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To obtain the 12 volts needed to unlock (or lock) the flash I constructed a capacitive voltage multiplier. The circuit uses the 5 volt supply and is able to supply the current need by the circuitry on the main board. The circuit could be built on a small printed circuit board and mounted inside the Tuxscreen, with a 2 pole push button switch (switching both the 5V and 12V) exposed through the case, or it could be external with a connector wired to the pads under the SODIMM.

Here is the circuit

http://gateway.bedroom.gen.nz/~clive/vpump.png

R2 and R3 could be scaled up by a factor of 10 and C6 down by the same factor. The 555 is oscillating at about 35kHz.

D1 is a 1W (12V zener), but could be 0.5W. C4 can be larger that 1uF, but rated at least 16V.

Q1 is just a small NPN (switching) transistor.

A bypass capacitor (maybe 0.1uF) could be connected between pin 1 and 8 of the 555, just to keep noise off the 5V supply.

The circuit consumes about 3mA. All the resistors can be 1/8W or smaller! The diodes D2 through D5 could be small Schottky hot-carrier types.

The TLC555 is a low power cmos type, which should function with a supply voltge down to 3.3V. The 5V circuit could use a standard 555.

The 5V supply does not appear on the main board, so it would be better if the circuit was modified to be driven from a 3.3V supply (it may need a extra stage or 2 of diode and capacitor).

Before connecting the 12V of this circuit to your Tuxscreen, check that it is less than 12.5V and more than 11.5V!



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