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Here’s a project to really catch your eye! It uses 10 light emitting diodes (LEDs) & flashes them around in a clockwise direction. By DARREN YATES I F YOU’RE new to electronics then you’re probably looking for a sim ple but eye-catching project to build that won’t cost the earth. This LED chaser uses 10 LEDs which flash in rotation around the outer edge of the board. It’s ideal for shop front ‘attention-grabbing’ displays or if you just want to learn more about digital electronics and have some fun along the way. Light chasers have been around for a long time. Originally they used a motor-driven rotary switch and incandescent lamps but these days you can do it quite simply with an IC or two and some light emitting diodes (or LEDs). Light chasers can have a variety of different ways in which the patterns of light chase around a loop. In this design, we have two lights chasing around a loop of 10 LEDs. The rate at which they run can be varied simply by adjusting a trimpot. How it works The circuit shown in Fig.1 has two ICs, five transistors and 10 LEDs. IC1 is a 555 timer connected up as an astable mul­tivibrator or oscillator. Its frequency is controlled by the 100kΩ trimmer potentiometer (or ‘trimpot’ for short), the 22kΩ and 2.2kΩ resistors and the 2.2µF capacitor. The frequency of this oscillator is worked out from the following formula: Frequency = 1.44/[(R1 + (2 x R2))C1] where R1 = the value of the trimpot + 2.2kΩ; R2 = 22kΩ; and C3 = 2.2µF. The output of the 555 is taken from pin 3 and this pulse waveform is con- BUILD THIS SIMPLE LED CHASER 62  Silicon Chip A 100 16VW 2.2k A A  A  A    6V LED1-10 100k VR1 4 7 22k 3 IC1 555 6 2  16 8 14 CLK 0 0.1 1 15 1 IC2 4017 10k RST 2 470 B 3 Q2 BC547 B C Q3 BC547 10k 7 470 C E 5 K Q1 BC547 10k 4 B C E K 4 CLK EN 13 10k 10  K 470 470 E B A 2 B  K C E 2.2 25VW E C VIEWED FROM BELOW 3 10k  K 470 5 1  K Fig.1: the circuit uses 555 timer IC1 to clock IC2, a 4017 decade counter. Its outputs each go high in turn & control LED driver transistors Q1-Q5. When output ‘5’ goes high, the counter is reset & so the sequence is continually repeated. Q4 BC547 B C E 8 Q5 BC547 SIMPLE LED CHASER nected to the clock input at pin 14 of IC2. This is a CMOS (Complementary Metal-Oxide Silicon) 4017 Johnson decade counter. This IC has 10 outputs, each of which go high in turn. While these 10 outputs are available to drive LEDs, we’ve used only the first five outputs; ie, those labelled ‘0’ to ‘4’. Reset pin Most Johnson counters also come with a RESET pin. In normal operation, this pin is held low but when it is taken high, it resets the counter to its initial condition with the ‘0’ output high and all of the others low. In our circuit, you’ll see that we’ve connected the ‘5’ output back to the RESET input. What happens now is that each output will cycle through from ‘0’ to ‘4’ but when the next rising edge appears at pin 14, the ‘5’ output goes high and this goes straight to the RESET input. The counter resets itself and sends the ‘0’ output high and the counter cycles around again. Even though there is a small delay while the ‘5’ output resets the circuit, it happens so quickly that it is not notice­able. This same principle works with the other outputs as well. If you connect output ‘7’ to the RESET input, the output cycle would be 0-1-2-3-4-5-60-1-2 and so on. Each of the five outputs we’ve used is connected via a 10kΩ current limiting resistor to the base of a BC547 NPN transistor. When one of the outputs goes high, it turns on its associated transistor which in turn switches on the two LEDs connected in series with its collector. This continues for each output and its associated transistor. By mounting the two LEDs connected to each transistor diagonally opposite each other, we can make it look as though there are 10 separate PARTS LIST 1 PC board, code 08103941, 133 x 82mm 4 stick-on rubber feet 1 100kΩ trimpot 1 6V lantern