lacks a UART device and is ideal for the UART link used in this project. The link connects to the Commodore by way of the computer’s User Port. The User Port is a bidirectional parallel port, though this project uses it as an output device only—the UART connected to the Commodore 64 is a transmit-only device. You can convert to transceiver operation by rewiring it.

The circuit in ill. 14-12 shows how to connect the transmitter UART to the User Port of the “host” Commodore 64. The circuit, with a parts list provided in TABLE 14-6, is shown connected to the laser diode PFM modulator (introduced in the last section). Alternatively, you can connect the transmitter UART to an audio amplifier and transformer or transistor modulator to operate a helium-neon laser tube. The receiver UART (with amplifier), shown in ill. 14-13, can be used as a stand-alone remote-control device. Or, it can be connected to another computer, such as a Commodore 64, for the purpose of receiving signals from the host machine. See TABLE 14-7 for a list of parts for the receiver UART.

The output of the receiver UART is an 8-bit binary code. This code can be used to remotely control up to 256 functions. One way to operate up to 16 devices, such as solenoids, alarms, or motors, is shown in ill. 14-14. The UART is connected to a 4028 1-of-lO decoder. The first 10 digits (00000000 through 00001010) are decoded and applied to relays or opto-isolators where they can be used to drive any of a number of output devices.

How the UART Works

In the schematic in ill. 14-12, 8-bit parallel data from the Commodore 64 (or other computer) is routed to the data lines on the UART. When the computer is ready to send the byte, it pulses the STROBE line high (the line might be called DATA READY or something similar). The UART converts the data to serial format and sends it through the serial output (SO) pin. The speed of the data leaving the output is determined by the baud- rate generator. The C0M8116 dual baud-rate generator sets the speed of the transmission and reception, and it's hooked up here to be rather slow—about 300 baud. This means that the UART sends serial data at the rate of roughly 300 bits per second (equivalent to 30 bytes per second).

The receiving UART is connected almost in reverse to the transmitting UART. The receiver uses a baud-rate generator that's operating at the same frequency as the transmitter. The receiver is equipped with an IR photodetector. If you could see infrared light, you’d see the LED flash on and off very rapidly as the data passed. The ON and OFF periods are equal to 0’s and l’s, or “spaces” and “marks” as they are called in serial communications.

ill. 14-13. Schematic diagram for UART receiver.

Table 14-7. Receiver UART Parts List

• IC1 AY3-1015D UART IC
• IC2 AY5-81 16 baud-rate generator
• IC3 4049 CMOS hex inverter IC
• R1 22 kohm resistor
• R2 82 kohm resistor
• R3 100 kohm resistor
• C1 1 uF tantalum capacitor
• Q1 2N2222 transistor
• D1 1N914 diode
• S1 SPST switch (momentary, normally closed)
• XTL1 3.57 MHz (colorburst) crystal

All resistors are 5 to 10 percent tolerance, ¼ watt. All capacitors are 10 to 20 percent tolerance, rated 35 volts or more.

The amplified output is applied to the serial data pin on the UART. When an entire word is received, the UART places it on the parallel data output pins and pulses the DATA AVAILABLE pin. In this circuit, a short time delay is used to automatically reset the UART so it processes the next word.

ill. 14-14. Four lines from the UART receiver can be decoded into 10 lines using a 4028 IC.

ALTERNATE HE-NE LASER PHOTOSENSOR

Conventional photodiodes are engineered to be most sensitive to light wavelengths in the near-infrared region. That automatically reduces their effectiveness in receiving a modulated signal over the red beam of a helium-neon laser. This loss in sensitivity isn't a major concern in most laser communications links, but it can present problems if the laser-to-receiver distance is very long.

More importantly, an infrared-sensitive phototransistor or photodiode is susceptible to swamping by the infrared radiation of the sun. Even with red filters, it’s difficult to “tune out” the sun while receiving the modulated signal over the laser beam. A better approach is to use a red light-emitting diode. Though not specifically designed to detect light, an LED can be easily adapted for the purpose.

ill. 14-15. Ways to conned an undiffused red LED to an opamp. (A) Adjustable sensitivity; (B) Preset sensitivity.

Sensitivity in the red region of the visible spectrum is accentuated, because the LED is designed to emit red light. You obtain best results when using clear, non-diffused red LEDs. Sample hookup diagrams appear in ill. 14-15. In my tests, the white light and infrared content of a nearby desk lamp changed the output of the circuit by a few tenths of a volt. But shining a red He-Ne laser at the LED caused the output to swing to about a volt or two of the supply voltage.