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Rotating Light for Models Here’s a simple circuit that has various applications, such as for a model lighthouse, or as a siren on top of a model emergency vehicle. It sequences eight LEDs, using PWM brightness control, to form a pretty convincing imitation of a rotating light. T his project originated from a family member’s desire for a white revolving light atop a miniature lighthouse. Kits to build this sort of thing are available, but we hadn’t published such a circuit, and I thought it might have other uses. I also thought it could be done simply, on a tiny PCB. We could use a single logic IC if all we wanted was essentially a circular LED chaser. However, I have seen that approach used (eg, on garbage trucks); while eye-catching, it doesn’t provide a convincing illusion of rotation. Moreover, most digital logic ICs can’t deliver much current, meaning the LEDs wouldn’t be that bright without extra transistors. With a microcontroller, we can fade the LEDs in and out, creating a much more impressive effect, even with just eight LEDs at 45° intervals. We can also make it adjustable; not just the rate of rotation, but also the direction and the brightness/beam angle. We can even have multiple ‘beams’ by lighting opposite LEDs, as shown in Fig.1. Adjusting the beam angle effectively controls how many LEDs can be lit at once. It can range from just one (with varying brightness) up to almost all of them being lit at once, with just a dim spot rotating. If you build it with white LEDs, it’s suitable for a model lighthouse, and with a compact, black PCB that’s just 20mm in diameter, it will fit in most models unless they are tiny. If you want to make a siren, you could use amber, blue, red, yellow or green LEDs, or even unusual colours like cyan or pink (yes, they’re available). Fig.1: by fading in one LED at the edge of each beam and fading out the opposite one, we create the illusion of a smoothlyrotating light with just eight fixed LEDs. The ‘beam’ brightness and width varies depending on how many LEDs are lit at any given time. 58 Silicon Chip Australia's electronics magazine siliconchip.com.au Features & specifications » Simulates a rotating light with one or two ‘beams’ » Adjustable rotation speed, from about 10 rotations per second to 30 seconds per rotation (0.03Hz to 10Hz) » Reversible rotation direction » Adjustable beam brightness » Adjustable beam angle from 45° to nearly 360° » Runs from 5-12V DC, typically drawing 10-20mA » Compact circular PCB: 20mm in diameter and less than 10mm tall (with SMD LEDs) » Can use SMD (M3216/1206/SMA) or through-hole (3mm/5mm) LEDs » Use any colour LEDs Project by Nicholas Vinen Suppose you want a really special effect. In that case, you could build it with differently coloured LEDs, so the colour shifts as it rotates! It can run from a small DC supply from 5V up to about 12V, including most small battery packs, such as standard 9V batteries or 6V batteries of four AA/AAA cells. The current draw depends on the brightness, but it’s typically around 10mA. So four AA cells would power it for quite a while; possibly as long as two weeks for really high-capacity cells. Four AAAs might last 5-7 days. You can see a video of our prototype in operation at siliconchip.com. au/Videos/Rotating+Light Circuit details The complete Rotating Light circuit is shown in Fig.2. A 14-pin, 8-bit PIC16F15224 was chosen as it has just enough pins, is inexpensive and draws very little current. It can drive the LEDs directly with fairly decent brightness (its maximum per-pin current is 25mA). It’s also easy to program with the free version of Microchip’s XC8 compiler and MPLAB X IDE. The eight LEDs are connected to eight of its digital outputs via 68W current-­ l imiting resistors. Their anodes connect directly to the 5V rail, and they light when the microcontroller pulls that output pin low, to 0V. This configuration was selected as the micro’s output transistors are better at sinking current than sourcing it, as is typical. So they will deliver a higher maximum current like this. Assuming white or blue LEDs with a forward voltage of around 3.3V and a 5V supply, there will be around 1.7V (5V – 3.3V) across the combination of the 68W resistors and the micro’s output transistors. With a 3V supply, the data says that those output transistors can sink 10mA with a 0.6V saturation voltage, implying an output impedance of 60W (0.6V ÷ 10mA). It might be lower with the higher 5V supply voltage used in this circuit, but let’s use 60W as the worstcase value. That means the 1.7V is across 128W (68W + 60W), so we can expect the LEDs to be driven with a peak current of about 13mA (1.7V ÷ 128W). LEDs with a lower forward voltage, like red or amber, would receive more current, likely around 20mA. So the peak current is limited to a safe level. The microcontroller can control the average current using pulse-width modulation (PWM). One nice feature about this microcontroller is that its two PWM peripherals can be dynamically mapped to any of its I/O pins. So as the light ‘rotates’, we can determine the two edge LEDs and assign them to the PWM peripherals to dim them. The other LEDs are either fully off or full on, as determined by the states of the other digital outputs. That means that all the LEDs are controlled by hardware, with the software just needing