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FIBRE OPTIC TWO-WAY INTERCOM Two assembled boards, two loudspeakers and a single optical fibre cable between them produce a duplex intercom which will give hands-free communication. By LEO SIMPSON Here’s your chance to experiment with fibre optic cable and circuitry. This communications link is full duplex, meaning both parties can talk at the same time, just as on the telephone. Two boards are linked by one optical fibre to provide a voice quality hands-free link. These days, optical fibres are widely used in telecommunications and in computer local area networks (LANs). In both of these cases though, the information transmitted is digital. But optical fibres will transmit analog signals just as well, as this project demonstrates. Two identical boards are used and each one accommodates one half of the duplex link. Each has an electret 14  Silicon Chip microphone and preamplifier driving a red LED which shines down the cable. At the other end of each respective cable is a Darlington phototran-sistor which receives the modulated light and turns it into a fluctuating DC signal which is amplified and fed to a small loudspeaker. No buttons or switches need to be pushed to speak. You just speak and you will be heard at the other end. Each PC board has a call button which you press to alert the party at the other end that you wish to “trip the light fantastic”. Each board can be run from a 9V battery or AC or DC plugpack. In fact, this fibre optic kit does not use ordinary LEDs or phototransistors. The red LED specified is actually a Motorola MFOE76 fibre optic emitter, in a special purpose housing designed to mate with low-cost (100 micron core) plastic fibre using the common FLCS connector. Similarly, the specified Darlington phototransistor is a Motorola MFOD73 photodetector, again intended to mate with plastic fibre via its integral FLCS connector. The really tricky part of this project is not even shown on the circuit of Fig.1. It involves combining the optical transmit and receive signals of one board into one cable and then separating the optic receive and transmit signals at the other end, on the second board. The two optical signals are combined in the optical equivalent of a directional coupler. This takes the form of a Y-piece with two short lengths of optical fibre cut at an acute angle and then joined and held together via a length of heatshrink tubing. In the tail of the Y-piece is a socket which accepts the common cable connection. Circuit description Fig.1 shows the circuit for one of the duplex channels but remember that the sender and receiver sections are actually on separate boards. The electret microphone is biased by the 22kΩ resistor and its audio signal is amplified by op amp IC1a which has a gain of 23. Its output signal is fed to IC1b which, together with transistor Q1, provides a current drive signal to LED1, the MFOE76. When the call pushbutton is press­ ed, capacitor C4 and resistor R7 apply positive feedback around IC1a so that it oscillates audibly. This becomes the calling tone, heard in the speaker at the other end. On the receiver side, phototransistor Q2 is AC-coupled to op amp IC1c, connected as a unity gain buffer. It drives volume control VR1 and then IC2, an LM386 power amplifier which drives the loudspeaker via a 47µF capacitor. Power to the circuit can come from a 9V or 12V battery, via diode D1 or via a 9V or 12V AC or DC plugpack, via bridge rectifier BR1. Battery operation is not recommended by the way – this circuit would “eat” batteries. The transmitter LED and the power amplifier both consume more current than can be economically provided by a standard 9V battery. Following diode D1 or bridge BR1, the DC supply is regulated to 9V by a Fig.1: the intercom is an analog-only circuit, with no digital processing. The electret microphone is amplified by IC1a, while IC1b and Q1 together provide current drive to the fibre optic emitter, LED1. The optical signal is sent down the cable to fibre optic detector Q2 and its associated audio amplifier IC2. May 1996  15 16  Silicon Chip Fig.2 (facing page): this diagram shows how the boards are connected together optically to provide the full duplex intercom. 