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SILICON CHIP Mini Projects #011 – by Tim Blythman This simple circuit causes an RGB LED to constantly shift between various colours using just three transistors and a handful of passives. No-IC Colour Shifter S ometimes, ICs and microcontrollers make things too easy. If you want to understand electronics better, using simpler components can help reveal how things work at a lower level. This circuit is one of the simplest versions of an ‘astable multivibrator’. That is a circuit that changes state continuously. Similar circuits form the basis of a bistable multivibrator, also known as a flip-flop or latch, the basis of many types of computer memory. So these types of circuits are all very important, even to modern digital technology. A ‘monostable multivibrator’ provides a single pulse of a known duration when it is activated. That is another similar circuit used where a timing feature is needed. Variations of this principle using valves or vacuum tubes date back to 1919, well before the invention of the integrated circuit (IC). We published a Circuit Notebook entry that uses the same principle (December 1995 issue; siliconchip.au/Article/6078). Three transistors, six resistors and three capacitors are all it takes to make an RGB LED flash and change colour. Fig.1 shows how we have laid it out on a breadboard, while Fig.2 shows the equivalent circuit diagram. You can see the layout in the photos and this video (siliconchip.au/link/abwi). It would be pretty straightforward to solder these components to a Jaycar HP9570 protoboard since it has much the same layout as the breadboard. We supplied 5V power by running some jumper wires from an Arduino board plugged into our computer, but you might have something else on hand to use. The circuit uses PNP transistors to allow us to use the common-­cathode version of the RGB LED module Figs.1 & 2: this shows how we laid out the components on a breadboard. If you leave off the capacitors and yellow wires, you’ll have three identical sections, each feeding one of the individual LEDs of the RGB LED module. The capacitors between stages are what cause the colour to shift constantly. You can see how it works a little more clearly in the circuit diagram. One capacitor charges until it switches on a stage that is currently off, and in doing so, switches another stage off. That causes it to cycle through three colours: cyan (blue + green), mauve (blue + red) and yellow (green + red). 68 Silicon Chip Australia's electronics magazine siliconchip.com.au (marked BRG−), which is what we got from our local Jaycar store. If you have a different version with a common anode (perhaps marked with something like BRG+ or BRGV), you can use NPN transistors (such as BC547s) instead. They have the same pinout (but opposite polarity), so they are placed the same way. In that case, the capacitors need to be reversed, as do the supply connections (the red and black wires) and positive and negative breadboard connections. The alternative breadboard layout is shown below in Fig.3 (note the slightly different labelling on the RGB LED module), while the resulting circuit is shown in Fig.4. In this version, all currents will flow in the opposite direction. Circuit details Imagine the circuit (Fig.2) without the capacitors connecting between the stages, which would have three identical but otherwise unconnected sections. You could simulate that on the breadboard by removing the yellow wires that link the stages. The 10kW resistors allow current to flow from the emitter of each transistor and out of the base to ground. This biases on the transistors and allows current to flow out of the collector, via the 220W resistors and one of the LEDs in the RGB LED module to ground, lighting it up. The RGB LED would appear white, as all three elements would be lit. Ensure the red wires go to your 5V supply and the black wires go to ground. We’ve used BC557 PNP transistors to make the circuit work with the common-cathode RGB LED module we purchased. You can build and test part of the circuit by fitting all the components and wiring shown here except the yellow wires and capacitors. Powering up the circuit at that stage lets you confirm that the RGB LED is working and shows a solid white colour. You can build the circuit like that, leaving out the yellow links and confirming that is what happens. It’s a good way to check that the wiring is correct so far. Now add the two shorter links and all three capacitors, then power on the circuit. The RGB LED might flicker