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Build this infrared light beam relay This simple project allows you to monitor a doorway or a path using an infrared light beam. When someone walks through the beam, it triggers an alarm.for a 1-second period. By DARREN YATES These days, security is a big issue. If you're running a business, then you'll know that it's impossible to keep an eye on the door at all times. A properly installed door monitor will let you know that someone has entered the shop if you're working out the back and can serve as a deterrent to shoplifting. A light beam relay can also be used to monitor sensitive areas around a business or your home, particularly where young children are involved; eg, a computer room or the access gate to a swimming pool. The SILICON CHIP Light Beam Relay is based on two common ICs (an LM324 quad op amp and an NE555 timer) and can either sound a buzzer or trigger a relay. Apart from the relay and the infrared (IR) LEDs, you will probably already have most of the parts in your junkbox. Fig.1 shows how the project works in principle. The Light Beam Relay is placed one side of the doorway and uses three IR LEDs to transmit highpower bursts of light across the doorway to a mirror. This mirror then reflects the IR light pulses back to the unit where they are picked up by an IR photodiode. Block diagram The block diagram of Fig.2 s_h ows how this reflected signal is processed. First, the detected signal from the IR photodiode is amplified by IClb and fed through a 2.ZkHz bandpass filter to extract the wanted frequency. This filter attenuates signals at all other frequencies that may be floating around, to eliminate false triggering due to noise. From there, the signal is fed to a DC WALL i______T!_!1R~AN~SM~IT!....___ _ _ LIGHT-8EAM I RELAY _ _J ----RECl~-----1 REC Fig.1: how the Light Beam Relay works. A mirror mounted on the opposite side of the doorway reflects bursts of infrared light back to a detector for processing. If the beam ·is interrupted, the alarm sounds. x121 AMPLIFIER IC1c,d INFRARED DIODE 02 2.2kHZ BANDPASS VOLTAGE DOUBLER FILTER D3,D4 MONOSTABLE AND RELAY DRIVER IC2 Fig.2: this block diagram shows how the reflected infrared light is processed. 26 SILICON CI IIP amplifier stage consisting of IClc & ICld. This amplifier has an overall gain of 121 and amplifies the small signals from the IR diode (which are in the order of a few millivolts) to a series of pulses which swing to both supply rails. The following voltage doubler stage converts these pulses to a DC voltage which is then used to control a monostable. If the pulses disappear at any time, as when someone walks through the beam, the DC voltage on the monostable trigger input drops to OV. This triggers the monostable which then turns on the buzzer or relay. Circuit details Fig.3 shows the complete circuit diagram. As you can see, there isn't a great deal to it. ICla is one section of an LM324 quad op amp and is connected as a Schmitt trigger oscillator which has a frequency of about 2.ZkHz. Diode Dl provides the .0047µF capacitor with a fast charge-up path, which results in a very low duty cycle; ie, the output at pin 1 consists of very narrow positive-going pulses. Conversely, when the capacitor is discharging, Dl is reverse biased and current flows through the 220kQ resistor tu the low output at pin 1. The output ofICla drives transistor Ql which turns on three series connected IR transmitter diodes (LEDs 13). These LEDs are driven with about 400mA of-current when the transistor turns on. Normally, this would blow the LEDs but because they are only on for about 8% of the time, the current averages out to be only about 35mA, which the LEDs can handle quite comfortably. By doing this, we can get much greater IR light output and thus much greater range than ifwe ffild them with a steady DC voltage. Again, because the duty cycle is so low, we can get away with using a BC337 transistor "' • At.ARM TRIGGERED ~f . Ne.:, -3 .,.,.i!g cz X N ... ~ ~. UGHT BEAM RELAY The circuit is housed in a low-cost plastic zippy case, with the three IR LEDs mounted on one side. A hole drilled in the case about 50mm below the IR LEDs allows the reflected light to reach the photodiode. for Ql since it doesn't have to dissipate any heat and can handle currents up to 1A. DZ is a LT536AB or similar IR photodiode which picks up the pulses ofIR light from the transmitter section of the circuit. However, the voltage developed by the diode is only in the order of a few millivolts at most. For this reason, its output is fed to IClb which operates as a high gain amplifier by virtue of the 10MQ feedback resistor connected across it (ie, it has very little feedback). The signal is then passed through a simple bandpass filter formed around the 8.ZkQ and 10kQ resistors and the two .OlµF capacitors. Because the IR diode can detect IR light that is modulated at varying frequencies, the bandpass filter attenuates all unwanted frequencies and allows our frequency of interest (ie, 2.ZkHz) to pass through. IClc and ICld are both non-inverting DC amplifier stages with gains of 11 each. These simply multiply together to give an overall gain of 121 , as shown in block diagram Fig.2 . The output signal appears at pin 14 of ICld and swings between the two supply rails (ie, ground and +12V). Diodes D3 and D4 form the voltage ~ ..J --1.