Multimeter display mod project

[steve30]

I like bench top multimeters, and I got this nice little Thurlby 1603 for about £10 on ebay a few years ago. I had to do a minor repair and now it works fine.

But, it has an LCD display. This is quite slow, small, and hard to read, so I decided to mod it to use LEDs.

I have it working (though I may modify it further to improve it), but the LEDs are too big to go in the existing case. I also wouldn't mind adding some indicator lights for things like Volts, Amps and Ohms, AC/DC etc.

Should I replace the case with a bigger one? (If so, any recomendations?)

Or should I try and use much smaller LED displays and retain the existing case?

Here's some pictures:





Thanks

steve30

[alm]

Is the display isolated from the input terminals or do they share a common ground? The latter might make for a bad day if you measure some dangerous voltage and accidentally touch the display terminals, and would require more isolation between the display and the user.

I would not replace the case unless you are very mechanically inclined: getting the front panel right would be a lot of work, unless you can reuse the front panel. I would either try to move the display board back to create more space inside the case or make a smaller PCB with smaller 7 segment LEDs and surface mount resistors on the back. This board should fit through the hole for the LCD so the LEDs would stick out in front of the case. I would only do this if the digital circuit is connected to physical earth and completely isolated from the inputs. Getting a filter (with markings for annunciators) in front of the LEDs would be tricky in this case.

[steve30]

The display and inputs are common. Can't see that being a problem as ultimately, it will be enclosed in a case, so I doubt I will touch the terminals. I tend to use my analogue meter for high voltage anyway though.

I've just been looking at the available LED displays, and have found some nice small ones, which will fit in the front panel, and leave room for annunciators. I will have to make the annunciators myself as there currently aren't any, but I'm sure I'll manage it.

I'll probably be able to free up some room in the case as the (currently unused) battery holder is rather large.

[Unclegummers]

Put it in a hobby box. Mount it to the front of the meter.

[steve30]

I got some Avago HDSP-C363 displays from Farnell. They are nice size, and confirm that LEDs of this size should work nicely in the existing case. Unfortunately, they are too dim, or rather, require too much current to be used in the way I want, so I will be buying some brighter displays now.

[AndyC_772]

Are you trying to run LEDs from a product that's only designed to provide enough current for an LCD (ie. none)?

I'd be worried about the driver being overloaded. Can you not buffer the signals to take the load off the LCD driver and provide a bit more current to drive your LED display properly?

I'd personally mount the display in a separate little box, with the buffer / driver logic behind it, and stick the box over the space in the front panel of the instrument.

[steve30]

At the moment, yes, but fortunately, the LEDs in the photograph use next to no current.

I figured out how to buffer it and I think the board for that will fit in the back of the meter where the battery pack was, as I don't intend to run it from batteries. Then the LED board (with small LEDs) will fit nicely in the front.

[steve30]

I'm making some progress with this now.

I've used 74HCT86 XOR gates to buffer the LCD signals and to eliminate the flickering. I have some new kingbright LED displays which are slightly too big, but i can always cut a little bit out of the front panel.

The display looks nice. Unfortunately, I managed to fry about 10 of my ICs. So I'll wait till I get some more before I continue. Also unfortunately, either I fried the multimeter, or I wired something up wrong -- I have three decimal points and no measurement. Should be easy to sort out though.



Who likes wires? 

[sorin]

you can use parallel to serial converter to reduce the number of wire .

[mariush]

( f*@# i just realized I wrote something stupid about using resistor networks but they wouldn't work in your case)

As for all those green wires... IDE hard drive cables are cheap and they have 80 wires in them... that should be enough for you. If I remember correctly they holes are also spaced at 0.1" it would be great for your needs. Or you could just buy regular cable at meter lengths:

http://uk.farnell.com/amphenol-spectra-strip/191-3003-068/ribbon-cable-68way-per-m/dp/1207472
http://uk.farnell.com/amphenol-spectra-strip/191-2801-160/ribbon-cable-60way-per-m/dp/1207449
http://uk.farnell.com/amphenol-spectra-strip/135-2801-060/ribbon-cable-60way-per-m/dp/1207461

[steve30]

yeah, I just used what I had available. I always have a ribbon cable shortage here.

But I think I'm going to scrap this idea and may be try something different. This turned into more of an educational/proof of concept thing for me than actually being useful.

I did manage to learn quite a lot about how the multimeter works. One thing I really like about this meter is how it is all made in 4000 series chips and other bog standard stuff.