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Make sure your house is easy to find LED-LIGHT: THE HOUSE How would you like a house number that glows in the dark? Visitors would no longer have to peer through the murk with torches to find your abode. Your house number would welcome them with a cheery glow. By JOHN CLARKE & LEO SIMPSON Most homes are hard enough to find during the day when you can see the house numbers but at night it's a different matter. How many times have you peered through the dark trying to find a particular street number? It's even worse on wet nights when rain and darkness combine to make street . numbers virtually impossible to see, even if you have torch. Now, at least as far as your own home is concerned, you can do something about it. You can build the LED-Light, a street number that glows in the dark. It turns itself on automatically at dusk and then turns itself off again about 4.5 hours later. We've even incorporated an automatic brightness feature so that the LEDs (light emitting diodes) are brightest at dusk and then dim down significantly so that they are not too bright on the visitor's eyes. That might seem like gilding the lily but when you have upwards of 50 or more LEDs glowing in the dark they can seem painfully bright. But more of that later. The LED-Light is powered from the mains via a 12V AC plugpack rated at 300mA or more. Using low voltage AC means that you could mount the LED-Light on your letter box and bury the wires carrying the 12V supply directly beneath the soil. In some areas, mounting the LED-Light high up on your home's front wall might be a better idea, in case some light-fingered technofreak takes a fancy to it. Either way, running the wires to it will not be a problem. We built our prototype LED-Light into a standard plastic jiffy box measuring 198 x 113 x 63mm and the LEDs are all fitted onto the lid, as can_ be seen from the illustrations. A light dependent resistor (LDR) senses whether it is daylight or not and operates the circuit accordingly. Our prototype was made for one of our staff members who lives out in the sticks; hence Lot 14. The circuit A light dependent resistor senses the approach of darkness and turns the LEDs on for a preset time. Make the figures big enough so that they can be easily seen from the street. 64 SILICON CHIP You might think we've gone right over the top with the circuit for the LED-Light considering that it uses five ICs, four transistors and five diodes. Well, they're real cheap ICs so that is not a problem. The circuit can drive up to 64 LEDs which is enough to cater for 4 or 5-digit street numbers (or Lot or RSD numbers, if you're out in the bush). The circuit has three functions: light sensing, a long interval timer and a brightness modulator. Light sensing is provided by the light dependent resistor LDRl and ICl ; the long interval timer consists of IC2 and IC3, while the brightness modulator is IC5, Q3 and Q4. IC4 lets the three circuit functions work together, to determine whether the LEDs are on or off, bright or dim. The light dependent resistor NUMBER THAT GLOWS 01-04 4x1N4002 12VAC FROM PLUG-PACK + + 470 10 16VWr 16VW.:r ~t}' J .,. 100k +12V 4xRED LEDS PER ROW (64 LEDS TOTAL) 16 4011 10 CK 4 8 .,. .,. 10 + 2 16VW:r 3 +12V +12V LDR1 ORP12 .,. 8 56k IC1 555 2 + .,. .,. 15k B E~C VIEWED FROM BELOW ~- .,. 470k 10k .,. .o~I .,. LED-LIGHT HOUSE NUMBER Fig.I: the circuit can drive up to 64 LEDs which is enough to cater for 4 or 5-digit street numbers. LDRl and ICl provide the light level sensing, IC2 and IC3 form a long interval timer, and IC5, Q3 and Q4 vary the LED brightness. LDR1 and a series 33k0 resistor form a voltage divider which is monitored by pin 2 (and pin 4) of IC1, a 555 timer. When LDR1 is exposed to light, its resistance is low and therefore pins 2 and 4 of IC1 are held high. As the level of illumination decreases (like, when it gets dark) the resistance of LDR1 increases and so the voltage on pins 2 and 4 begins to fall. When the voltage on pin 2 drops to about 4V, IC1 's output at pin 3 goes high. It remains high for as long as pin 2 is at 4V or below. In pitch dark, the resistance of LDRt is likely to be several megohms which means that pins 2 and 4 of IC1 will be below 1V. Pin 3 of IC1 is connected to pins 12 and 13 of NAND gate IC4c and pin 6 of IC4b, another NAND gate. NAND gate IC4c functions simply as an inverter, so that when pin 3 of IC1 goes high, pin 11 of IC4c goes low, pulling the reset pin (11) of IC3 low too. This lets the timing interval start. It also turns on the LEDs via IC4b, Qt and Q2. We'll come back to IC4b later. Long interval timer IC2 is another 555 timer IC which is connected to oscillate at about 0.5Hz. This means that it produces a train of positive pulses at its pin 3; one pulse