--11· w a: :lE <( w m 1::z:: (!) ::i I I I ~ g L - -.....,,.,,+-l,<lf4-,,,.J,. ,. i:c 'l / I ,______..........H•· ...... C, C, I a:--./ N:aa::5 c~c:e !:; en DECEMBER1991 27 the resistance in ohms and the C is the capacitance in Farads. Power supply The circuit runs from a standard 12VDC 300mA plugpack and no regulator is required. Supply bypassing on the PC board is provided by the 100µF and 0.1µF capacitors connected across the 12V rail. Construction A cardboard light barrier must be placed between the IR LEDs & the photodiode diode (D2), otherwise light leakage from the back of the LEDs will upset the circuit operation. The leads between the board and the IR LEDs are fed through a small notch cut into the top of the barrier. doubler stage and this converts the output signal at pin 14 of IC1d into a steady DC voltage. This voltage is proportional to the peak voltage of the pulses on pin 14 and is applied to the pin 2 trigger input of 555 timer stage ICZ. ICZ is wired as a 1-second monostable. Normally, when IR pulses are present, pin 2 of IC5 is held high and the monostable is held reset; ie, its pin 3 output is low and thus the buzzer (or relay) is off. Now consider what happens when someone breaks the path of the IR light beam. When this occurs, the signal voltage at all points in the receiver circuit drops to 0V and so the .047µF capacitor at the output of the voltage doubler quickly discharges via the parallel 100kQ resistor. As soon as the voltage on pin 2 of ICZ drops below 1/3Vcc, (ie, below 4V), ICZ is triggered and its output at pin 3 goes high. This then turns on the "Alarm Triggered" LED (LED 4) and sounds the alarm at the output. Diodes D5 and D6 protect the output stage of the 555 from large negative voltages which would otherwise occur when an inductive load (eg, a relay) is switched off. Switch S1 allows the circuit to be adjusted without having the buzzer constantly going off, or can be used to disable the buzzer when it isn't required. The 100kQ resistor and the 10µF capacitor on pins 6 & 7 of ICZ set the alarm period to about one second. This period can be easily adjusted to suit your particular application by changing either the resistor or the capacitor, or both. The alarm time in seconds is simply 1.1RC where the R is Most of the parts in the Light Beam Relay are mounted on a single PC board measuring 133 x 82mm and coded SC0311291. Fig.4 shows the wiring details. Before you begin construction, check the board carefully for any shorts or breaks in the tracks. If you find any, use a dash of solder or a utility knife where appropriate to fix the fault. When you're satisfied that everything is OK, start by soldering in the wire link and the resistors. If you're not sure about the resistor colour codes, check them on your multimeter before soldering them into circuit. The remaining components can now be installed on the PC board. Make sure that the diodes, transistor, ICs and electrolytic capacitors are all oriented correctly. The IR photodiode (DZ) should be mounted about 10mm proud of the board (see Fig.3 for the pinout details). If you use the equivalent BP104 CAPACITOR CODES 0 0 0 0 0 Value IEC Code EIA Code 0.1µF .047µF .01µF .0047µF 100n 47n 10n 4n7 104 473 103 472 RESISTOR COLOUR CODES 28 0 No. Value 4-Band Code 5-Band Code 0 0 0 0 0 1 8 3 2 220kQ 100kQ 10kQ 1kQ 15Q red red yellow gold brown black yellow gold brown black orange gold brown black red gold brown green black gold red red black orange brown brown black black orange brown brown black black red brown brown black black brown brown brown green black gold brown SILICO N C/111' S1 [DJJ Fig.5: this section shows how to wire in a relay instead of the buzzer. The switched output leads can be wired to a socket. Fig.4: make sure that photodiode D2 is correctly oriented when you install the parts on the PC board (see Fig.3 for pinout details). photodiode, first install a couple of PC stakes at the diode position. The diode can then be fitted by soldering its terminals to the PC stakes. If you intend using the buzzer, it can also be mounted on the board at this stage as shown in Fig.4. If you intend using the relay instead, then refer to Fig. 5 for the