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The Night Keeper Lighthouse By Andrew Woodfield The Night Keeper Lighthouse briefly lights up the darkness, to keep children’s dreams from running aground on dangerous shores. This is an excellent project for beginners; it’s easy to build, and you will learn several important aspects of electronic circuit theory. M any readers will have children or grandchildren who from time to time peer enquiringly at electronic parts and gizmos you’re working with on the bench. At moments like these, it’s useful to have a simple project available to encourage the next generation to take up the hobby. When my grandchildren were planning a visit recently, I was asked if I could help the 8-year-old build ‘something electronic’. Does this sound familiar? Searching for a suitable circuit suitable for children, it’s essential that they can build it reasonably quickly, before they lose interest. Equally, it should be useful enough to gain parental approval. I have had a blinking light circuit running on the shelf above my workbench for several years. I built it while testing some ideas for discrete high-efficiency boost power supplies. The “rat’s nest” of parts was built on a scrap of prototype board. These days, I use it for the occasional end-of-life 1.5V cell. It’s a simple way to use up the very last whiff of energy from such near-dead batteries. Rather than just building a blinking light, I thought I could make it a little more useful and exciting with a few simple improvements. First, I designed a printed circuit board (PCB) to make it easier for children (and parents, grandparents or caregivers) to build. That PCB allowed me to mimic a widely recognisable object, and make it more attractive and interesting. It also suggested a few other applications, 68 Silicon Chip which will be noted later. This, then, is the “Night Keeper”. Building it is well within the capabilities of a bright 10to 12-year-old, or perhaps even younger with some adult assistance. Since a soldering iron is required, they will need close adult supervision and a well-ventilated workspace. A kitchen table with a similar clear workspace of about one square metre is perfect; cover it with a cloth or some cardboard to protect the surface. Circuit description This simple and well-known oscillator circuit (shown in Fig.1) consists of two transistors, a white LED, and a few passive components. It brightly flashes the LED once every second for many months from a single 1.5V cell. Even a near-exhausted battery can power the LED for a month or two. The two transistors forming the heart of the device operate as a highly efficient regenerative oscillator. When power is first applied, the voltage on the base of Q1 (Va) begins to rise slowly as the 10MΩ resistor charges the 330nF capacitor from the battery. When Va reaches about 0.6V, the base-emitter junction of Q1, which acts much like a silicon diode, becomes forward-biased and begins to conduct. Meanwhile, the 10kΩ resistor has quickly charged the 100µF capacitor to close to the battery voltage. That’s about 1.5V for a new cell. This produces a voltage across LED1 (Vc) very close to 1.5V. However, LED1 cannot light up yet, because white LEDs need more than 2.5V to operate. Australia’s electronics magazine siliconchip.com.au      3.5V 6.3mA 2.8V 5.4mA 2.1V   4.5mA 1.4V 3.6mA 0.7V 2.7mA 0V SC  NIGHT KEEPER 1.8mA -0.7V Fig.1: the Night Keeper uses a two-transistor oscillator to drive a charge pump based on the 100µ µF electrolytic capacitor and the diode junction of white LED1. Once per second or so, the point labelled “Vc” will shoot up to around twice the battery voltage (about 3V), providing enough voltage to light the LED brightly for a few tens of milliseconds. 0.9mA -1.4V 0mA -2.1V -0.9mA -2.8V -1.8mA -3.5V -4.2V 4.8s 5.1s 5.4s 5.7s 6.0s 6.3s 6.6s 6.9s 7.2s 7.5s -2.7mA 7.8s As soon as Q1 begins to turn on, its increasing base- Fig.2: this simulation shows how the voltages at Va (cyan), Vb emitter current causes its collector current to rise still (green) and Vc (red) in Fig.1 change over time. Va ramps up, and faster due to the transistor’s current gain (beta or hFE) then all three voltages suddenly shoot up, at which point the being greater than unity. In turn, this results in Q2’s current through LED1 (blue) spikes, until the voltages drop and base-emitter junction starting to conduct too. The the process begins again. instant Q2 begins to conduct, voltage Vb starts to rise due to the current passing from Q2’s emitter to its collector. This causes Q2 to abruptly turn off too. The result is Vb Q2 amplifies Q1’s collector current still further, as a result suddenly falls from 1.5V to 0V. Va, via the 330nF capacitor, of its own current gain. The increasing voltage Vb causes then drops from 0.5V to -1V. Va to rise in ‘lock-step’ as the rise is coupled through the It goes negative because, just before Q1 and Q2 switch 330nF capacitor. This triggers a swift ‘avalanche’ effect off, Va is at around 0.5V while Vb is about 1.5V. So when through Q1 and Q2, causing them to both switch on fully Vb drops to 0V, that is coupled through the capacitor and as a result of their combined current gain. 0.5V – 1.5V = -1V. Consequently, the voltage at Vb rises suddenly and At this point, the entire cycle begins again. The result is abruptly up to the full battery voltage, around 1.5V with a very efficient regenerative oscillator which produces a a new cell. Since Vb is now suddenly at 1.5V, Vc rises in brief, but bright flash from the white LED about once every ‘lock-step’ via the 100µF