to periodically recalculate which LEDs should be lit. It can then update the PORT and PWM registers to advance the rotating light to the next position. Two trimpots, VR1 & VR2, connect across the 5V supply with their wipers Fig.2: the circuit is little more than eight LEDs driven by the microcontroller via current-limiting resistors, two potentiometers to set the parameters and a simple linear power supply. siliconchip.com.au Australia's electronics magazine April 2025  59 Parts List – Rotating Light 1 double-sided black PCB coded 09101251, 20 × 20mm 8 high-brightness LEDs, 3mm/5mm through-hole or SMD (SMA, M3216/1206 or M2012/0805 size), colour to suit application 1 PIC16F15224-I/SL 8-bit micro programmed with 0910125A.HEX, SOIC-14 (IC1) 1 MCP1703AT-5002E/CB 5V 250mA low-dropout linear regulator, SOT-23 (REG1) 1 RB491D 20V 1A schottky diode, SOT-23 (D1) 2 1μF 16V X7R ceramic chip capacitors, M3216/1206 size 2 10kW TC33X-2-103E SMD trimpots (VR1, VR2) 1 5.1kW SMD chip resistor, M2012/0805 size 8 68W SMD chip resistors, M2012/0805 size 1 length of light duty figure-8 wire, to supply power 1 5-12V DC 100mA power source At upper right, the top side of the PCB is shown at actual size. The underside views of the SMD and through-hole versions of the Rotating Light project are shown enlarged. LED selection SMD LED kit (SC7462; $20 + postage) | TH LED kit (SC7463; $20 + postage) Both kits includes all the parts listed above, except the power supply and wire going to pins 8 & 11 of IC1. These are both analog-capable pins, so we can use the micro’s internal analog-to-­ digital converter (ADC) to measure these voltages. VR1 controls the speed & direction of ‘rotation’, while VR2 controls the beam width & brightness. Usually, you would put capacitors on these pins to keep the AC impedance low, making the ADC results more precise, but there isn’t a lot of room on the PCB, so we’ve left them off. We don’t need to make super accurate measurements, and we can compensate for the lack of capacitors either by tweaking the software or by eye when making the adjustments. In practice, we found that the ADC measurements were close enough to what you would expect based on the trimpot positions without these extra capacitors. The 5.1kW pull-up resistor on the MCLR pin (pin 4 of IC1) prevents spurious resets, while the 1µF capacitor across its supply pins provides 60 Silicon Chip If you’re going to power it from a regulated 5V supply like USB, you could omit REG1 and solder a bridge between its input and output pads. You could also bypass D1, or replace it with a 0W resistor, if you are certain that the supply polarity can’t be reversed. The maximum recommended supply voltage is 12V due to REG1’s absolute maximum rating of 16V. With a 12V supply and 50mA average current draw, REG1 will dissipate 350mW ([12V – 5V] × 0.05A), giving an expected temperature rise of nearly 120°C, which would put it close to its shutdown temperature of 150°C at an ambient temperature of just 30°C. The PCB draws enough heat away from REG1 that it’s unlikely to shut down unless the current draw exceeds 50mA. Still, if you intend to run the Light with a bright, wide beam, you’d be better off with a supply voltage below 12V; 6-9V would be ideal. If you manage to overheat REG1, it won’t be damaged; the light will just shut off and then restart when it cools down. bypassing for stability. The 5.1kW value is not critical; it could be 4.7kW, 10kW or another similar value. All that remains is the simple power supply. 5V low-dropout regulator REG1 ensures IC1 receives a steady and safe voltage, even if the incoming supply at CON1 varies. Schottky diode D1 prevents any damage from occurring if the supply polarity is accidentally reversed at CON1, while also having a modest (~0.3V) voltage drop. REG1 requires an input bypass and output filter capacitor for stability, so we have provided 1μF in both cases. That is the minimum value for unconditional stability on the output, and is more than enough for input bypassing. The circuit can be run from a 5V supply (eg, from USB), although the LED brightness will be reduced somewhat as IC1 and the LEDs will only have a supply of about 4.6V, ie, 5V minus D1’s forward voltage (~0.3V) and REG1’s dropout voltage (<100mV). Australia's electronics magazine The LEDs are arranged around the outside and can be through-hole (3mm or 5mm) or SMD types (M3216/1206 or M2012/0805). While side-emitting SMD LEDs exist, we reckon it’s easier just to use regular SMD LEDs and mount them on their sides, with the emitters facing out. That’s how we built our SMD prototype, shown in the photos. Some reasons we don’t think it’s worth getting side-emitting SMD LEDs are: 1. They are many times pricier than the normal top-emitting type. 2. They aren’t that much easier to solder than a top-emitting type facing sideways. 