Fullscreen HTML5Med Res (??MB)HTML4

Build this simple 12/24V light chaser Looking for a simple circuit to sink your teeth into? This 12V light chaser has four separate channels, variable chase rate, fuse protection and uses just two ICs. By DARREN YATES Light chasers using light-emitting diodes (LEDs) or low voltage light bulbs have been around for ages. You don't have to look far for examples. If you go down to your local video store, chances are you'll see a line of flashing lights in the window. Believe it or not, our august publisher recently travelled in a "stretch-limo" which had little LED chasers running down the interior of the car. We wanted something a bit more potent than a LED chaser, so we designed this circuit to drive 12V light bulbs. There are four channels and each channel can handle up to 36W with only a modest amount of heatsinking. A single rotary control lets you vary the chase rate from about one flash every two seconds to four flashes per second. To make the circuit as versatile as possible, we also designed it to run from a 12V to 24V DC supply. This means that you can power it from the mains via a 12V DC plugpack supply, or you can use a 12V or 24V battery if no mains supply is available. A small PC board holds all the parts except for the external pot. If you don't need to vary the flash rate, the pot can be replaced by a fixed value resistor. So, if you want to jazz up the interior of your purple Monaro or panel van, this circuit is the one to go for. For 12V supplies, we recommend that you use the MES (miniature Edison screw) lamps which are rated at 3W. This means that you can use 12 lamps per channel, or a total of 48 lamps for the entire chaser. How it works If you look at the circuit diagram of Fig.1, you'll see that there are only a handful of components to the 12/24V Light Chaser. In fact, the circuit is based on two very common ICs - a 555 timer and a 4017 CMOS decade counter. Let's see how it works. ICl is the 555 timer and is connected as a simple variable frequency oscillator. Its output frequency is determined by the l0kQ and 1.8kQ fixed resistors, the lO0kQ potentiometer (VRl) and the l0µF electrolytic capacitor. The lO0kQ pot varies the frequency from about 0.5Hz to 4Hz. The output at pin 3 is a pulse waveform which is fed directly into the clock input (pin 14) of IC2 , a 4017 CMOS decade counter. On each rising edge of the incom ing clock pulses, one of 10 outputs of IC2 goes high in turn, starting at pin 3. Since we only need the first four outputs and not all 10, we connected output 4 (which is actually the fifth output since we count from zero) to the reset input. When this output goes high, it resets the chip and the first output at pin 3 goes high again. There is a finite delay as output 4 resets the IC but because we are operating at such a low frequency, it is almost instantaneous in effect. So the sequence of events is this: at switch on, the Q0 output (pin 3) is high. When the next clock pulse from ICl arrives, Q0 switches low and Ql switches high. At the next clock pulse, Ql switches low again and Q2 APRIL 1991 53 F1 5A 100U 10 + 16VWJ 16 IC1 555 14 i : : - - - - -""1 CLK 36W MAX IC2 4017 Fig.1: the circuit is based on two low-cost ICs. ICl is a 555 timer wired as an astable oscillator. This clocks decade counter IC2 which switches its outputs high in succession. These outputs then drive transistors Q1-Q4 to switch the lamps on and off. ~ 021"4_ _ _ _~2..,.2k~-------"if-1 15 RST 04 B0649 QJ;l'-7_ _ _ _ _ _..._2Y,.2k,.__ _ _ _ _---"t-t. BCE 13 ~ 12V/24V LIGHT CHASER switches high and so on until Q4 switches high and resets IC2. In other words, the transistors act like switches which are opened and closed one at a time in sequence. Each output ofIC2, at pins 3, 2, 4 & 7, is fed to the base of an NPN Darlington transistor (Ql-Q4) via a 2.2kQ resistor which limits the base current to about 3.5mA. Thus, as each output of IC2 goes high in turn , the corresponding Darlington transistor turns on and switches on the lamp(s) in its collector circuit. The Darlington transistors are TO TO BD649s. These can dissipate up to 60W with suitable heatsinking and also have a gain of at least 750 (but typically above 1000). This means that our 3.5mA of base current is turned into about 3A of collector current running through each transistor. However, since only one transistor is on at any one time, 3 amps is the maximum current drain of the circuit, so the load on a car battery is minimal. To protect the ICs from the spikes and surges that occur in most car electrical systems , we've included a 12V LAMPS-Fi-. q_ 8 E C~~ 100~1 N ~ ZD1 (t~ s1:c::;~SIS TO TO E C~ ~ N ' k IC2 Fig.2: follow this layout diagram carefully when installing the parts on the PC board. Fig.1 shows the pinouts for the four power transistors (Q1-Q4). The lOOQ resistor should be changed to lkQ 