Backlight Hacking - Mastech MS8218 Multimeter

Back in 2012 I obtained a Mastech MS8218 multimeter. It is a decent multimeter, but the screen is hard to read under low light. Fortunately, it has a nice bright, even backlight, but you have to press a button to activate it, and it only comes on the a few seconds. This was irritating; I would put my probes down to press the button, pick the probes back up, fiddle around to take the measurement, and by the time I look at the screen, the light had gone off!

I needed something that was on all the time. I wired the two backlight LEDs up to the power switch (via a resistor), so they were on whenever the multimeter was on. This worked for a while, but due to the LEDs drawing 15mA, and the meter being powered only by AAA cells, the cells soon died. I needed something which used much less current.

To find out how to build a backlight from scratch, visit the BT Decor page.

Firstly, here are some photos of the multimeter with the original backlight, off and on.

Modding the backlight

Basically, I had to remove the existing surface mount whiteish/blueish LEDs, and replace them with some 'Superbright' green ones, available for 24p each from Spiratronics. These give much more light per milliamp than the original ones. I originally tried to install two LEDs, but one of them got in the way of the case, so I just used one.

Here, the new Superbright green LED is fitted into the existing backlight panel. The job of the backlight panel is to diffuse the light across the whole LCD. Wires were soldered on and heatshrunk.

A resistor was chosen to allow approximately 1mA to flow. This allows the LED to be bright enough to be seen in a variety of lighting conditions, but will affect the batteries much less than the previous LEDs. They are hooked up in series with the power switch, so the LED is on whenever the multimeter is on. Heatshrink was used for neatness and a help prevent any shorts.

The backlight panel can be seen working with the new LED here. As there is only one LED, there is a bit of a hotspot, but the panel still diffuses it well, and it is more than adequate for this application.

The following photo shows the screen all nicely lit up :). These things are hard to show properly on camera, but it is highly readable, both in the dark, and in light.