78L09 three-terminal regulator. Note that one op amp in each LM324 is unused; pin 14 is connected to pin 13 while pin 12 is not connected. Construction As noted above, two identical PC boards are required for this project. Both should be assembled completely before making any optic fibre cable connections. Fig.3 shows the component layout. We suggest installing the resistors first, followed by the capacitors, diodes, transistors, three terminal regulators and trimpots. Do not confuse the three terminal regulators with the BC547 transistors; they come in the same TO-92 package. Make sure that all the electrolytic capacitors and semiconductors are installed the right way around. IC sockets are included in the kit and should be installed with the correct orientation; upside down with respect to the board labelling. Note that our published circuit (Fig.1) and PC board layout (Fig.3) are different from that indicated in the information supplied in the kit. Specifically, we have changed C2 from 0.1µF to 2.2µF, R8 from 100Ω to 1kΩ and R7 to 390kΩ. These changes must be made on both boards. Fibre optic connections Terminating the optical fibre cable Y-piece into the FLCS connectors can be a fiddly process if they have already been soldered to the PC boards. Therefore we suggest that the two legs of the supplied Y-piece be pushed into the respective FLCS connectors and the cylindrical sleeves screwed on; do not overtighten. This done, solder the FLCS connectors to the boards. The Y-piece should be anchored to the board with a wire link which is adjacent to the bridge rectifier, BR1. The link should be inserted through the board to anchor the Y-piece and the wire ends twisted underneath, not soldered. If they are soldered, there is a risk of heat damage to the Y-piece. Now you are ready to wire the two boards together, along with the speakers and AC plugpacks, as shown in the diagram of Fig.2. You will be supplied with a length of optical fibre and each end should be cut cleanly and squarely with a utility knife. Push one end of the cable into the Y-connector on one board and the other end into the remaining Y-connector on the other board. Note that each speaker should ideally be mounted in a small box to baffle it. Operating the speakers without any baffling gives a tinny sound, easily subject to overload. The speakers should also be kept as far as possible from the electret microphones, otherwise acoustic feedback, in the form of severe squealing, will result. Adjust volume control VR1 on each board for a comfortable listening level. Check that each call button produces a tone in the speaker for the other board. Now you can sit back and have hands-free communication via optical SC fibre! PARTS LIST for duplex link (two boards required) 2 PC boards (DIY kit 39) 1 length of plastic fibre optic cable 2 fibre-optic Y connectors (see text) 2 14-pin IC sockets 2 8-pin IC sockets 4 2-way PC-mount terminal blocks 2 2.1mm DC power sockets 2 momentary contact PC-mount pushbutton switches (S1) 2 76mm 8Ω loudspeakers Semiconductors 2 LM324 quad op amps (IC1) 2 LM386 power amplifiers (IC2) 2 BC547 NPN transistors (Q1) 2 MFOE76 fibre optic emitters (LED1) 2 MFOD73 fibre optic Darlington detectors (Q2) 2 78L09 3-terminal 9V regulators (REG1) 2 1N4148 diodes (D1) 2 W02 bridge rectifiers (BR1) 2 electret microphone inserts (MIC) Capacitors 2 100µF 25VW PC electrolytic 2 47µF 25VW PC electrolytic 10 10µF 25VW PC electrolytic 2 2.2µF 25VW PC electrolytic 4 0.1µF monolithic 4 .01µF monolithic or ceramic 2 .001µF ceramic Resistors (0.25W, 5%) 2  680kΩ 4  22kΩ 2  390kΩ 2  10kΩ 2  220kΩ 6  1kΩ 6  100kΩ 2 100kΩ preset trimpots (VR1) Kit availability Fig.3 (above): the parts layout. Note that the values we show for C2, R7 and R8 are different from those appearing on the boards supplied in the kit. This duplex fibre optic intercom is designed and produced by DIY Electronics, of Hong Kong. The kit is available in Australia from Ozitronics, 24 Ballandry Crescent, Greensborough, Vic 3088. Phone/ fax (03) 9434 3806. Their price for the kit is $116.85 plus $4.00 postage and packing. They also have a simplex kit (one way communication) priced at $41 plus $4.00 postage and packing. May 1996  17