NerdKits - electronics education for a digital generation

You are not logged in. [log in]

NEW: Learning electronics? Ask your questions on the new Electronics Questions & Answers site hosted by CircuitLab.

Support Forum » Power Supply Problem - Boxes on the LCD

October 29, 2011
by axle38
axle38's Avatar

I posted a video that hopefully will explain both verbally and visually the question I have. I connected the 9v battery to a switch so that I don't need to constantly disconnect and reconnect the battery. The problem is that when I do this, the MCU seems to not have enough power, and the LCD displays only boxes on the screen. When I hooked a multimeter into the circuit to measure the current, everything worked fine. Why is it doing this and is it a problem of a low battery?

Here is the video link

http://www.youtube.com/watch?v=bqBVAziOYKE

October 29, 2011
by dgikuljot
dgikuljot's Avatar

Dear Axle38, When you run the current through the multimeter do you also run it through the switch at the same time?

IF you are only running it throught the DMM and not the switch when it does work, it might be that the switch is shorted out possibly? As in if you are trying to put a switch between the postiive wire, you accidentally conencted the other end to ground?

October 30, 2011
by axle38
axle38's Avatar

Hi dgikuljot,

When I run the current through the multimeter it is still connected to the switch. I'll add that when I measure the voltage drop across the regulator pin ground and the regulator pin that goes to the positive rail I measure 4.97v but still get the boxes on the LCD. The whole time everything is connected through the switch.

October 30, 2011
by Rick_S
Rick_S's Avatar

Your video doesn't do a very good job of showing the wire connections between the knife switch you have and the battery/NK board. Nor do you state what mode your meter is in (voltage/current/diode check/resistance/capacitance). Without knowing more info, it's hard to troubleshoot your situation. The video idea was good, you just needed better details of your circuit in it.

Rick

October 30, 2011
by axle38
axle38's Avatar

Hey Rick, I do apologize, I realize it was not the most comprehensive video. The meter is always in current mode throughout the video and even though the knife switch is dpdt I only use one side of it and the battery, meter, and mcu are connected in series. If someone would like I'd be happy to do another video or draw a schematic and post it.

October 30, 2011
by missle3944
missle3944's Avatar

Does it work without the switch? You could try using one of the extra programming switches that the nerdkit came with. I think that is SPDT

October 30, 2011
by mongo
mongo's Avatar

Can you put a picture up with all of the wires and connections? That some times helps if we can see the whole circuit.

November 03, 2011
by axle38
axle38's Avatar

Hopefully this new video does a little more justice explaining my problem than the first one.

http://www.youtube.com/watch?v=FDxaaMHFMNM

I really just want to know why when I connect the battery directly to the voltage regulator through 22 gauge wire the MCU won't function properly but when I insert a multimeter in-between the battery and the breadboard, the MCU functions normally. Does the multimeter supply a certain amount of current to the circuit? I thought meters were supposed to be neutral when measuring anything.

November 03, 2011
by mongo
mongo's Avatar

Hey 38, The circuit actually looks like it is working without the meter but since you are not introducing extra noise in the line, it is just more stable.

Here is a little experiment. As an example, years ago, I had a Simpson 260 meter connected to a power supply in my test bench setup. I tested smoke detectors and when I had one fail on power-up, or during operation, I had a simple pushbutton in line which put the meter in the circuit. (Normally closed button right across the meter). The resulting current draw was logged and I had a chart for all the different failures and their current characteristics. The other guys called it my "black magic" button because I was getting 3 or 4 times as many units tested and repaired as they were.

So, while the meter is in the circuit and the display appears to be working, go ahead and short the wires to the meter. It won't hurt anything but the meter should go to zero. See what the display does. Essentially, you would be doing the same thing as connecting it without the meter but without any interruptions.

November 03, 2011
by BobaMosfet
BobaMosfet's Avatar

axle38-

I watched your video then went over to Sears website to make sure I could see your meter clearly (#82344).

A 9V battery can generate approximately .5A (somewhere around 1/2 an Amp normally). A little more, a little less. So, approximately 500mA- which is within what the LM7805 will pass if needed (albeit you're circuit shouldn't require more than about 100mA at this stage).

Your meter is not adding any voltage or power to the circuit. If anything it acts as a resistor. In voltage mode, it's a very high resistor (so only a little current goes into the meter, and it has little impact on your circuit). However, in ammeter mode, it is a very low resistor, and passes virtually all the current the load is requesting to pass between the power-supply (9V Batt.) and the load (your circuit)-- In this case 20.83mA.

Go ahead and plug your circuit into the battery normally, not through the meter. Then set your meter to volts and measure the voltage across the battery terminals. This is the only way you can accurately test a battery- while it is under load. It's voltage should be close to what you would expect, if it drops a lot, it's weak.