0.5W if a 24V supply is used. 54 SILICON CHIP zener diode across the supply line. This also regulates the supply rail to the ICs to +12V if a DC supply of greater than 12V is used. Added protection is provided by the 5 amp fuse which fits directly on the PC board. The circuit also has protection against reverse connection of the battery supply, although it may not be apparent. Each of the Darlington transistors incorporates a reverse-connected diode from collector to emitter and these will conduct if the supply is reversed, turning on all the lamps and (possibly) blowing the fuse. The two ICs are also protected against reversed supply by the 12V zener diode which acts as a forward biased diode if the supply polarity is wrong. Construction The 12V Light Chaser is built on a small PC board coded SC08106911 and measuring 105 x 67mm. Whether you buy or make the board, check that there are no shorts or breaks in the tracks. If you find any, use a small knife to scrape away the excess copper or a dash of solder to bridge the gap a~ required. Fig.2 shows the parts layout on the PC board. Begin by installing 12 PC stakes at the external wiring points, then solder in the wire links and the two fuse clips. Make sure that the fuse clips are M205 versions. These are smaller than the standard 3AG types. Next, install the resistors. Table 1 shows the resistor colour codes or better still, use your multimeter to check the values. Note that the 100Q resistor will have to be changed to lkQ 0.5W for 24V operation. The three capacitors can now be installed. Check the polarity of the two electrolytics carefully (they both face in the same direction). Now for the semiconductors: install the two ICs and the zener diode as shown in Fig.2, then mount the four power transistors (Ql-Q4). Check Fig.1 for the transistor pin connections. When they are installed on the board, their metal tabs should face the centre. The 100kQ pot is connected externally as shown on the wiring diagram. It can be replaced by a fixed value resistor if you don't need to vary the flash rate. If you like, you can mount the completed assembly inside a folded aluminium case. Each transistor should be fitted with a small TO-220 clip-on heatsink for loads greater than 12W. Make sure that the heatsinks don't short together. It's up to you how you wire up the lamps. If you are using the MES (miniature Edison screw) lamps, then matching MES sockets are probably the most convenient. However, these sockets cost as much as the lamps themselves and 48 at around 50 cents each is getting a bit expensive. As an alternative, you might also consider using 12V mini lamps which come with 10mm leads. These are more expensive than the MES lamps but may be more suitable for some applications. Make sure that the lamps are correctly rated for the voltage you are using. If you are using a 12V supply, then you can wire groups of 12V lamps in parallel. If a 24 V supply is to be used, you can still use 12V lamps but these should be wired in series pairs of two, with each pair then wired in parallel. PARTS LIST 1 PC board, code SC081 06911 , 105 x 67mm 10 PC pins 2 M205 PCB fuse clips 4 TO-220 clip-on heatsinks 1 aluminium box to suit 1 100kQ potentiometer (VR1) Semiconductors 1 NE555 timer (IC1) 1 4017 CMOS decade cou nter (IC2) 4 B0649 NPN Darlington transistors (01-04) 1 12V 1W zener diode (ZD1) Capacitors 2 10µF 16VW electrolytics 1 0.1 µF metallised polyester or ceramic (5mm lead pitch) Resistors (0 .25W, 5%) 1 10kQ 4 2.2kQ 1 1.8kQ 1 100Q Testing Once you've finished the board, you can install the fuse and power up the circuit. To do an initial test , use a LED and a lkQ resistor as the load for each transistor. If it is working cor- Miscellaneous Heavy duty cable, hookup wire, solder, etc rectly, each LED should come on in turn and the circuit should respond by either increasing or decreasing the chase rate as the 100kQ pot is rotated. If it doesn't work, check the board thoroughly for any solder splashes which may be causing shorts between the tracks. In particular, check between the IC pins. This done, check that all components have been installed correctly and that the correct value has been used at each location. Once the circuit is working with the initial LED load, you can then replace it with any load you choose up to 36W total for each transistor; ie, you could use 12 3W bulbs or six 6W bulbs, etc. SC Fig.3: here is the full size pattern for the PC board. Use it to check that your board has been correctly etched before mounting any of the parts. TABLE 1: RESISTOR COLOUR CODES 0 0 0 0 0 No. Value 4-Band Code (5%) 5-Band Code (1%) 1 4 1 1 10kQ 2.2kQ 1.8kQ 100Q brown black orange gold red red red gold brown grey red gold brown black brown gold brown black black red brown red red black brown brown brown grey black brown brown brown black black black brown APRIL 1991 55