View Full Version : My LCD screen likes to share.

08-02-2005, 07:45 AM
When my demo board is not plugged in, and I connect my back lit (externally powered) LCD screen to the in/out pins on my stamp, the power led on the board lights up a little with one pin in, and even more with two pins.· Even with both pins it still is not up to the intensity of when the board is plugged in, but I cant help to think that it lighting up at all is good.· So is it something to worry about?

08-02-2005, 08:39 AM
I imagen that would be bad.








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Bruce Bates
08-02-2005, 12:04 PM
Cheech -

Not only would I think that's bad, I'd troubleshoot the problem IMMEDIATELY, before BLUE SMOKE starts escaping from the Stamp! This kind of remote troubleshooting is a bit difficult, but here's what I'd check, and the order that I'd check it:

For this first two tests, the LCD does NOT need to be connected to the "demo board".

1. Check the polarity and voltage of BOTH the LCD and the backlighting circuit for the LCD. Many LCD contrast circuits require a NEGATIVE voltage to operate, while the LCD requires a POSITIVE voltage to operate.

2. Make sure the GROUND terminals of the LCD are common with the Stamp GROUND terminal(s).

Now remove all power from the LCD, and connect it to the "demo board". Make sure the Stamp is properly inserted in its socket and/or is wired correctly at the power terminals. Now apply power ONLY to the "demo board", and NONE to the LCD.

3. Place one probe (black/neg/ground) of your voltmeter on the GROUND (Vss - Pins 4 and 23) terminal of the Stamp or the ground teminal of the power supply, and carefully probe all the terminals on the LCD, looking for voltages. Note any that are present, and the recorded voltage.

4. Without removing the BLACK probe as positioned above, check Pins 21 and 25 on the Stamp for voltage, if it is a standard BS-2. Only one of those pins should indicate a voltage. Note the voltage, and the polarity of the voltage ( i.e. did the meter get pinned in the negative direction?).

Report back what you found, and we'll go from there. Is this a Parallax "demo board" you're using, and if so, which one?


Bruce Bates

08-03-2005, 09:03 AM
http://www.parallax.com/detail.asp?product_id=27238 is my board/kit
http://www.parallax.com/detail.asp?product_id=30058 is the LCD

1. +5.10 volts going into it, I don't know what is the backlighting circuit and what is the LCD it self, and I'm not about to go poking around with my multimeter.

2. I use the +5 and ground of my USB ports on my computer to power the backlight, and the grounds are not connected, should I still do the rest of the steps?

Loopy Byteloose
08-04-2005, 10:26 PM
The above is good advice.· And yet, there are so many kinds of LCDs that it is hard to say what is going on.

I have two LCDs that operate on the Hitachi 4780 format. They look exactly the same and they program exactly the same, but.....

One is from Parallax without any backlighting.· It has 16 pins on the board, but only 14 are connected to the cable.· I tried powering the other two [as I was confused and thought it was my backlit LCD].· Nothing happened.

With my other LCD that is backlit, everything looks the same.

Oddly, I get Backlighting with only the 14 pins plugged into my Parallax Demo board and the other two unattached.

Obviously there are bridges on both the ground and the supply voltage. I am not even sure pin 15 and 16 go to anything.· It is possible that they use on printed circuit board for many arrangements [Such as 8 bit data only, 4 and 8 bit data, 14pin with backlight, 16 pin with controlable backlight, 14pin without backlight, and so on.]

Since it is already backlit, I have not bothered to connect pins 15 and 16 or tried to use diminished voltage on those pin·to dim them.· Separate backlighting is a great feature as it saves battery power [just like on your cell phone].

I have been wondering if it is a defect or a design feature.· Someday, I would prefer a backlit screen that can turn off and can dim and possibly be dimmed remotely via a digital potentometer.