3. Many of them have a central pad for extra support that could short out the anode and cathode pads. 4. There are much more limited choices of size and colour compared to regular SMD LEDs. 5. Only the largest component sellers stock them. Through-hole LEDs can be soldered on either side of the board, while SMD LEDs have to go on the top. You could perhaps get away with soldering smaller SMD LEDs across the pads on the bottom if you have a particular reason to do that. siliconchip.com.au PCB design The PCB is circular with a 20mm diameter (10mm radius). By making it black, we can hide it inside models, so you only see the light when it’s on. In the middle of the top side of the PCB is the microcontroller, the two small SMD adjustment trimpots, one of the 1μF capacitors and the 5.1kW resistor. All the other components, like the LED current-limiting resistors and the remainder of the power supply, are in the middle of the underside. The power connections are two solder pads to which wires can be soldered from either side of the PCB, to suit the installation. Software The software (from siliconchip.au/ Shop/6/1837) is just under 200 lines of C code. The PIC runs at 8MHz with its internal Timer0 used to control the rotation speed of the light and Timer2 to run the PWM peripherals used for LED dimming. At power-up, it sets the pins as analog inputs and digital outputs as required. It then initialises the two timers and the ADC. The main loop waits for Timer0 to roll over, which happens every 4ms or so. Each time, it adds the rotation speed/direction to a 16-bit accumulator. It uses the accumulator value to calculate the brightness for the eight LEDs, then updates the output and PWM states. The 8-bit PWM runs at around 2kHz. When Timer0 rolls over, it also measures the voltages at the two analog inputs and applies a low-pass filter to remove noise and glitches from those readings. The new readings are used for future light update calculations. The code compiles to 1276 instruction words, taking up 2552 bytes of the 8kiB of the available flash memory (31.2%). The pro version of the XC8 compiler is not required. The critical part that generates the ‘rotating’ light is actually quite simple. If you mentally unwrap the circular light pattern into a line, you end up with a bi-directional chaser that ‘wraps around’ from one end to the other. The mathematics to calculate that, even with the LED brightness smoothly changing, is relatively simple. In twin-beam mode, with VR2 closer to the clockwise end than anti-clockwise, the chaser shifts so that there are two lit areas exactly four LEDs apart. Many lighthouses and sirens that use siliconchip.com.au actual rotating lights will emit light from both ends, so this mode better simulates that appearance. Construction The Rotating Light is built on a double-­ sided PCB coded 09101251 that measures 20 × 20mm. The top and bottom component overlay diagrams are in Fig.3, with two versions shown to suit SMD or through-hole LEDs. Refer to those during construction to see which parts go where. If you find the small board slides around while working on it, use a little Blu-Tack to temporarily stick it to your work surface. None of the components are terribly hard to solder individually. We found the main challenge to assembly was to avoid accidental bridges between adjacent pads because they are quite close together due to the small size of the PCB, especially the two trimpots and the two SOT-23 devices that mount side-by-side. So it’s best to feed in solder carefully and use the minimum necessary to form good fillets. The microcontroller IC has fairly widely spaced pins, on a 1.27mm pitch and other parts have larger or more widely spaced pins. So the actual soldering of individual components is not too difficult. As there are parts on both sides, once you have fitted the parts on one side, the PCB won’t easily sit flat and will tend to rock as you work on it. To deal with this, you can either use Blu-Tack as mentioned, or you could do what we did and place the PCB on top of a roll of solder-wicking braid. This has a hole in the middle for the components to fit in, so it can rest on its edges and sit flat. Of course, that depends on you having a similarly sized roll of braid to us, but it worked surprisingly well for us. Fig.3: the top side of the PCB has the microcontroller, both trimpots, one capacitor, one resistor and either SMD or through-hole LEDs, although TH LEDs can also be inserted from the bottom side. All the currentlimiting resistors are on the underside, along with most of the power supply. Australia's electronics magazine April 2025  61 There is no provision to program the microcontroller on the board, so you’ll need to either purchase a pre-­ programmed micro (from our Online Shop, either individually or in a kit), or program it yourself before soldering it. Our article on the PIC Programming Adaptor in the September 2023 issue (siliconchip.au/Article/15943) explains how you can do it. Once programmed, make sure you have identified pin 1 on the chip and lined it up with the marking on the PCB (very important!). Also check it against Fig.3, then tack-solder one pin. Adding a little flux paste will help the solder flow. Check the alignment of all the other pins with their pads (now is also a good time to double-check that pin 1 is in the right place!). If the positioning is not perfect, remelt the solder joint and gently nudge the chip into position. Once it’s located correctly, solder the diagonally opposite pin, then spread a little flux paste down both sides of the chip, over all the pins, and solder the remaining pins. You can drag solder them, or do them one at a time. If solder bridges have developed between any pins, clean them up by adding a dab of flux paste and then using a clean piece of solder wick to remove the excess solder. Clean off the flux residue with a suitable solvent, then inspect