When you want to power an electronic device from solar panels, broadly speaking there are currently two approaches. The first, very conventional method (described elsewhere in this double issue) consists of employing a combination of solar panels (or an array), an automatic charger, and a battery (or an array). This combination then powers the device concerned, which has its own voltage regulating circuits. The second, which we are proposing in this project, consists of building a ‘solar’ PSU directly. It is of course based on the same concept as the one described above, but having been designed for this purpose right from the start, the elements it composes are integrated to a higher extent, leading to improved efficiency.
Our suggested circuit is intended to power a number of current electronic devices directly, and can provide three different voltages: 3.3 V, 5 V or 12 V, depending on component selection; all at a current of 400 mA, which can even be increased to 1 A if necessary (details below). It’s primarily based around IC3, a high-performance switching regulator from Linear Technology. Depending on whether you choose an LT1300  or an LT1301 (*)  you will have a choice of two output voltages: 3.3 or 5 V for the former, and 5 or 12 V for the latter. For both ICs, the voltage is selected by fitting jumper S1 or not, as indicated in Table 1. Look carefully into the output voltages you will require and then select the appropriate ICs for the project. When jumper S2 is fitted, the output current of these ICs is internally limited to 400 mA. It can be increased to 1 A by omitting the jumper, but we don’t really recommend this as the rest of the circuit has been optimized for an output current from a few mA to 400 mA maximum. The primary power source is the NiMH rechargeable battery pack, which in the case of the LT1300 will comprise two 1.2 V cells, or three cells in the case of the LT1301. The solar panel should be chosen to deliver a voltage of the order of 9 V at an output current of around 100 mA. Such panels are available commercially. IC1 acts as a constant current charger to limit the current to around 60 mA.
To avoid overcharging the battery in the event of low current draw by the device powered on the one hand and constant sunshine on the other, the circuitry around IC2 and T1 has been added. IC2 is just a variable zener which will turn on T1 harder as the voltage at the wiper of P1 increases. In this way, when the voltage at the battery terminals rises too high, as at the end of charging, T1 will be turned on harder and harder, bypassing part or all of the charging current to ground via R5 and R7, and lighting the LED as it does so. This is simply a contemporary variation of the traditional shunt voltage regulator. The whole of the project fits easily onto a compact printed circuit board of which the component mounting plan reproduced here. The copper track layout is a free pdf download as usual. Building up the board should not present problems as there are no oddball components to solder or mount. An 8-pin DIL socket should be soldered in the IC3 position to allow fitting of one or the other of the intended ICs. If your usual retailer doesn’t have them in stock, you should be able to obtain them from mail order suppliers, for example, from Farnell. Take care choosing the choke L1 (22 μH for the LT1300 or 33 μH for the LT1301). It must be able to handle a current of 800 mA without saturating, which is far from being the case with many ordinary moulded types.
Our 22 μH one comes from Radiospares (RS Components) and is an ELC08D from Panasonic. It is vital that diodes D1 and D2 are Schottky types, to minimize forward voltage drop, and in the case of D2, to be fast enough in terms of recovery. AA or even AAA 1.2 V batteries will be suitable, given the impressive capacity of current types on the market. The circuit should work the moment it is powered; all that remains is to adjust potentiometer P1. To do this, temporarily disconnect the solar panel and batteries, replacing the latter with an adjustable stabilized power supply unit, across which you should also connect a voltmeter. If you are using the LT1300 version, i.e. with two 1.2 V cells, set your PSU to 3.2 volts and then adjust P1 to obtain definite illumination of the LED. If you use the LT1301 version (and hence three 1.2 V rechargeable cells), you’ll need to set your PSU to 4.8 V and again adjust P1 till the LED lights.