You may search and search for a problem in the BasicStamp and find nothing.· [Then it may never cause damage to other parts - just have its own personality]

Or, you may have it getting power from the BasicStamp via the data and control pins [This may harm something other than the LCD].· If that is the case, remove all but the supply and the ground [pins #1 and pins #2] and see what happens.· Then, try just the other pins on an individual basis to locate the bad pin.· While you may find a bad pin, I·am not sure what you could do about it other than try to return the merchandise as defective.· Nonetheless, you would know where the problem lies if not why is it.

And, YES - you should be sharing a common ground between the LCD and the BasicStamp. NOT doing so is a receipe for some very mysterious disasters

G. Herzog in Taiwan

Post Edited (Kramer) : 8/4/2005 3:34:15 PM GMT

Loopy Byteloose
08-04-2005, 10:41 PM
Just an after thought.
If you mean that the power LED on the unpowered BasicStamp board lights up from the LCD, that would most likely indicate that there is a bridge between PINS 15 and 16 and the respect PINS 1 and 2.

The power goes in PINS 15 and 16 and out PINS 1 and 2 into the BasicStamp board and proves just enough current to light up the LED, but not enough to power the BasicStamp [you are in a 'brown out' condition for the BasicStamp.

Measuring the input voltages to the BasicStamp will tell you this.
Complete removal of the BasicStamp from the board and observing the LED will allow you to trace everything down without any data or control lines contribuitng to the question.

Good luck and don't worry. I think you may have a LCD like mine.

G. Herzog in Taiwan

Tom-Matrix Orbital
08-05-2005, 02:31 AM
Hi Cheech,

From what I see from your provided link, you have a Matrix Orbital serial interface LCD. The backlighting circuit is on the display unit itself. You turn off and on the backlight by issuing commands to the unit.

I also strongly advise you have the grounds commonly hooked up or you can do some damage. Let me get this straight. When you plug in the RX and TX lines of the display to the stamps input/port ports, your power light gets dim?

08-05-2005, 08:29 AM
That's close Tom, when I hook up the TX and RX lines to the stamp i/o pins, the led on the board (the one that Kramer talks about in his second post) goes from off to dim (meaning its getting power). Now if I have the stamp plugged in the wall with my ac adaptor, then the led is so birght, you couldn't tell it being plugged in or not.

If I can't use my PC's power, because I am not going have have this attached to my pc all the time, then can the LCD and back light run off the 5v that my demo boards regulator is supplying?

Tom-Matrix Orbital
08-05-2005, 10:39 PM
Your display will be able to run off the 5V unit you provide ,when you power up the kit with a battery. The display does still draws 90mA of current, so if you are running out of power from your battery the power led light will go dim. Change the battery, and the power led will be bright again. If you are more worried about power consumption, we do offer a 2x20 pled display that draws half of the current that your display draws. There is even a smaller pled display that draws even less current, which is only 10mA. You can find these displays at http://www.matrixorbital.com/index.php?cName=lcd-character-pleds

Best Regards,

Tom Lam

Post Edited (Tom-Matrix Orbital) : 8/5/2005 4:46:06 PM GMT

08-06-2005, 12:17 PM
Woah there Tom, I think you missed the boat again. I am talking about the 5v regulator that is heatsinked on my demo board (you can see it by looking for the links in a previous post). My board usually gets its power from an ac adaptor, and my LCD gets power from my USB port (which I now know is not a good idea to have separate grounds). Now there is no battery involved in this whole system, thus I don't care how much current it draws as long as my regulator can handle it along with my stamp.

I still have yet to hear a convincing reason why my led lights it self when it is externally powered, I have yet to test it with the common grounds, because I'm waiting for a OK from someone that knows the limits of the regulator on my board, and if it has enough current.

Tom-Matrix Orbital
08-08-2005, 11:39 PM
The regulator can output a maximum of 1000mA, so you should have sufficient current. If you aren't using the AC adaptor to power the stamp, what else are you using? I know you are using a usb power cable to power up the LCD, but what about the stamp? The regulator will still need an input minimum of 6V, if you want to regulate the output at 5V. You can look up the specification of the regulator at http://www.national.com/pf/LM/LM2940C.html