battery, Eveready 509 or equivalent Semiconductors 1 NE555 timer (IC1) 1 4017 Johnson counter (IC2) 5 BC547 NPN transistors (Q1-Q5) 10 5mm red LEDs (LED1-10) Capacitors 1 100µF 16VW electrolytic 1 2.2µF 25VW electrolytic 1 0.1µF 63VW MKT polyester Resistors (1%, 0.25W) 1 22kΩ 1 2.2kΩ 5 10kΩ 5 470Ω Miscellaneous Tinned copper wire, solder, battery clips. outputs with the two LEDs diagonally op­posite chasing each other. If you take a quick look at the overlay diagram, you’ll see that the LEDs are set around the PC board in an oval shape. If you run the chaser in a dark room, you will see the oval shape appear as the LEDs chase each other. Power is supplied by a 6V battery and we suggest you use a lantern battery; eg, Eveready 509 or equivalent. They are rela­tively cheap and can last for years when used at low currents. Construction All of the components for the LED chaser are installed on a PC board measuring 133 x 82mm and coded 08103941. Before you begin construction, check the copper side of the board for any shorts or breaks between tracks. If you find any, they should be repaired before you proceed further. Start off by installing the wire links. Make sure that you make them as straight as possible. Use the overlay wiring diagram of Fig.2 to make sure that they go in the correct position. After the wire links, you can continue by installing the resistors and the trimpot. Again, make sure that you install them in the correct location. Next up, you can solder in the ICs, transistors and the capacitors, followed by the PC pins. Be careful not to apply too much heat to the ICs and transistors or you may damage them. The last job is to install the LEDs. It’s important that they go in the March 1994  63 LED2 LED9 A 100uF K A LED4 A K K BATTERY 2.2k IC1 555 Testing 1 Q1 Q2 Q3 Q4 470  22k 470  K 470  A 470  A LED6 VR1 470  LED7 0.1 correct way around. Use the overlay diagram and the pictorial diagram of the LEDs on the circuit to check which way they go. Lastly, make up a set of clip leads for the supply to the 6V battery. Make the negative lead from black wire and the posi­tive lead from red wire. K Q5 2.2uF 10k 10k 10k 10k 10k LED5 LED8 Now that you’ve finished the construction, A K A K IC2 switch your multi­ meter 4017 to a low current range (say 1 200mA) and connect it in series between the battery LED3 LED1 LED10 and the circuit. A A A K K K As soon as you make a complete connection, you should see the LEDs jump into life, with opposite LEDs lighting up in turn. The current consumption should be less than 10mA. If the LEDs fail to light, disconnect the battery and check your board thor­ oughly against the overlay diagram for any possible errors. In particular, make sure that the ICs, transistors & LEDs are correctly oriented. Note that if you inadvertently connect the battery around the wrong way, there is unlikely to be any damage since it’s only 6V. If you find that the circuit appears to be working but two LEDs opposite Fig.2 (top): install the parts as shown here, taking care to ensure correct polarity of the ICs each other fail to light, & LEDs. Trimpot VR1 adjusts the chaser speed. Fig.3 above shows the full-size PC pattern. check to make sure that they are both correctly installed. You may find that one of them pot VR1. If you want the LEDs to go a board in a box or simply put some is installed the wrong way around. lot faster, reduce the 2.2µF capacitor rubber feet at the corners as we have done and amaze your friends with your Note that you can change the speed to 1µF. SC of the chasing LEDs by adjusting trimTo finish off, you can install the PC new-found knowledge. RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ No. 1 5 1 5 64  Silicon Chip Value 22kΩ 10kΩ 2.2kΩ 470Ω 4-Band Code (1%) red red orange brown brown black orange brown red red red brown yellow violet brown brown 5-Band Code (1%) red red black red brown brown black black red brown red red black brown brown yellow violet black black brown