You’re doubtless familiar with the little solar-powered automatic lighting units found in DIY stores each year as summer approaches, sold in packs at ridiculously cheap prices. They certainly work, but their electronics and most of all their housings, manufactured for extreme cheapness, have a life expectancy that’s proportional to the purchase price… Our project here adopts a slightly different approach. It’s intended for use in conjunction with existing or still-to-be-built garden lighting systems, which in particular may be more powerful than the cheapo stuff mentioned above. The project described here cannot operate alone, but must be used in conjunction with the ‘Solar-powered Battery Charger’ project described elsewhere in this issue. This charger has a connector already provided to interface directly with the garden lighting systems described here. So the charger handles the ‘intelligent’ charging of the battery by the solar panels, while the circuit shown here takes care of the control of the lighting part. Naturally, it includes a photocell, in the form of an LDR (light dependent resistor), to measure the ambient light and, to avoid wasting the precious energy stored in the batteries, a presence detector so as to only light up when there is a need. Furthermore, the detector has a time delay, which makes the lighting unit very convenient in practice.
Given that it has to be used in conjunction with the solar-powered battery charger, the circuit is obviously very simple, as you can see from the schematic diagram. It uses just a single IC, a Microchip type 12C671 PIC microcontroller – i.e. the same type used in the charger, to make your buying easier. Let’s remember that this IC includes an analogue- digital convertor with several inputs, which is obviously going to be put to good use here. It’s powered from the stabilized 5 V supplied by the charger, via pins 3 and 4 of the connector provided for this purpose. Take a look for a moment at the charger circuit and note that, when it is used in conjunction with the automatic lighting system, the jumper between pins 1 and 2 of its connector has to be removed. This allows relay Re2 in the charger to be driven by our automatic lighting system, instead of directly by the charger itself. So, the load fed by the automatic charger here comprises the lamps or other lighting devices to be controlled. However, the excessive battery discharge protection is retained, as this information, from output GP4 of the charger’s 12C671, is fed to input GP4 of IC1 via pin 2 of the connector. This same input also receives override (optional) switch S1, which makes it possible to force the lighting off. Input GP3 also receives a switch making it possible to force the lighting on all the time, for example, when you want to admire or show off your garden at night, by overriding the presence detector circuit. The latter employs a ready-to-use offthe shelf module, since these days it’s no longer worthwhile nor sensible to build such a unit from scratch. It’s powered at 5 V and provides a logic high output when a presence is detected, which is connected to input GP3. Watch out! Different modules of this type currently on the market exist with various supply voltages and generating high or low levels during detection.
One module suitable for this application is available for example under reference PI8377 from Lextronic (www.lextronic.fr) where the author got it from. Some judicious online shopping may be in order to find a local equivalent The ambient light level is measured using an LDR connected to analogue input AN2, while adjustable potentiometers are connected to both inputs AN1 and AN0. Preset P2 allows adjustment of the day/night threshold according to the characteristics and positioning of the LDR used, while P1 allows adjustment of the duration of the lighting following presence detection, from a few seconds to around ten minutes or so. The program for the 12C671 PIC is of course available for free download from the Elektor website or from the author’s own website: www.tavernier-c.com. The project works immediately and only requires correct setting of P1 and P2 as indicated above. Finally, it should be noted that that before it can be used with the automatic charger described elsewhere in this Summer Circuits issue, the charger must first be adjusted on its own, as described in the relevant article. In other words, do not connect up the two projects as you will be struggling with an equation with at least two variables that may interact in unexpected ways.