Electronic ballasts (EBs) for fluorescent lamps, also known as electronic control gear (ECG), have advantages over their conventional cousins: higher efficiency, flicker-free start-up, no 50 Hz (60 Hz) flicker and longer tube life. Moreover, they allow the light to be dimmed. Suitable EBs with a 1– 10 V analog control interface are available from all the usual manufacturers, including Osram and Philips. An internet search for ‘dimmable EB’ will turn up a large number of on-line sales outlets for the devices. For the purposes of this circuit EBs with a digital interface (known as DALI, for Digital Addressable Lighting Interface) are not suitable. Osram provides an excellent technical description of the 1–10 V interface on their website at . The interface provides an interference-proof DC voltage of up to 10 V, which, when loaded, delivers an essentially constant current of 0.6 mA: in other words, it is a constant current source with an open-circuit voltage of 10 V. If a resistor is connected across the interface then the lower its value, the lower will be the voltage across it, and this controls the dimming of the connected lamp. When the control input is open circuit and the voltage across it is 10 V, the lamp is driven at full brightness (100 % of nominal power). If the control input is shorted the control gear dims the lamp to 3 % of nominal power. Between 3 % and 100 % the behavior of the controller is logarithmic. The very simple circuit described here to drive the interface has several features that will be of particular interest to aquarium owners. The circuit is connected across the control input of the EB and therefore the control voltage appears across it. The brightness of the tube can be adjusted using P1.
S1 allows electrolytic capacitor C1 to be connected across P1: the charge current (0.6 mA) is very small and the capacitor very large (10000 μF) and so it charges very slowly. This means that the voltage across it, and hence the brightness of the fluorescent tube, will increase only slowly. The larger the value of C1, the slower the rate of brightness increase; with the suggested value the simulated sunrise takes around 12 minutes to complete. As can be seen, the circuit does not need its own power supply. When the EB is switched off C1 discharges into P1 (assuming S1 is closed); when it is next switched on the brightness of the tube will rise slowly as before. An optional extra is the circuit consisting of relay RE1 and resistor R1. If the contacts of RE1 close C1 will be slowly discharged into R1. The control voltage will fall gradually and the tube will slowly dim. The larger the value of R1, the slower the simulated sunset will be. When the contacts of RE1 are closed the value of R1 will also affect the maximum brightness that can be achieved by adjusting P1: the greater the value of R1, the higher the maximum brightness.
One possible arrangement is to plug the aquarium light into one timeswitch and drive RE1 from a mains adaptor plugged into a second timeswitch. The relay contact is made to close say 30 minutes before the first timeswitch turns the aquarium light off. When the simulated sunset is complete the relay contact can be allowed to open again.