every two seconds. These pulses are fed to the clock input (pin 10) of IC3, a 14-stage divider. This OCT0BER1988 65 5-second delay set by the 470k0 resistor and 10µF capacitor connected to pins 6 and 7 of IC1. Brightness modulator The LEDs are inserted into tight-fitting boles in the lid of the case and wired in series groups of four. Our prototype used 55 LEDs but you will probably need a lot less than that for your house number. As noted above, the brightness of the LEDs is varied depending on whether it is dusk or pitch dark. At dusk, the LEDs are at maximum brightness and as it gets darker, they are dimmed down significantly. This function is performed by IC5a, a dual operational amplifier. IC5a monitors the voltage across LDR1 and its output at pin 1 is used to vary the voltage at pin 5 of IC5b which operates as a Schmitt trigger oscillator. It runs at up to about 2kHz (depending on the degree of darkness) and its pulse output drives transistor Q3 which drives Q4 and the LEDs. Thus IC5 controls the duty cycle of the LEDs. At dusk, when the LEDs are brightest, they are on 100% of the time. When it is pitch dark, they are on about 50% of the time. This is when they are being pulsed on and off at about 2kHz. Transistor Q4 switches on and off the positive supply to the LEDs and is driven by Q3 via oscillator IC5b. Power supply Power for the LED Light is derived from a 12VAC plugpack. This feeds a bridge rectifier consisting of diodes Dl to D4. The rectifier output is filtered with a 1000µF capacitor to supply the LEDs. For the remaining circuit, the filtered rectifier output is regulated with a 12V 1W zener diode, with further filtering provided by a 4,70µF capacitor. The 10µF capacitor is for extra supply bypassing adjacent to the ICs. The light dependent resistor (LDR) is mounted at full lead length on the PCB. When the PCB is later clipped into the place, the leads of the LDR are bent so that its face protrudes through a hole in the side of the box. divides the incoming frequency by 214 or 16,384. The result is that after about 4 hours 30 minutes the output at pin 3 (Q14) of IC3 goes high. The signal from pin 3 ofIC3 is fed to NAND gate IC4a, connected as an inverter. So when pin 3 of IC3 goes high, the output of IC4a, pin 3, goes low. This drives pin 5 of NAND gate IC4b and thereby turns off Qt, Q2 66 SILICON CHIP and the LED display. IC4a also drives pin 4 of IC2 which then inhibits any further oscillation. The output of IC3 stays high until next morning when light hits LDR1 and IC1 delivers a reset pulse to pin 11 (ofIC3). IC1 will not respond immediately if car headlights strike LDR1 for & few seconds. This is because of the Construction We built our LED-Light into a plastic utility case measuring 198 x 113 x 63mm. All the circuit components, with the exception of the LEDs, are mounted on a printed circuit board coded SC03-1-0988-1 and measuring 104 x 103mm. Assembly of the PCB is a simple matter of inserting the parts into the board and then soldering."Begin by installing the wire links, resistors and diodes. Then do the capacitors and transistors. PARTS LIST 1 plastic utility case, 198 x 113 x 63mm 1 PCB coded SC03-1 -0988-1 , 104 x 103mm 1 12VAC 1 A plug pack 64 5mm red LEDs (see text) 1 ORP12 cadmium sulphide cell (LDR1) Semiconductors 1 4020 14-stage ripple carry binary counter 1 4011 quad 2 input NAND gate 2 555 timers 1 LM358 dual op amp 1 BC558 PNP transistor 1 BC548 NPN transistor 1 BC338 NPN transistor 1 BC328 PNP transistor 4 1 N4002 1 A diodes 1 1 2V 1W zener diode Fig.2: here's how to mount the parts on the PCB. Because the LEDs are wired in groups of four, you might not need all of the 4 700 resistors shown. Note that the corresponding 4700 resistor must be increased to 8200 for a one or two-LED set, as outlined in the text. Note that the diodes, transistors, electrolytic capacitors and ICs must be oriented as shown on the overlay diagram. Don't bend the leads of the 1W zener diode, D5, close to its body to fit it into the holes on the board. This component can get warm depending on whether the LEDs are off (which means that the supply is lightly loaded) and whether the mains voltage is high. To enable D5 to cope with expansion when it does get warm, install it with an expansion loop at cine end. The light dependent resistor LDR1 is soldered into circuit with its leads sufficiently long so that it can be mounted with its face protruding from the the side of the box. Checking the circuit Before wiring all the LEDs into the circuit and installing the unit in the box, it is a good idea to check that the circuit board functions properly. If you check it out now it will be much easier than trying to troubleshoot it later. So leave off the rainbow cable connecting the LEDs until the following checkout procedure