On Sun, 31 Aug 1997 15:11:16 -0700 Tim writes: >>I have been told that those watches use a chip Motorola designed >>a few years ago. It is able to raise the voltage delivered by >>the battery with an efficiency which is far better than the one of a >>coil based system to feed the EL backplane. > >>Just for notice, Casio offers now in most of its wrist watches >>a system similar to Indiglo called "Illuminator". > >I believe these EL systems use the parts from Sipex. These are a good >solution in an 8-pin SOIC with minimum external parts and decent >efficiency. > The made-in-China watch I have has a backlit LCD similar to Indiglo, maybe not quite as bright but adequate. The circuit is very conventional. It uses two SOT-23 transistors, a coil, and a diode. The transistors are driven by two outputs from the CMOS watch chip. When the light button is pressed, two outputs from the chip produce sequences similar to these: A: 1010101010101010101010101010101000000000000000000000000000000000 B: 0000000000000000000000000000000011111111111111111111111111111111 The frequency of A is I believe 32 KHz direct from the crystal. When output A is one the first transistor turns on. It pulls 3V across the coil. When A goes to 0, the transistor turns off, the coil volatge flies up, turns the diode on and is applied to the EL panel. After several cycles the voltage across the EL panel reaches about 60V. The only thing limiting the rise of voltage is the capacitance of the panel. Just energizing an EL panel with DC will not make it light up. It is necessary to use AC, preferably 400 to 1000 Hz. Therefore, the other transistor, controlled by output B, is used to discharge the panel back to 0V. I forget exactly how this is wired to prevent shorting out the battery through the coil and diode during the discharge phase. This is the gist of the circuit though. No exotic chips, etc. just a coil, 2 transistors, and a diode. The coil looked like the same type that boosts the voltage for the alarm beeper. I think the transistors were bipolar though they could be FETs. Of course a PIC could generate sequences A and B easily. I don't know if EL's can electrolyse like LCDs. If they do, leaving an EL on constantly with this type of drive may damage it due to the DC component. Adding another capacitor to the circuit to couple the voltage to the EL would solve that.