What can be done, when two light bulbs in one light fitting are to be switched separately, but only one switch circuit is available? Simple: build a ‘smart chocolate block’ into the ceiling rose! The circuit is built from discrete components and with a bit of ingenuity can be fitted onto a printed circuit board measuring just a centimetre or two square. When light switch S1 is operated for the first time lamp La1, which is connected in the usual way, lights; La2 remains dark. Electrolytic capacitor C1 starts to charge via rectifier diode D1 and resistors R1 and R2 until zener diode D3 conducts, limiting the voltage to about 6.8 V. This voltage is used as a supply for the rest of the circuit. The second lamp is connected via a triac and a fuse (1.5 A, medium speed recommended). The triac is triggered by T4, which can only happen when T3 does not pull its base down to ground. The first time the circuit is switched on this is the case, as we shall see below. . Smart Chocolate Block Schema...