the pins under magnification to ensure all the solder joints are good (solder has flowed onto both the pin and pad) and no bridges remain. Solder the two trimpots similarly, being careful to avoid bridges to adjacent pads due to their proximity to IC pins and LED pads. We found the trimpots were the trickiest parts of all to solder because the pads didn’t stick out very far from underneath them. We’ve extended them in the final version of the PCB, but there was limited space available to do so. Add flux paste on both the PCB and component leads before soldering need to be careful to check that the solder has flowed down on the PCB pads before moving on to other components. With the trimpots soldered correctly, add the sole top-side capacitor and one resistor. Finally, if you are fitting SMD LEDs, you can do that now. Soldering the LEDs We recommend soldering standard SMD LEDs on their side, like in our photos. First, figure out which end of 62 Silicon Chip the LED is the cathode. You can do this with a DMM on diode test mode. When the LED lights up, the black probe is on the cathode. It must go to one of the pads marked “K” in Fig.3. Start soldering each SMD LED by adding solder to one of its pads. Due to the through-holes, you’ll need to add more than you might expect until you get sufficient solder on the top surface. You want a visible bulge so enough solder will reach the pad on the side of the LED, rather than the bottom as usual. The hardest part of soldering the SMD LEDs on their side was picking them up with the tweezers. We found the easiest way was to pick them up from the bench with one hand, rotate them on their side, then grab them with tweezers using the other hand. Make sure the tweezer tips don’t extend past the bottom of the LED or you won’t be able to get it to sit flat on the PCB. Once we had picked them up correctly, we found that soldering them wasn’t too hard. Position the LED with tweezers while keeping the solder on that initial pad molten with your soldering iron. Remove the iron for a few seconds to let it solidify, then check if the position is good. If it is, add a fillet to the other pad. The LEDs don’t need to be perfectly aligned but it helps if they are close. If you aren’t happy with the LED position, you can grab it again with the tweezers, reheat the initial joint and nudge it into place. Once both sides are soldered, you may need to add a dab of flux paste to the first pad and heat it to reflow the solder and form a nice, shiny fillet. With all the top-side components fitted, flip the board over and add the remaining SMDs, as shown in Fig.3. Don’t get D1 & REG1 mixed up. None of the other components are polarised. If using through-hole LEDs, bend their leads consistently and solder them in place now. You can insert them from either side of the PCB but make sure when you bend the leads that the shorter (cathode) lead will always go into a pad marked “K” in Fig.3. Now solder the power leads to their pads. They are marked with + and – symbols on one side of the PCB. You can solder them from either side. Testing If you have a current-limited supply, set it to 6V and 25-50mA. Otherwise, Australia's electronics magazine you could include a series resistor (eg, 100W 5W from a 12V supply) to limit the current in the event of a fault. Apply power and check the current flow. Depending on the trimpot positions, it should be around 10-20mA and should definitely not exceed 50mA. Verify that the LEDs light up and start to sequence. If the current draw is too high, switch off and inspect the board for faults, such as solder bridges between pads or pins, or incorrectly placed or orientated components. Perform similar checks if there is no current draw or nothing happens. Also check that all solder joints have been made correctly. If it operates but some LEDs don’t light, likely they are faulty, their solder joints are bad, or they are shorted to an adjacent pad. If it appears to be working, try adjusting VR1 & VR2 to verify that you can change the rotation speed, direction and beam brightness/ width as expected. We found that many of our Phillips head screwdrivers of various sizes failed to actually rotate the trimpot. We had to search around until we found a slotted screwdriver of the perfect size before we could get sufficient purchase. After that, we could make easy and precise adjustments. Usage With VR1 centred, rotation is very slow; if it is perfectly centred, the light will not rotate, or just barely. It ‘accelerates’ in either direction as you move towards the clockwise or anti-­ clockwise extremes. This gives a reasonable range of speed options without making the adjustment super fiddly. With VR2 centred, you will have a narrow (45°), dim beam. As you move it anti-clockwise, the beam will first start to brighten, then widen. At about halfway between anti-­clockwise and the centre, you will have a bright 45° beam. As you move closer to anti-clockwise, the beam will get wider and wider until it occupies almost the whole circumference. Rotating it from the centre clockwise is similar except that you will have two opposing 45° beams that get brighter, then wider. If you want to power this board from a USB supply, we have an upcoming article on USB Power Adaptors. You would need to join the two boards with a short length of light-duty twin lead or similar. SC siliconchip.com.au