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2 x 16 lcd display problem

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 27 June, 2012 » 2 x 16 lcd display problem « Previous Next »

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noel
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Username: noel

Post Number: 122
Registered: 06-2005

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Posted on Wednesday, 13 June, 2012 - 01:54 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I have a 2 x 16 lcd display, manufactured by powertip, farnell stock no. 1671498, which I am using in a project.
The display is not working, so I had a look at the datasheet, which appears to indicate the need for a 'negative' voltage to pin 3 of the display. Is this correct ?
I would be grateful if someone could take a look at the attachment (diagram on bottom of sheet) and tell me how to connect pins 2 & 3 properly.

Thank you.
Noel.
application/pdfExtract from datasheet
Farnell 1671498 display.pdf (70.8 k)
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epithumia
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Post Number: 886
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Posted on Wednesday, 13 June, 2012 - 02:32 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Noel,

If I didn't have a negative voltage available I think I'd start by using ground for the negative voltage, adjusting the pot and seeing if it worked.

Then maybe try up to -5V* for the negative voltage and try that. A 3 to 4.5V battery with + connected to ground would do the trick.

(* -5V because the limit is -10V with respect to Vdd, rather than ground)


If that works, I'd consider using a chip such as *7660* (e.g. Farnell 9762680) to convert your 5V supply to -5V for the final circuit.
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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vlf
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Post Number: 334
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Posted on Wednesday, 13 June, 2012 - 02:57 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi noel... this is not my direct field of experience but, some odd kits I have made up just for the low cost of their parts, and display in the standard and current 16x1 displays just show the pot between ground and Vcc pin 2, with the wiper of pot, to pin 3.

However having said that, many years ago it was a requirement to provide a negative voltage for the older displays.

Here is a USA Kit, using a 16x1 display URL:

http://electronics-diy.com/70v_pic_voltmeter_amperemeter.php

Also here 16x2 display circuit:

http://electronics-diy.com/schematics/70V_PIC_Voltmeter.jpg

Although this might further confuse you !

Regards, Brian L.
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zeitghost
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Post Number: 1720
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Posted on Wednesday, 13 June, 2012 - 03:16 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Some of those displays require Vdd - 6V to get reasonable contrast.

Which really is a bit of a pain.

One of my circuits had a single AA glued on to add an extra -1.5V to the contrast voltage just so I could read what it was saying.
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james
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Post Number: 64
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Posted on Wednesday, 13 June, 2012 - 06:35 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi all,

Geoff Graham's GPS Car Computer (EPE Jan 2012) gives an interesting solution to this sort of problem by jacking up the LCD's Vdd supply voltage slightly. This gives a slightly higher -ve voltage available with respect to Vdd for pin 3, the Vcon pin. Apparently the extra -0.2V gained is sufficient for the LCD display, (which Geoff uses in this design), to operate correctly and the higher Vdd is still within spec'.

Description of solution on page 14.

Cheers

James
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gordon
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Post Number: 816
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Posted on Wednesday, 13 June, 2012 - 09:29 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Most of the 2x16 LCD displays are pretty standard, and normally have the pot across the normal 5V supply that is used for the contrast. Could be worth double checking all the connection pins, I miscounted in the past and got a couple of pins wrong.
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dave_g
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Post Number: 744
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Posted on Wednesday, 13 June, 2012 - 10:34 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Pin 2 is +5v. Ideally put a 10k pot across pins 1 and 2, connect the wiper to pin 3. You will almost certainly find you get a decent result near 0v on pin 3.

Generally if that's all connected OK, you will see a number of squares at switch on, which then means its an interface problem.
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noel
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Post Number: 123
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Posted on Wednesday, 13 June, 2012 - 11:05 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I have changed the ground connection on the display to a -4.5 (negative) voltage as epithumia suggested and all the squares on the top row of the display appear.
It seems I do need a negative voltage, of 1 or 2 volts will probably do - what a pain!!!

Thank you all for your comments.
Noel
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5uco
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Posted on Wednesday, 13 June, 2012 - 11:11 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello, first post ...

I have used that same ext temp LCD. It seems to want some -0.2V instead of 0V.