capacitor to give about 3V at Vc. second or two. This is largely determined by the time the This is enough to forward-bias LED1, lighting it up. The 330nF capacitor takes to charge from -1V to about 0.6V via charge stored in the 100µF capacitor is then dumped into the 10MΩ resistor. LED1, giving a brief bright flash of light. Note that while the parts list suggests BC54x and BC55x This process is demonstrated in the simulation traces types, you could also use a 2N3904, 2N2222 or 2SC1815 shown in Fig.2. Va is shown in cyan, Vb in green and Vc in for the NPN transistor; and a 2N3906, 2N2907 or 2SA1015 red. The current through LED1 is in blue. You can see that for the PNP. Almost any pair of NPN and PNP transistors all three voltages rise rapidly at the same time, coinciding will work, but keep in mind that pinouts can vary. with the spike in LED1’s current. While LED1 is lit, the 330nF capacitor keeps Q1 switched Construction on and in doing so, discharges through its base-emitter If all of the parts are ready to hand, the Night Keeper junction. It manages to keep Q1 on for about 30ms. How- should take about an hour or so to build. Expect younger ever, as soon as Va falls below 0.6V, Q1 begins to turn off. children to take longer. Splitting the build into two parts, Parts list – Night Keeper Lighthouse 1 PCB, code 08110201, 64 x 91mm 1 BC547, BC548 or BC549 NPN transistor [Jaycar ZT2154 or Altronics Z1042] 1 BC557, BC558 or BC559 PNP transistor [Jaycar ZT2164 or Altronics Z1055] 1 5mm white high-brightness LED [Altronics Z0876E or Jaycar ZD0190] 1 100µF 16V electrolytic capacitor [Jaycar RE6130 or Altronics R5123] siliconchip.com.au 1 330nF MKT, ceramic or greencap capacitor (code 0.33, 330n or 334) 1 PCB-mount AA or AAA cell holder [AA: Altronics S5029 or Jaycar PH9203; AAA: Altronics S5051; Jaycar PH9261] Glue or double-sided foam tape to fix cell holder to back of main PCB Resistors (all 1/4W, 1% or 5%) (see overleaf for colour codes) 1 10MΩ Australia’s electronics magazine 1 10kΩ 2 1kΩ September 2020  69     +   Fig.3: the PCB is made of two parts, the lighthouse itself and its round base, complete with dangerous rocks! Snap or cut them apart before fitting the components where shown here.  Ratherthan attaching the cell holder via wire leads (as shown here, which you could do), we instead recommend mounting the holder on the back of the board. fitting the resistors and capacitors in one brief session and the remaining parts in a second, makes construction easier and suits the shorter attention spans of young children much better. Completing the project with the addition of the battery holder and base could be managed in a brief third session. The Night Keeper Lighthouse is built on a PCB coded 08110201, which measures 64 x 91mm. Before starting, snap or cut off the circular base from the side of the lighthouse, and file or sand both edges smooth. It’s a good idea to score along the cut line before snapping it. To do that, run a sharp knife along the line joining the small ‘mouse bite’ holes several times. Set the base aside for now, then refer to the PCB overlay diagram (Fig.3) and construction guide (Fig.4) to see which parts need to go where. All of the parts, except for the battery holder, mount on the top side (the side with the component outlines and part numbers), with their leads soldered on the opposite side. The battery holder is mounted the other way around, and that should be done last. Begin by fitting the four resistors, which can be identified by the coloured bands as shown. 1% resistors usually have five bands, while 5% resistors typically have four. Both possibilities are shown. Bend the legs of each resistor in turn with a pair of fine needle-nose pliers or a bending jig, so they neatly fit through the holes for each component in the PCB. Insert them, one by one, in turn, spreading the wire leads apart slightly to hold them in place. They can be fitted either way around. Turn the PCB over and solder both leads to the pads. Then trim off the leads flush with the solder joint using a pair of sharp side-cutters. 70 Silicon Chip Next, fit the 330nF capacitor. It may be either a mylar, MKT or ceramic type. Then install the electrolytic capacitor, and solder and trim the leads in the same manner. Make sure that the longer lead of the electro goes into the pad marked + on the PCB. The striped side of the can should be opposite the + symbol. Now it’s time to fit the two transistors. Q1 is an NPN transistor while Q2 is a PNP transistor. Each transistor must be fitted in the correct location. They are generally not pushed right down on the PCB, but rather, left with leads sticking out by about 5-10mm. This distance is not critical. You will probably find it helpful to spread the three leads of each transistor slightly apart before inserting them into the PCB, making sure the flat face is orientated as shown. Once you have pushed the leads through the PCB, spread them apart a little more on that side to hold them in place before inverting the PCB to solder them to the PCB. Again, trim the leads once soldering is completed. Now mount the white LED at the top of the board. It has a slight flat edge on one side. The LED should be inserted so this matches the shape printed on the PCB overlay for the LED. The longer anode lead will be on the opposite side to the flat. Carefully check that all of the parts are correctly located, and that all of the component leads have been soldered and trimmed. Check also that there are no solder splashes which would cause short circuits. The battery holder can then be mounted on