shop window lighting

An expensive item in any shop- keeper"s budget is the cost of illumi- nating the window display. One method of reducing costs is to keep the number of lights down to a minimum. Unfortunately, this could well discourage nocturnal window shoppers. Some time ago an important retailer specialising in domestic appliances came up with an economical alterna- tive. The shop window lighting was installed in such a way that the passer by was able to switch it on himself. After a certain period of time the lights would then go out automati- cally. This system appears to be an interesting prospect, but installation is by no means cheap. The outside switch would have to be vandal proof and water proof at least and this is expensive to start with. Hardly cutting costs! The circuit described here is also operated by the window shopper, but without using an external switch. A proximity sensor fitted inside the window controls the lighting via a triac. The drawing in figure 1 shows a temperature c...