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CIRCUIT NOTEBOOK Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number. A reliable solar lighting system The steps to my shed do not all have the same tread depth and riser height. I am used to them, but I still have some difficulty on moonless nights. A few years ago, when solar-powered garden lights became available cheaply, I bought a few of them and poked them into the ground alongside the steps, and I could finally see the steps at night. The problem is, these things are only made to have a life of about 12 months if you’re lucky, and soon they needed repair or replacement. After going through a couple of repair/replacement iterations, I decided that I needed a more permanent solution. I had collected a few 18650-size Li-ion cells which still seemed to have some life left, so I decided to use these in a lighting system for the steps. I needed to mount the actual lights on something solid so they wouldn’t be destroyed by a brush-cutter. I mounted the LEDs in a length of rectangular steel tube from a discarded swing set. I mounted this so that it also formed a handrail for the steps using tubular steel supports concreted into the ground with welded brackets. The four LEDs came from a defunct garden light. Once they were mounted siliconchip.com.au to the underside of the handrail, I ran a cable into the shed. The accompanying circuit diagram shows the solar panel, battery charger and battery manager for the lights. The circuit switches on the lights at nighttime but switches them off once the battery is flat. The solar panel is a 10W, 12V type left over from a previous project, already mounted on the shed roof. The XL6009 buck-boost regulator module reduces the voltage from the solar panel to the 5V required to run the TP4056-based Li-ion battery charger. The low voltage cut-off circuit below prevents damage to the Li-ion cells from over-discharge. Op amp IC1a acts as a voltage comparator with the reference coming from VREF1, an LM385BZ. This device only requires tens of microamps to operate and produces an accurate 1.2V which is fed to pin 2 of IC1a. Its other input pin samples the battery voltage via a 120kW/100kW divider. When the battery voltage falls to 2.7V, the output of IC1a goes low and so NPN transistor Q2 switches off. The gate of Mosfet Q1 is then pulled up to its source voltage by the 100kW resis- Australia’s electronics magazine tor, so Q1 turns off, preventing current from flowing from the battery to the LEDs. About 200mV of hysteresis is built into the switching point by the 560kW positive feedback resistor. The LEDs are also switched off during the day due to the action of NPN transistor Q3. When the solar panel voltage rises above about 1.2V, Q3’s base-emitter junction is forwardbiased, and so it switches on, pulling the battery-related voltage at pin 3 of IC1a low, close to 0V. This is sensed by IC1a as if the battery is flat, so again it switches the LEDs off. The rest of the time (ie, when the battery is above 2.7V and the solar panel is in darkness), the LEDs are connected across the battery and so they light up. The garden light LED modules incorporate current-limiting resistors to prevent them burning out with a fully charged (4.2V) battery, not shown on the circuit diagram. Note that Q1 was chosen so that it would present a low channel resistance with its gate at -2.7V compared to its source, ie, just before the low-battery cutout activates. It must also have a sufficiently low on-resistance to avoid getting too hot (~20mW in this case). K. G., One Tree Hill, SA. ($80) January 2021  75