problem with LCD module

[samcheetah]

i have a 2 line 16 character LCD module. when i connected it in a certain circuit it didnt work at all. so i disconnected it from the circuit and applied 5V to Vdd and GND to Vss. but nothing appeared on the display. what has happened? is it dead?

and one thing more it has an additional pin with the usual 14 pins. the pins are somewhat like this;

14 13
12 11
10 9
8 7
6 5
4 3
2 1

0

the "0" is the 15th pin. it is a bit far from the other 14 pins and its hole is a bit smaller too. there is a potential difference of about 2.6V between this pin and Vss (GND). do i have to connect something to this pin.

plzzzzzzz help

 

[Nigel Goodwin]

Check my PIC tutorials for more details of how to connect them, but you MUST connect the contrast pin or you won't see anything. You can test the module without connecting it to anything else, just supply power and a contrast control - turning the contrast up should result in a row of black squares. The LCD hardware page gives everything needed for this simple test - so all you need is a connection to pins 1, 2 and 3, plus a 5K pot.

The 15th pin doesn't sound like it's anything!.

 

[samcheetah]

i have seen your tutorial about using an LCD with a PIC. infact the "certain circuit" i was mentioning was actually that tutorial board.

i connected 5V to Vdd and GND to Vss and i connected a 5K pot to the Vee pin. but there were no squares on the display. i varied the resistance but nothing became visible. i read in the article "How to use intelligent LCDs" from EPE and it said that connecting the Vee pin directly to GND will also do. i did that but still nothing happened.

what should i do???

 

[Nigel Goodwin]

i have seen your tutorial about using an LCD with a PIC. infact the "certain circuit" i was mentioning was actually that tutorial board.

i connected 5V to Vdd and GND to Vss and i connected a 5K pot to the Vee pin. but there were no squares on the display. i varied the resistance but nothing became visible. i read in the article "How to use intelligent LCDs" from EPE and it said that connecting the Vee pin directly to GND will also do. i did that but still nothing happened.

what should i do???

It sounds like it's not working, I'm presuming you're counting the pins from the correct end?.

 

[samcheetah]

yeah i think its dead.

im counting the pins from the correct end. pin 1 is labelled 1 and pin 14 is labelled 14 just like in the pin configuration im my first post. i have connected pin 1 to GND pin 2 to 5V and pin 3 to the movable terminal of a 5K pot.

it looks like i will have to say goodbye to my LCD. i think im gonna cry :cry:

 

[samcheetah]

the EPE article i talked about in a previous post said that some modules have to be given as less as -7 Volts on the Vee (contrast) terminal. so i tried this with my module. i used an old PSU salvaged from a computer because it had both 12 and -12 volts. well, what do u know, it worked. the squares become visible when a voltage for about -6V is applied to the Vee terminal (pin 3).

oh i really love my switching PSU!!!!!!!!!!!!!!

8)

 

[samcheetah]

but now there is one more problem. how would i produce -6V without my switcher?

actually i have a circuit in which the contrast pin is connected to one fixed terminal of a 10k pot. and the other fixed terminal and the movable terminal are connected to GND. the PCB has already been etched and i have soldered all the components on it. and now i need to have about -6V for my LCD.

the circuit im talking about has a connector for charging a 12V battery. i wont be connecting a 12V battery because i havent bought it yet :lol: but the terminals give about 12-12.5V volts. so i can connect a 7906 to this connector and give -6V to the contrast pin. or i can make a completely independent supply for the LCD. for that i would need a 7805 for the Vdd of the LCD and a 7906 for the contrast.

does anyone have a better solution?????????

 

[Nigel Goodwin]

does anyone have a better solution?????????

Buy an LCD that isn't an antique :lol:

Text LCD's requiring a negative supply are really old, I've never even seen one - only read about them.

 

[samcheetah]

Buy an LCD that isn't an antique

LOLzzzzzzzz :lol:

i think ill give it to some museum. or better yet, ill make my own museum. i have one near me and u know what its called, the dust bin. but i think i shouldnt be that harsh on the poor LCD.

so ill try out a few things and see whether a solution comes out that wont destroy the look of the circuit im going to use it in.

 

[Exo]

Found this once. it's supposed to produce a negative voltage sufficient for an lcd.

Never tested it...

 

[Oznog]

I came up with a circuit using:
an inductor, cap, diode, and MOSFET driven off the PIC's PWM pin to generate the negative voltage;
a couple of resistors to form a voltage divider so the PIC can take an ADC reading to adjust the PWM period to regulate the neg voltage;
a single digital output to switch on and off the pullup for the voltage divider. This is necessary in case you want to shut off the PWM, otherwise the voltage divider will reverse the voltage on the -vee cap.
A zener diode on the -vee in case your code fails to regulate -vee properly.

It's not too complicated, but it does tie up several good PIC resources. The great thing is you have full digital control over contrast inside the PIC without an exotic external component. So you don't have to mount a user-accessible contrast knob, which is a huge pain in the ass. I have another ADC pin reading a thermistor and automatically adjust the LCD contrast based on temp. This is also VERY useful if you put it in a car or anyplace where the temp can vary a lot. Twenty deg will make a display disappear or turn all black without a contrast adjustment.

 

[samcheetah]

Oznog can u give some code and schematics?

as i said that i have already made the circuit and ammending the PCB would be a bit difficult and there is no space for a add-on module type of thing. but controlling the LCD contrast with a PIC is a kool idea. and i would love to include that in my projects. [/list]

 

[andrew2022]

Found this once. it's supposed to produce a negative voltage sufficient for an lcd.

Never tested it...

have tried this design b4 but it didnt work (or i dont summit wrong)

 

[Oznog]

Oznog can u give some code and schematics?
as i said that i have already made the circuit and ammending the PCB would be a bit difficult and there is no space for a add-on module type of thing. but controlling the LCD contrast with a PIC is a kool idea. and i would love to include that in my projects.

**broken link removed**
The transistor there is a PMOS. Eagle just has some obscure symbols for MOSFETS in their libs. The source is hooked to 5V, drain to the inductor.

My code's in C (and on another machine) so I don't know if I'd make sense to you, but I'll try to copy it here sometime soon.

 

[samcheetah]

okay thanx.

i havent started programming PICs in C. but im learning it. so id be glad to see that code

thanx again