Assuming you are using a 5V pic or the like, put a 1n400x to the DC -ve, take the 10k pot to the cathode, and the anode will be to the 7805 gnd and is then the legitimate 0V to the pic etc.

Hideous diagram att.

Dave
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noel
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Post Number: 124
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Posted on Wednesday, 13 June, 2012 - 11:37 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks Suco, a very simple solution - I will try it.

regards,
Noel
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5uco
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Posted on Wednesday, 13 June, 2012 - 11:48 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Just a quick caveat, that diode takes the return current of everything ... if you have eg. motors etc. it's a no go ....

Dave
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noel
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Post Number: 125
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Posted on Thursday, 14 June, 2012 - 12:00 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

no motors etc. Current drawn is quite small, probably 40 or 50 ma, much less than the 1n400x rating.

Thanks again Dave.

Noel
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gew
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Post Number: 30
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Posted on Thursday, 14 June, 2012 - 08:02 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi, Noel,
I have got a Powertip LCD Pc1602 ARS (not ARU)and the datasheet says contrast adjust requires 20K VR between Pin 1 and 2, adjust to Pin 3, it does work when I manaully put litters in, but not yet able to get it to work with PIC, I think it timing problem, 1, waiting for the LCD to setup on switch on, 2, getting delays right when sending data and enable signal, if anyone can help with this?
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noel
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Post Number: 126
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Posted on Thursday, 14 June, 2012 - 10:32 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Gew, thanks for the comments.
My problem is definately a contrast one. I tried Suco's solution and the text on the display becomes visible but is still very faint.
I get a proper contrast when I use zeitghost's solution of using a 1.5 volt (AAA batery) in series with a 3K3 resistor to provide a negative supply.
Im not sure how long the batery would last, so Im thinking of just buying another display that doesnt require a negative supply.

Noel
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terrym
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Post Number: 1170
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Posted on Friday, 15 June, 2012 - 03:00 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If you've got room, you can use a 555 ic to generate the -ve voltage. I've also seen it done with a spare pin on a PIC.

TM
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cjaysharp
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Posted on Friday, 15 June, 2012 - 08:01 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Noel, I wonder if you could add on Suco's idea and use a second diode in series?

WIthout thinking too hard about it I'd expect the negative voltage to be the diode forward voltage so two diodes, twice the forward voltage?
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noel
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Post Number: 127
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Posted on Friday, 15 June, 2012 - 03:20 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

terrym: I could add an negative supply, but I would have to add another board, components etc (not much space on existing board).

cjaysharp: I tried using a second diode to Suco's suggestion, but there was no noticable difference. I think Suco's suggestion would probably be enough if I more than 5 volts supply available.

I have used 2x16 diaplays in the past, but I never came across one requiring a negative supply. The display only cost €3.50, the cheapest I could find - so I will get another one that doesnt require a negative supply.

I guess you get what you pay for!

Thanks for the suggestions though, I appreciate the help.

Noel.
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rich13
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Post Number: 69
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Posted on Saturday, 16 June, 2012 - 01:08 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Noel

I was reading back issues of Everyday Electronics, as you do, and in volume 3 May 2001, there is a project ,Pic Graphics LCD Scope.
Anyway, the LCD, a Powertip PG12684-F, requires a negative line to its CX point, (contrast).

The chip used is 7660 negative voltage converter, using hardly any components,(2 elec capacitors and a 22k preset), running from 5v.

As you know, copyright wont allow me show you, but if Alan permits, maybe just the part where the 7660 resides.

Hope this helps.

(Message edited by rich13 on 16 June, 2012)
__ Rich __
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zeitghost
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Post Number: 1726
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Posted on Monday, 18 June, 2012 - 08:24 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Ah. 7660. I remember working on those.

Dear dead days beyond recall.

Wrong Metal can.

Entire test wafer built into same cans.

Substrate Shorted output to ground.

Oh dear.
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noel
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Post Number: 128
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Posted on Monday, 18 June, 2012 - 09:24 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks Rich13 (and everyone else), but i've ordered a replacment that doesnt require a negative supply.

Kind regards,
Noel.

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