but will settle back to a solid white. Adding the longer link should cause the RGB LED to cycle through the colours, changing about once per second. Before adding the link, all three collectors are near 5V since all the transistors are on. Adding the last link pulls the base of the right-hand transistor (Q3) up to 5V too, switching it off. The 10kW resistor slowly charges the associated capacitor until Q3’s base voltage drops far enough to allow it to switch on. However, Q3’s collector is connected to Q2’s base via another capacitor. So Q3 switching on causes Q2 to switch off. Now the middle 10kW resistor slowly charges up the next capacitor, and the cycle continues around the loop of three subcircuits. The colour Figs.3 & 4: this layout is similar to Fig.1 but suits a common-anode RGB LED module in case you come across one. The circuit at right is basically the same as in Fig.2 but flipped upside-down, with the PNP transistors switched to NPN and the polarised electrolytic capacitors reversed. siliconchip.com.au Australia's electronics magazine September 2024  69 Silicon Chip PDFs on USB ¯ A treasure trove of Silicon Chip magazines on a 32GB custom-made USB. ¯ Each USB is filled with a set of issues as PDFs – fully searchable and with a separate index – you just need a PDF viewer. ¯ Ordering the USB also provides you with download access for the relevant PDFs, once your order has been processed ¯ 10% off your order (not including postage cost) if you are currently subscribed to the magazine. ¯ Receive an extra discount If you already own digital copies of the magazine (in the block you are ordering). A top-down view of the finished Colour Shifter. Note the capacitor orientations; they are reversed on the version using NPN transistors (such as Jaycar ZT2152). showing on the RGB LED changes as it does. The cycle will start even if the circuit is powered on with all three links in place. That’s because there is enough variation in the component values to ensure that one transistor switches on before the others, which will start the cycle. You might have realised that the electrolytic capacitors will sometimes be reverse-biased, with the positive end actually being more negative. This will be at most around -0.7V (limited by the 0.7V across the transistor base-emitter junction). Reverse voltages below 1V are generally not a problem for electrolytic capacitors as the voltage is not high enough to affect the insulating oxide layer. Tweaks EACH BLOCK OF ISSUES COSTS $100 NOVEMBER 1987 – DECEMBER 1994 JANUARY 1995 – DECEMBER 1999 JANUARY 2000 – DECEMBER 2004 JANUARY 2005 – DECEMBER 2009 You could replace the RGB LED with three individual LEDs if you like, or even a discrete RGB LED like Jaycar’s ZD0270. If you do this, double-check the pinout and make sure the cathode or cathodes (for individual LEDs) all go to the black wire in Fig.1. You could even add some jumper wires to place the LED module fur- ther away from the main board. You can modify the capacitor values if you want to change the cycle speed. Higher capacitor values will slow the rate (as they take longer to charge and discharge), while lower values will speed it up. Experimentation You might be wondering if the circuit will work with more than three stages. We tried it with four & five stages and found that the cycle did not start reliably. If it did start, two or more impulses travelled around the loop! However, it works well with two stages. If you build the circuit without the third stage, you should see the two lights alternate, making it useful for something like a model railway level crossing. Earlier, we mentioned that devices like timers are closely related to this circuit, even though they have different functions. If you swap one of the capacitors with a wire link, the cycle will run until it stalls on one colour. If you remove that link, the colours will change a few times, then stop again, making it a very basic countSC down timer. Parts List – Colour Shifter (JMP011) WWW.SILICONCHIP.COM. AU/SHOP/DIGITAL_PDFS 1 30-row breadboard [Jaycar PB8820] 1 RGB LED module [Jaycar XC4428] 3 BC557 45V 100mA PNP transistors (Q1-Q3) [Jaycar ZT2164] 3 100μF 16V electrolytic capacitors [Jaycar RE6130] 3 10kW ½W 1% axial resistors [Jaycar RR0596] 3 220W ½W 1% axial resistors [Jaycar RR0556] 1 5V DC power supply (eg, USB/serial adaptor plugged into USB supply) assorted solid-core wire [Jaycar PB8850] 70 Australia's electronics magazine JANUARY 2010 – DECEMBER 2014 JANUARY 2015 – DECEMBER 2019 OR PAY $500 FOR ALL SIX (+ POST) Silicon Chip siliconchip.com.au