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Build this light beam relay extender This simple infrared transmitter circuit is designed to go with the Light Beam Relay project published in the December 1991 issue. It’s based on a 555 timer IC & will more than double the effective range. By DARREN YATES The Light Beam Relay published in our December 1991 issue has proven to be a popular project. In most applications, it is used to monitor a path or a doorway (eg, to a shop) using an invisible infrared light beam. When someone walks through the beam, it briefly sounds an alarm. To simplify construction, the original project housed the transmitter and receiver circuits in the one case. This meant that the infrared light from the transmitter had to be reflected back to the detector in the receiver using a mirror mounted on the opposite side of the doorway. But what if you want greater range, or a unit that can be moved to another location and quickly set up without critical alignment? The answer is to disable the internal transmitter circuit and use this external transmitter circuit (or Light Beam Relay Extender) instead. It uses a 555 timer IC and a transistor to pulse two IR LEDs at a frequency of about 2kHz. This external circuit increases the working range to about five metres –2.5 times that of the original. That’s mainly because the light no longer travels over a double path length and because scattering losses at the mirror are eliminated (since the mirror is no longer required). Further improvements in the range are derived from increas­ing the gain of the receiver and by moving the detector diode (D2) right up to its viewing hole in the side of the case. These last two modifications must not be applied to the original pro­ject however, as this would cause false triggering due to the close proximity of the IR LEDs and the detector. Refer now to Fig.1 for the circuit details. IC1 is a 555 timer and is connected as an astable oscillator. Its frequency of oscillation is about 2kHz, while the duty cycle of the output waveform at pin 3 is about 100:1. The output signal at pin 3 drives transistor Q1 via a 100Ω current limit­ ing resistor. Since Q1 is a PNP type, it only turns on during the narrow low-going pulses from pin 3 (ie, its duty cycle is about 1%). Each time Q1 turns on, about 200mA is pulsed through the two IR LEDs to turn them hard on. Although this may seem a very high current, the LEDs are only on for about 1% of the total time and so the current averages out to about 2mA which is well within their rating. Power for the circuit is derived from the same source that’s used to power the receiver (ie, a 12V DC plugpack). Diode D1 provides reverse polarity protection, while the 10µF capacitor provides supply line decoupling. Construction All the parts for the Light Beam Relay Extender are in­stalled on a small PC board coded 03106931. Fig.2 shows the parts layout. No particular order need be followed when installing the parts but take care to ensure that all polarised parts are correctly oriented. These D1 1N4004 68k 4 7 3.3k IC1 555 6 2  3 100 A IRLED1 B E C VIEWED FROM BELOW 10uF +12V C D1 .01 1 0.1 K IRLED2 IRLED1 100  Q1 A  2xCQY89 A LIGHT BEAM RELAY EXTENDER 38  Silicon Chip 0V Q1 BC327 E B 1 .01 47  3.3k 0.1 8 IC1 555 10 16VW 68k 12V K IRLED2  K 47W Fig.1 (left): the circuit uses astable oscillator IC1 (555) to pulse two IR LEDs on & off via driver stage Q1. Fig.2 (above) shows how the parts are installed on the PC board. Make sure that the LEDs are correctly oriented. K ALEX The UV People ETCH TANKS ● Bubble Etch ● Circulating LIGHT BOXES ● Portuvee 4 ● Portuvee 6 ● Dual Level TRIMMER ● Ideal PCB DRILL ● Toyo HiSpeed MATERIALS The PC board fits inside a small plastic utility case, with the two IR LEDs protruding through holes drilled in one end. ● PC Board: Riston, Dynachem ● 3M Label/Panel Stock ● Dynamark: Metal, Plastic ✸ AUSTRALIA’S NO.1 STOCKIST ✸ PARTS LIST 1 PC board, code 03106931, 56 x 41mm. 1 plastic case, 83 x 54 x 30mm Fig.3: the full-size etching pattern for the PC board. include the two IR lEDS, the semiconductors and the 10µF electrolytic capacitor. Mount the two LEDs at full lead length so that they can later be bent to protrude through one end of the case. A small plastic utility case is used to house the transmit­ter circuit. Drill two holes in one end for the LEDs plus four mounting holes in the base, then secure the PC board using ma­chine screws and nuts. Power for the transmitter circuit can be obtained by run­ ning a long lead back to the DC socket inside the receiver. This lead can be hidden by running it over the top of a doorway, for example. Alternatively, you can power the transmitter from a separate plugpack supply. Receiver modifications In order for the extender circuit to do its job, you need to disable the transmitter in the original project. This is done by removing the 100kΩ Semiconductors 1 NE555 timer IC (IC1) 1 BC327 PNP transistor (Q1) 1 1N4004 silicon diode (D1) 2 CQY89A infrared LEDs (IRLED1, IRLED2) K ALEX 40 Wallis Ave, East Ivanhoe 3079. Phone (03) 9497 3422, Fax (03) 9499 2381 Silicon Chip Binders Capacitors 1 10µF 16VW electrolytic 1 0.1µF MKT polyester 1 .01µF MKT polyester Resistors (1%, 0.25W) 1 68kΩ 1 100Ω 1 3.3kΩ 1 47Ω Miscellaneous Hook-up cable for power leads, machine screws & nuts. resistor between the +12V supply rail and pin 2 of IC1. If you are building the project from scratch, just leave out the transmitter components around IC1a. You should also connect pin 2 of IC1 to ground and connect pins 1 & 3 together. The gain of the receiver circuit is increased by reducing the 10kΩ resistor on pin 9 of IC1c to 1kΩ. Note that you can save a few dollars by transferring the IR LEDs to the external trans­mitter SC circuit. These beautifully-made binders will protect your copies of SILICON CHIP. They are made from a dis­tinctive 2-tone green vinyl & will look great on your bookshelf. Price: $A14.95 (incl. postage in Australia). NZ & PNG orders add $5 each for postage. Not available elsewhere. Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097. Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. July 1993  39