Fluke 8050A LCD display
replaced with LED seven-segment

Recently I was
able to cheaply buy a second hand Fluke 8050A made in the early eighties. It is
a nice 4.5 desktop digit meter, but a typical problem seems to be that the LCD
wears out and goes black. Replacement LCD displays are hard to find, not cheap
and probably lead to similar problems in due time.

The LCD display is
interfaced by standard components (CD4054 and CD4056) that drive each segment
individually. This allows it to be replaced with a LED 7-segment display.
However LCD displays are driven normal/inverted about 30 times/second. LED
displays can be used by disabling this clock and connect it to a fixed level
(high).

Since my Fluke
does not have the battery option I was pretty confident that the power supply
would be able to provide the additional LED current. The LCD drivers are able to
sink current per segment in the order of 2mA. Sinking capability is somewhat
higher than sourcing, so I decided on an LED display with common
anode.

Since I had no
idea how bright a LED display is with this amount of current, I ordered the
brightest 9mm displays I could find. If they turned out too bright I could
always reduce the current. And that is what happened. In the end a series
resistor of 2k2 showed proper results. With 1k the display was brighter, but
some bleeding to neighbouring segments started to become noticable.

The LCD display
also has a couple of special symbols for high-voltage(HV), low battery(BT),
decibel (dB) and relative measurement (REL). Of course these have to be handled
differently and I simply printed them as inverted text on paper. The paper is
then lit from behind with separate high bright LEDs. I blackened the
laserprinter output a bit more with a felt pen.

Another special
case is the plus (+) and minus (-) of the LCD display. Personally I do not see
the need for a plus symbol, so I decided to use the minus only since the LED can
provide this nicely. The most significant digit segment g is used for this
purpose. To “disconnect” it from the digit I tipped a small peace of the segment
with a black CD felt pen.

However the
behaviour of the +/- signals is a bit strange. I have no knowledge of how this
worked originally on the Fluke. After playing around what seemed to work in all
measuring modes (R/V/A/dB and REL combinations) is to use the inverted '-'
signal and disable the segment when '+' is high (??!!). Without using the '+'
signal, measuring ohms has the minus sign always on. Oh, well, this can all be
done using a single small BS170 mosfet.

Below is shown the
detailed images of the whole process, the circuit diagram of the display board
and the circuit modifications.

The new LED
displays in stages

The bad LCD display

Replaced it with 9mm LED displays and 3
LEDs

With printed symbols on paper

Paper is made more black with felt
pen

The selected LED
display details:

Kingbright
SA36-11SRWA

Red 9mm
display

Common
anode

Brightness
2200-9000 uCd @ 10mA

Conrad order code:
158780

Dimensions:

The Fluke 8050A
display board circuit (photo)

Circuit
modifications:

Note that each individual segment has a 2k2 resistor
between it and the driver pins. I soldered the cut resistors directly to each
driver IC and wired them from there to each segment. This was a bit more work
then I expected... The symbol LEDs have 1k in series. For a Fluke with the
battery option you would need an additional LED.

The
results:

Further ideas
RS-232 interace:

Now I understand
the circuit en have seen the signals on an oscilloscope, I was thinking of
making an isolated RS-232 interface to this meter. It is a desktop model that is
mains powered, so keeping it switched is not a problem. This would allow long
duration measurements like rechargable battery charge/discharge curves etcetera.
To isolate the circuit to be measured from the computer would also require an
opto isolated interface. It might never happen, but if it does I will update
this page.

Last updated
4-september-2005.