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Problem:

I am thinking of using a chip that gives out 4-6V on 7-segment output pins but the LCD only wants 3-5V input, so I need to convert somehow.

The display is a TN type, drawing virtually no current. The LCD data specification is tiny and poor and does not actually mention current consumption, but a similar device's spec mention max 2 µA/cm2, and the screen is 4 cm2. Since that is for the whole device, it is approximately 0.3µA per pin (8µA / (4*7)). (Are my assumptions flawed here?)

The solution have to be cheap since it should be applied to 25 individual pins, and also preferably also not take up a lot of space. One obvious, simple solution is to use a resistor voltage divider (10k + 47k would transform to 3.3-4.9V).

Do I have any other option?

My first thought was to put a diode (or two) in series, but that will not work because the voltage drop over the diode will not be that great for so low currents, right?

Since the LCD will draw so little current, would there be any problem using 100k + 470k instead of 10k + 47k?

Or can I just skip doing anything and feed the signals directly to the LCD, hoping it will be ok?

Project details:

Building a combined voltmeter/ampere meter similar to this and this, but using an ICL7106 instead of ICL7107.

Device specs: ICL7106, display (similar display).

Dave Tweed
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hlovdal
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  • Rather than trying to mess around with interfacing why not just find a suitable/compatible display e.g. http://www.futurlec.com/LED/35LCD.shtml (3 1/2 digit display, 3 - 6V) – JIm Dearden Jun 29 '13 at 05:39
  • Yes, that is certainly an optimal solution. I did try to shop around at several places to find such one, but I do not have a good overview of where to buy electronic components, so I did not find anything. But thank you, I might quite possibly use that one. – hlovdal Jun 29 '13 at 11:05

1 Answers1

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If the LCD is the "wide voltage" version which can handle up to 5V, simply run both the LCD and the positive rail of the ADC from 5V.

If it's the 3V version, then this is probably not a good idea (although the LCD datasheet is typically unhelpful on range/abs max details) and either dropping the ADC positive rail to 3V (if possible, I haven't checked the datasheet to see if this is in spec) or changing ADC to a lower voltage version would be options.
The other option as you mention is to use voltage dividers (or level shifter ICs, etc) on the outputs of the ADC to drop the 0-5V down to 0-3V. This is cheap and easy but will add a lot of components to your board.

Unless you are absolutely stuck on using this LCD and ADC combo, I would probably look at making the LCD and ADC run from the same rail (i.e. swapping one or both if necessary)

Oli Glaser
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  • The LCD is "wide", but the ICL7106 is supplied with 9V, and can go down to minimum 6V, so running both at 5V is not possible (actually there is no supply voltage on the LCD, there is only input pins). – hlovdal Jun 28 '13 at 21:08
  • Are you using a single supply for the ICL7106? – Oli Glaser Jun 28 '13 at 21:21
  • Yes, either a 9V wall DC converter or battery. – hlovdal Jun 28 '13 at 21:22
  • Okay, it says on page 8 that the ICL7106 generates an internal digital ground using a Zener, which I think is intended for the digital output returns. I have only skippeds through quickly, but I think it is the BP (backplane pin) You can confirm the voltage is ~5V by testing between the digital outputs and the BP pin with a multimeter/scope. – Oli Glaser Jun 28 '13 at 21:28
  • I have not bought the devices, so I cannot test yet. I am currently researching to see if it is a feasible design. Adding 10 SIL resistor nets of 5 resistors each will be a bit of a hassle (and I intend to build several units). – hlovdal Jun 28 '13 at 21:35
  • I see - I think it's certainly a feasible design, but you will have to read the datasheet carefully - there are a few examples of different ways of powering the chip at the end of the datasheet. On page 10 it says it can be run from a single +5V supply with an external reference if desired. – Oli Glaser Jun 28 '13 at 21:38
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    The single +5V supply option is only applicable for ICL7107. The data sheet is combined for both ICL7106 and ICL7107 so yes, it has to be read very carefully ;) – hlovdal Jun 28 '13 at 21:48
  • Whoops, you are right missed that :-) Still I think it's feasible providing the digital ground is within range - I would probably grab one of the chips to test initially, but if you don't wish to purchase first, satisfy yourself the datasheet confirms this. I can't see anywhere it mentions voltage levels for the digital outs re internal ground, but it should do somewhere. – Oli Glaser Jun 28 '13 at 21:54