the back of the PCB. A standard AA cell holder is sufficiently large that the end of the battery holder allows the lighthouse to sit it on the edge of a shelf or a book, as shown in the photo. The battery provides an ideal weight to hold the lighthouse vertical, useful for tight corners of a bedroom or office. The wire tails of some battery holders will fit precisely into the holes provided on the PCB. The positive (+) lead should go into the hole nearest the top of the PCB, adjacent to the LED. Other battery holder leads may need to be bent slightly to fit. Use a pair of needle-nosed pliers to bend the wires gently into the appropriate shape to fit neatly. Ideally, space the battery holder off the conductor-side of the PCB by about 3mm. This provides enough space to solder the two wire connections of the battery holder to the correct pads on the rear of the PCB. Attaching the base Alternatively, the circular base PCB can be added. This features a ‘rock-like’ overlay to add to the overall effect, and allows the Night Keeper to be placed on a flat surface. This part of the build may require additional adult assistance to complete – two hands to hold everything in the right place, the other two to apply solder and the soldering iron. Begin by briefly soldering two small ‘blobs’ of solder at each end of the lower tinned edge of the lighthouse PCB. Place this on the tinned strip located on the upper surface To join the two PCBs together, first “tack” them with solder and then run a bead of solder along the tinned copper tracks on the PCB. It won’t let go in a hurry! Australia’s electronics magazine siliconchip.com.au And here’s a side-on view showing the two boards soldered together and the battery holder in position. OK, we cheated a bit: we found that the stiff tinned wire was sufficient to hold it in place without glue or tape. You don’t have to solder the main PCB to the base: the weight of the AA battery holder will ensure it stays in place “hanging” over the edge of a bookshelf. of the circular base PCB. The main PCB should be approximately central and vertical on top of the base. Touch the soldering iron to the two ‘blobs’ of solder to ‘tack’ the two boards together. Repeat this if necessary, reapplying the soldering iron briefly to each tacking point while adjusting the main PCB slightly, until the main board is precisely vertical and centred on the base. Then apply further blobs of solder with the iron along the join, keeping the two boards in their final position. Finally, run the soldering iron down the tack seam to smooth the join and tidy its appearance. instead of the AA type. In that case, you can expect the cell to last closer to six months. The battery life you achieve will vary depending on the battery type (heavy-duty, alkaline etc) and on its condition when first inserted (new, slightly used, near-exhausted etc). Using the Lighthouse The Night Keeper makes a useful bright night-light for children. But keep in mind that flashing lights can disturb sleep, especially if they’re aimed at one’s face. Also, because of the brightness of some high-efficiency white LEDs, the Night Keeper should not be placed where the LED will shine directly into any young and especially sensitive eyes. It’s preferable to locate the Night Keeper lighthouse so that the LED light shines slightly upwards or at right-angles, perhaps onto an adjacent wall. Such arrangements are generally more effective for use as a night light anyway. Older constructors may find, as I did, that the Night Keeper can be useful for locating things in the night, for children and adults alike. Suitably mounted near a door, a light switch or placed on a shelf, it can help guide your way to a location or around furniture in the depths of the darkest of nights. SC Just like a real lighthouse! Operation Have you noticed that there’s no power switch? The circuit uses such a tiny current, a switch is unnecessary. The battery life in use is similar to that of the shelf-life of the battery. A new non-alkaline AA battery can run the Night Keeper for over a year. Hopefully, the faces of the new builders will light up as brightly as the Night Keeper just as soon as they insert the battery. As soon as the battery is inserted, the circuit will start to blink. Note that you could use a AAA battery holder and cell LED1 White LED Align flat on LED with PCB overlay 10k resistor, 5% or 1% Brown - Black - Orange - Gold or Brown - Black - Black - Red - Brown 10M resistor, 5% or 1% Brown - Black - Blue - Gold or Brown - Blk - Blk - Green - Brown BATT+ 100F electrolytic ‘can’ capacitor Align longer lead with PCB + (Stripe on opposite side from +) 1k resistor, 5% or 1% Brown - Black - Red - Gold or Brown - Black - Black - Brown - Brown + Q2 BC557 (PNP) Align shape with PCB overlay 557 330nF MKT capacitor Fit this capacitor either way Q1 BC547 (NPN) Align shape with PCB overlay 1k resistor, 5% or 1% Brown - Black - Red - Gold or Brown - Black - Black - Brown - Brown BATT- 547 Fig.4: in case it isn’t clear from Fig.3 which part goes where on the board, here is what each component looks like. Just follow the arrow to see where it goes. You can match up the part orientations to the drawings, too; the five components where orientation matters are LED1, Q1, Q2, the electrolytic (can-shaped) capacitor and the battery holder. The rest don’t care which way around they go. siliconchip.com.au Australia’s electronics magazine AA or AAA Cell Holder Glue or double-sided tape to the OTHER (copper) side of the PCB + (red) lead goes near LED1 - (black) lead goes to ‘Batt-’ September 2020  71