On 22/08/2011 16:16, Dave Plowman (News) wrote:
Bought a rather nice small LED voltmeter off Ebay for not a lot. It's a 3
digit type. For the old Rover which didn't have any battery indication -
intend fitting it in the middle of the rev-counter face. However, it's a
bit bright for night time. Any clues about dimming it? Can't get a data
sheet for it - came from the far east.
The PCB has a single processor and two regulators - one 5 volt and one
3.3v. I'd guess the latter is for the display. But being surface mount,
rather difficult to suss out the schematic.


LEDs normally have series resistors to limit the current, ( otherwise,
they represent almost a short-circuit in the forward direction! ) and
this controlls the brightness.

There may be a resistor bank somewhere near the displays.

THe current in the LEDs will be (supply voltage - 0.7 )/R
So dropping the voltage will also dim the displays.

However, that assumes that the displays have their own supply which does
not also supply ICs. If you drop the voltage to the ICs, then it will
stop working.

3.3 may well be the the IC supply, and the 5v for the display. You'd
need to trace it out on the PCB. If one supply does indeed supply the
displays only, then yes, you can rig up a dual-voltage arrangement, and
have the vehicle lighting toggle which voltage is used.

--
Ron