In article ,
Ron Lowe wrote:
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.

I do have a circuit for a similar device - a Vellerman LED voltmeter.
There are no external series resistors on that - just a driver chip. The
common anode on each display is fed via a transistor controlled again by
the driver.

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.

The PCB is surface mount and fitted to the display. It is at least two
layers, so not possible to trace it out without removal.

There are very few external components. The 5 volt reg is much smaller
than the 3.3v one, so my guess is 5 volts for the IC, 3.3 for the display.
Since it's a blue display, 3.3 could be used to drive it with no current
limiting resistors?

My thought are to replace the 3.3 reg with a variable one and try a lower
voltage? There's plenty space to add an extra PCB in this installation.

Unless anyone has a better idea?

--
*Don't squat with your spurs on *

Dave Plowman London SW
To e-mail, change noise into sound.