What voltage and current to control 1 segment of LCD ?

[hihihi]

Hi..

How high is the current needed to switch ON one segment of
a seven segment number on a LCD screen ?
What is the voltage needed for this ?

Is the current enough to light up a small LED ?

Thanks.

 

[John Fields]

Hi..

How high is the current needed to switch ON one segment of
a seven segment number on a LCD screen ?

 

[Jerry G.]

The current is usually in the nA "nano-Ampere" range. The voltage can be
anything from about 1 to 3 volts or so, depending on the design of the unit.
The voltage is not DC, but is usually at 15 to 30 Hz, depending on the
design necessity for the display and application.

--

Jerry G. GLG Technologies GLG
==========================


Hi..

How high is the current needed to switch ON one segment of
a seven segment number on a LCD screen ?
What is the voltage needed for this ?

Is the current enough to light up a small LED ?

Thanks.

 

[hihihi]

l though the current available from an LCD driver may be sufficient - not
sure why you would want to thoiugh...

I found a Philips PM 2544 Logic multimeter on a secondhand market.
Problem is the LCD is partly black.
One number and the volt/amper/ohm indicators are unreadable.

I thought about connecting the signal from the LCD driver to a external 7
segment number and a few leds.
That way i could read the measured info, missing from the LCD.

But it looks like i found a new way
The signal to the LCD driver is a serie of pulses.
Which contain all the data which would be displayed on the LCD.

I am trying to connect this signal, via a opto-coupler, to a serial or
parallel port on a computer.
Then make a prog which converts the pulses to a number on screen.

Who needs a LCD screen when you can have the measured data inside a
computer

 

[John Fields]

I found a Philips PM 2544 Logic multimeter on a secondhand market.
Problem is the LCD is partly black.
One number and the volt/amper/ohm indicators are unreadable.

I thought about connecting the signal from the LCD driver to a external 7
segment number and a few leds.
That way i could read the measured info, missing from the LCD.

But it looks like i found a new way
The signal to the LCD driver is a serie of pulses.
Which contain all the data which would be displayed on the LCD.

I am trying to connect this signal, via a opto-coupler, to a serial or
parallel port on a computer.
Then make a prog which converts the pulses to a number on screen.

Who needs a LCD screen when you can have the measured data inside a
computer

---
Good luck...

What you may not know is that in order to capture the LCD drive
properly there are two signals you need to consider, the segment drive
and the backplane drive.

The backplane (which is a transparent electrode deposited behind the
segments, on the inside of the display's rear glass) is driven by a
continuous square wave at about 30Hz. The segments are also driven
continuously with the "same" square wave, with the invisible segments
being driven in phase with the backplane and the visible segments
being driven 180° out of phase with the backplane.

So, what you'll need to do to decode what's going on is to compare the
voltage on the segments with the voltage on the backplane; if they're
the same, then that segment will be off, but if they're different that
segment will be on.

In addition, there's what's called "direct" drive (described above)
and then there's "multiplexed" drive which you can find about by
Googling for it. If you're lucky, your multimeter will be using
direct drive.

 

[hihihi]

oscilloscopeeJul 2004 14:16:27 +0200, hihihi


---
Good luck...

What you may not know is that in order to capture the LCD drive
properly there are two signals you need to consider, the segment drive
and the backplane drive.

Its easier then that.
You do not need to mess with the LCD or its drivers.
Here is a picture of the signal to the drivers.
http://home.wanadoo.nl/hihihi/tp5008/pictures/0volt.dat.png
Made with a very slow oscilloscope card

It contains all the info displayed on the lcd.
Its like a I2C or SPI signal.

I have allready connected it to the pin 13 of the parallel port.
Now i have to figure out what the signal says

 

[John Fields]

Its easier then that.
You do not need to mess with the LCD or its drivers.
Here is a picture of the signal to the drivers.
http://home.wanadoo.nl/hihihi/tp5008/pictures/0volt.dat.png
Made with a very slow oscilloscope card

It contains all the info displayed on the lcd.
Its like a I2C or SPI signal.

I have allready connected it to the pin 13 of the parallel port.
Now i have to figure out what the signal says

---
Toggle the segments on a single digit on and off to find out which is
which and how wide they are and how wide a digit is and which digit is
which, then turn the anunciators on and off to find out where they are
and you'll be done!

Like I said, good luck...

 

[hihihi]

Toggle the segments on a single digit on and off to find out which is
which and how wide they are and how wide a digit is and which digit is
which, then turn the anunciators on and off to find out where they are
and you'll be done!

I am making progress
Here is a picture of the signal for different range selections.
The data has been actually read by the computer from the pin 13 of the
parallel port.
So that part works.. I had never done it before..
http://home.wanadoo.nl/hihihi/pm2544/pm2544-amp-volt-ac-dc-ohm.dat.big.png

 

[CWatters]

I found a Philips PM 2544 Logic multimeter on a secondhand market.
Problem is the LCD is partly black.
One number and the volt/amper/ohm indicators are unreadable.

Good second hand ones seem to be around 160 Euro but perhaps you can get a
new meter that will do what you want a lot cheaper. Is it really worth
spending a few hours trying to fit it?

 

[hihihi]

CWatters casted

Good second hand ones seem to be around 160 Euro but perhaps you can get a
new meter that will do what you want a lot cheaper. Is it really worth
spending a few hours trying to fit it?

Yes..
There is now way i am going to spend that much money on a meter.
This one costed 3 euro on a secondhand market

I do not need such a meter.
But i am having fun trying to make it work