mounting details. Of course, you can use other devices here provided that you don't exceed the 555's maximum drive current rating of 200mA. The unit can now be made ready for testing by wiring up the LEDs and SC03112911 the DC power socket. Use 100mm lengths of hookup wire to connect the LEDs and 50mm lengths to connect the power socket but don 't worry about switch Sl at this stage. Note that LEDs 1-3 are all CQY89 infrared types, while LED 4 is a conventional red LED. Testing Before applying power, use your multimeter to check the polarity of the plug on your plugpack supply. You should find that the tip is the positive terminal. If it isn't, then you will either have to reverse the leach; to the plug or reverse the leads from the socket to the PC board. If you h ave any doubts about supply p olarity, the best procedure is to disconnect the leads from the socket, then apply power and use your multimeter to determine which terminal is positive and which is negative. You can then connect the supply leads from the PC board to the socket as appropriate. If you do get the supply leads mixed up, the two ICs will be destroyed at switch on. Once the connections are sorted out, apply power and aim one of the transmitting LEDs at the fro nt face of the photodiode . Check that the trigger LED turns on briefly and then goes out and stays out. If you now cover the photodiode , you sho ul d find that the trigger LED now lights up and stays lit. Thi s is because the circuit thinks that someone is standing permanently in Fig.6: check your PC hoard carefully against this fullsize artwork before installing any of the parts . Etched & drilled PC boards are available from the usual suppliers. Du:1,;,\ / BEil I WI l 29 PARTS LIST 1 PC board, code SC03112911, 133 x 82mm 1 front panel label , 93 x 155mm 1 plastic case, 160 x 90 x 55mm 1 SPST switch 1 3.5mm jack socket 8 PC stakes 1 12VDC buzzer (optional, see text) 1 12V single pole relay (optional, see text) 1 12VDC 300mA plugpack 4 LED mounting bezels Semiconductors 1 LM324 quad op amp (IC1) 1 NE555 timer IC (IC2) 1 BC337 NPN transistor (01) 1 1N914 signal diode (D1) 1 LT536AB or BP104 IR photodiode (D2) 4 1N4004 power diodes (D3-D6) 3 COY89 IR LEDs (LEDs 1-3) 1 5mm red LED (LED 4) + ALARM TRIGGERED Capacitors 1 470µF 35VW electrolytic 1 10µF 16VW electrolytic 3 0.1 µF 63VW 5mm fixed-pitch polyester 1 .047µF 63VW 5mm fixed-pitch polyester 2 .01 µF 63VW 5mm fixed -pitch polyester 1 .0047µF 63VW 5mm fixed-pitch polyester LIGHT BEAM RELAY Resistors (0.25W, 5%) 1 10MQ 1 8.2kQ 1 220kQ 2 1kQ 8 100kQ 1 15Q 0.5W 310kQ Miscellaneous Hookup wire, solder, screws, washers, nuts. front of the photodiode, blocking off the beam. If everything checks out so far, the circuit is working correctly and you can install it in a plastic case. The board will fit inside any of the available standard zippy cases which measure approximately 160 x 95 x 55mm. The board is mounted on the bottom of the case using machine screws and nuts. Use the board as a template for marking out the hole positions, then drill the hol es using a 3mm drill. This done, drill three holes in one side of case to accept the mounting bezels for the IR LEDs, plus another hole that aligns with the IR photo diode. Holes must also be drilled in either end of the case to accept the DC power socket and switch Sl. Finally, drill a 30 S ILICON CHIP circular pattern of small holes in the end of the case adjacent to the buzzer to allow the sound to escape. If you decide to use a relay instead of the buzzer, delete switch Sl and substitute a socket on the side of the case for the switched relay outputs. The PC board and the other hardware items can now be installed in the case. Note that the photodiode must be optically isolated from the transmitter LEDs, otherwise light leakage from the back of the LEDs will upset the circuit operation. This can be achieve d by installing a cardboard barrier inside the case. Make sure that this barrier is a tight fit and push it all the way down onto the PC board, as shown in the accompanying photograph. The leads between the board and the IR LEDs can be fed through a small notch cut into the top of the cardboard barrier. Setting up The easiest way of setting up the project is to mount it on one side of the doorway, opposite a small mirror. It's then simply a matter of adjusting the unit slightly until the mirror reflects the IR beam back to the photodiode (ie, the trigger LED goes out). It may take you a couple of attempts to find the optimum position but it shouldn't be too hard. Finally, switch the buzzer in and check that the unit triggers for a period of about one second if you momentarily interrupt the beam. Provided the unit is correctly adjusted, it should operate reliably over distances up to 1.5 metres. SC