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I purchased a TekPower TP4000ZC multimeter, which has no backlight for the LCD.
I wish to add one myself, perhaps a single led at low current, driven directly off of the two AA batteries. I'll add a small toggle switch as well for saving the battery.

I'm wondering if there's any reason not to do so.
Could this end up altering a reading?
I know the multimeter has to accommodate a battery that diminishes in power anyway, so I'm assuming the small drop won't be an issue. However I'm worried there may be something I'm not considering, as multimeters measure several parameters with high sensitivity and have many components in close proximity. I could see some sort of tiny EMF or other phenomenon occurring right next to a component and changing a reading, making this a waste of time.

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I wouldn't expect that adding an LED would affect the meter's accuracy.

However, I would suggest trying the LED backlight idea first, in a temporary fashion, to make sure that you can really add an effective backlight. If the display was not designed to have a backlight, it may not be possible to add one.

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  • \$\begingroup\$ Peter is right here. LCDs that do not have backlight most often are configured as transflective where the ambient light is meant to reflect off a background material behind the LCD crystals in a way that allows the LCD image to be visible. Trying to add backlight to these will most likely not give a useful result. LCDs with backlight are most often configured as transmissive where the back light source shines from behind the LCD crystals making them visible. \$\endgroup\$ – Michael Karas Dec 31 '14 at 1:04
  • \$\begingroup\$ Indeed. However, if I shine an LED in through the side of the glass, and put foil on the back and sides, it often reflects enough. Another method is diffusing the back of the LCD with a something such as "diffuser" gel so that the light is evenly distributed. Anything is better than nothing when working in the dark with no extra hands. \$\endgroup\$ – Bort Dec 31 '14 at 1:16
  • \$\begingroup\$ @Bort: I'm just suggesting you test possible solutions before making any permanent (or non-reversable) modifications to your meter. \$\endgroup\$ – Peter Bennett Dec 31 '14 at 1:41

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