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In the WEEE-k#1 follow up - testing the displays

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So I managed to get those displays tested and no luck on one of them and the other: the jury is still out to weather I have the right data on it, or if I'm barking up the wrong tree - or even barking the right questions.

Muttley Snickers:
With the first LCD in the same orientation run your finger down the right hand side edge of the display, if you feel a convex arch or bump then this edge is the indicator for pins one and forty, you will also see a difference when looking straight at the screen, I think it may be upside down.

The decimal points indicate that the orientation is correct but these displays are available in quite a number of segment configurations, minus symols, clock colons and so on. Flip the display, whack 470 ohm resistors on 1 and 40 back to ground and start probing the other pins with another 470 ohm in series with 3 volts, each individual segment should fire up so write them down.

If it doesn't fire up it could mean it's a common anode display and not a common cathode as pointed out in the PM I sent you, just reverse it if thats the case.

On your next video I expect to see the word nIcE on the display....... :)

Best Regards


Just read my messages - keep forgetting to do that! Another inbox to look at ::)

Yes - Doh! that could be why pin 1 & 40 are not connected and seems a sensible.

you want n1cE? - you got it! :-+

Bare glass LCDs don't really like DC bias - they will survive a short test but its not good for them.  Also the link between supposedly common pins is almost certainly Indium tin oxide which may well not be conductive enough for your multimeter's continuity range to register.

The best approach is to AC couple adjustable amplitude antiphase squarewaves of about 30 to 90 Hz to the segment and common or digit contacts you want to probe.  Start by providing a leakage path to ground for all pins e.g a 1M resistor per pin,  connect two pins to the antiphase signals and increase the amplitude from zero till one or more segments are visible.   If you see two segments, you've found two segment pins but haven't found the common or digit pin.  If you see lots of segments, you've found two separate common/digit pins. In either case turn down the amplitude till the segments are faint then move one probe about until a single segment appears with reasonable contrast.  Once you've got one segment, you should adjust for best contrast then start mapping out the display. 

Here's a fairly crude circuit using a quad OPAMP to generate adjustable antiphase drive waveforms for testing only. RV1 is a 10K pot for the amplitude and R12,R13 are actually the individual pin bias resistors, not part of the probing signal generator.

If you've got a square wave output signal generator that can go up to about 5V peak to peak, you can skip all that, and simply use 1K resistors and 470nF caps in series with each probe, and return all the bias resistors to the mid-point of a potential divider using 1K resistors across the signal generator output.

Application circuits:
For a  non-multiplexed bare glass LCD you can use a job lot of XOR gates to drive the segments and a 50Hz squarewave signal to drive the XORs and the common.  If the segment is in phase with the common, its off, and if antiphase (i.e. other input of XOR is '1'), its on.  The signals can be generated in a MCU on an I/O port rather than by XORs.  Care should be taken to keep the duty cycle at exactly 50:50 to minimise any residual DC bias.

Multiplexed bare glass LCDs are a *LOT* trickier to drive with multi-level stepped waveforms required, and although you *can* get there with divider chains, CMOS switches and OPAMP buffers, it makes far more sense to use a proper LCD driver chip or one integrated into a MCU.

Edit: Corrected the bias resistor designators in the description + some clarification.

I think I have read about this method a long, long time ago, you just reminded me.

So if I've got your attached circuit correct:

* U1A is basically creating a virtual earth point for the oscillator...
* ...U1B, which from my calculations is generating an approximately 30Hz square wave
* U1C is buffering (0o phase) whilst U1D is inverting the signal(180o phase) - hence anitphased signals
* R8A & R8B is the amplitude control, right? Which you've just used for SPICE simulation using the .step param
so R12 & R13 are the leakage path resistors right? cos I don't think I have 40 1M resistors! lol! ;D

I could build that, I have most of the parts (LT1014). Quite a few passives but all 80% are same values. Cheers Ian.M :-+


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