has been done. First check that all solder joints on the board are up to scratch and that there are no solder bridges between tracks or breaks in the tracks themselves. Then connect a 12V AC or DC plugpack capable of delivering 300 milliamps or more. You can use a 12V bench power supply if you like, as it will work just as well. If you are using a bench supply, adjust it to deliver about 14 volts which will cater for the voltage drop in the diode bridge (about 1.2 volts). Now use your multimeter to check voltages around the circuit. D5 should have close to 12 volts DC across it. With your multimeter's negative lead on the negative end of D5, check that + 12V appears at pin 8 of ICs 1, 2 and 5, at pin 14 of IC4, at pin 16 of IC3 and at the emitter lead of Qt. Now cover LDR1 to prevent it being exposed to light. If you have an analog multimeter (or a digital multimeter with an analog scale) you should be able to check that IC1 is oscillating at about 0.5Hz. Leave the meter set to the same DC voltage scale and place the positive Capacitors 1 1OOOµF 25VW PC electrolytic 1 4 70µF 16VW PC electrolytic 3 1 OµF 16VW PC electrolytic 1 .01 µF metallised polyester Resistors (0.25W, 5%) 2 X 470k0, 4 X 100k0, 1 X 56k0, 1 x 33k0, 1 x 15k0, 1 x 1 OkO, 2 x 5.6k0, 3 x 1 kO, 18 x 4700, 1 x 330 0 .5W Miscellaneous Solder, tinned copper wire, 300mm of 1 8-way rainbow cable. lead on pin 3 of IC1. The pointer should flick up the scale to 12V about once every two seconds. You check the frequency divider operation of IC3 in a similar manner. Place the negative lead on each output in turn. Pin 9 (the Qt output) should rise to 12V every four seconds or so; pin 7, every 32 seconds; and pin 5, every 64 seconds. You ·could check every available divider stage (not all stages have pin outputs) in this way if you wanted to but it would be very timeconsuming and a fairly pointless exercise. Once you've demonstrated that the early divider stages of IC3 are working you can be sure it is OK. OCTOBER 1988 67 Note that IC2 will oscillate whether or not LDR1 is exposed to light but IC3 will not work unless LDR1 is covered. This is because IC3's reset input, pin 11, is high (ie, at + 12V). You can check the action of LDR1, IC1 and IC4 by covering and uncovering LDRt. With LDR1 covered, pin 3 of IC3 and pins 12 and 13 of IC4 should be high. Pin 11 of IC4 and IC3 should be low. Five seconds after LDR1 is uncovered, all these points should change state;, ie, pin 3 of IC3 and pins 12 and 13 of IC4 should go low and so on. Having got this far, we've practically checked the whole circuit. Now wire one LED and a 10k0 resistor in series between one of the 4700 resistors and the common line from the collector of Q5 (this line is along one edge of the board, adjacent to the row of 4700 resistors). Note that the LED comes on when LDR1 is covered and goes out five seconds after it is uncovered. Good. Its brightness should also visibly dim as LDR1 is completely covered. I OOOIMIODD-GOOOOD-GO .,, I Fig.3: this is the full size pattern for the printed circuit board. Wiring the LEDs The LEDs are mounted on the lid of the case and can be arranged in any manner required. Up to 64 LEDs can be accommodated. We arranged the LEDs on our prototype close together but they can be spaced further apart for better readability at a distance. The LEDs are inserted into tightfitting holes in the lid. Wire the LEDs in series in sets of four with the anode of one connected to the cathode of another. If the last set of LEDs is not four, then the series resistor supplying that set will need to be altered. For three LEDs, the resistor can remain at 4700, for two and one LED sets, use an 8200 resistor. You can run the wires from the LEDs to the PCB using multi-way rainbow cable. A hole is required in the base of the case for leads from the 12VAC plugpack and another close-fitting hole is needed for LDR1. The unit is now ready for final testing. Connect up the power and test that the voltage across the zener diode is about 12V. Now test 68 SILICON CHIP Stout pieces of tinned copper wire can be used to connect the commoned anodes of the LEDs together. Keep the wiring tidy to avoid confusion. that the LEDs light when LDR1 is covered. They should remain lit for five seconds after LDR1 is again exposed. Finally, the case can be weatherproofed using silicone sealant over the cord entry and around the edges of the lid. Delete options Just like some new cars have "delete" options then so has this circuit. If you are not buying the kit you can save a few dollars by leaving out the automatic brightness feature. To do this, delete IC5, transistors Q3 and Q4 and the 10 resistors and .OlµF capacitor associated with these active components. A link should be wired in place of the collector and emitter leads to Q4. ~