What voltage do you see? Once you know the voltage, since you know the current, you can then get an idea of what 'resistance' your load represents to the battery.

If you have a second meter available, measure the voltage across the battery when your current meter is measuring amps, in series with the battery. That will give us an idea of how the ammeter is impacting the circuit if any.

Let us know what you find, please.

BM

November 04, 2011
by axle38
axle38's Avatar

I sincerely hope I'm not alone in spending 5 hours on a problem only to have no good answer...but I do have a solution to my problem and a theory.

First of all, thanks everyone for your suggestions, I certainly tried many different configurations of the meter, breadboard, etc.

If you'll look at the picture below, this is my problem...

Again, notice the boxes on the LCD and of course the MCU is not processing. Taking the multimeter, the readings are 20.82 microamps and 4.9 volts. You'll notice two red wires leading away from the voltage regulator, those two wires go to the 9V battery.

The first two hours I measured, re-measured, replaced the 9v battery with a 12 volt power output, added a capacitor, resistor, everything...and still nothing. I did notice though that when I used wires under an inch in length, the microcontroller DID work. So I thought, maybe the longer the wire, it possibly induced interference? So I moved the voltage regulator to another part of the breadboard shown below...

And as you can see, the LCD displays the correct characters and the MCU is processing.

I went on to move the voltage regulator to another breadboard and take the positive and negative output back to the breadboard containing all of the components and it still worked. I added more wire, a capacitor, and a switch and it still worked. Still 20.82 microamps and about 4.9 volts.

My theory is that somehow with longer lengths of wire, some sort of noise is being introduced into the circuit and moving the voltage regulator away from the other components reduces this noise. I don't have an oscilliscope or any other device to measure my theory but it's the best I could do. If someone would like to recreate this experiment, please do. I would like to know if it's something I'm doing wrong or if it truly is a result of this noise.

In the end I think I partly succeeded, I can now use a rectifier bridge I put together to provide a constant source of power to the breadboard with of course additional components being added if needed to the power supply (switches, relays, etc.) Thanks again for everyone's help!

Alex

November 06, 2011
by BobaMosfet
BobaMosfet's Avatar

Alex,

I wish I were there with a o-scope, your problem is fascinating. Annoying to you, but yes, fascinating. I'm wondering if your crystal is really sensitive. I like the idea that perhaps long wire is inducing possible a capacitive element into this.

You mention 20.82 microamps (uA)- don't you mena milliamps (mA)?

You might put two 22pF Capacitors on each leg of the clock crystal, to ground.

Please explain your connect on pin3 on the LCD, and pin 27 on the MCU. You have an LED on 27 with no resistor; for now, I'd just remove that LED. PIN3 on the LCD is for contrast for the display- you could use a trimpot here, but you have a resistor attached- where is the other leg of the resistor going, and what value is the resistor, please?

BM

November 09, 2011
by axle38
axle38's Avatar

BobaMosfet,

First to correct the meter reading, it is 20.82 milliamps (mA) NOT microamps. That was a typo.

Concerning the pins, I removed the LED on pin27 of the MCU to remove any confusion. It was just there from the LED blinking light project but had no discernable effect on the operation of the MCU. Pin3 of the LCD is connected to the resistor suggested by the Nerdkit guide and is connected to ground of the voltage regulator. If I'm not mistaken, a resistor with the color band BROWN-BLACK-ORANGE is a 10k resistor. As a side note, I also now know what a trimpot is...had to look up what that was.

I'm trying different lenghts of wire (while staying at or below 22 gauge) and am finding that I can only get to around 3 inches of wire on both the negative and positive side before the MCU locks. If I move the voltage regulator to the opposite side of the breadboard to pins I28,29,and 30 and connect the output to the rails on the right side then I can get up to wire lenghts of about 6 to 8 inches.

I did try your suggestion with the capacitors but I had no luck. If you have any other ideas, I'd be more than happy to try them. For now I'll move the voltage regulator and start using the bridge rectifier.

Thanks for your input!

November 09, 2011
by Rick_S
Rick_S's Avatar

Yes, Brown-Black-Orange is 10K. You need 1K though for the LCD contrast resistor (Brown-Black-RED). If you are using a 10K for your contrast resistor, that could be part of your problem. It's odd though that it would have ever worked right with that?? Confused

Rick

November 10, 2011
by axle38
axle38's Avatar

Rick,

Upon further review the resistor is brown-black-red, although it's the lightest shade of red I've ever seen on a resistor. I measured the resistor with an ohmeter and it comes out to 1k.

Has anyone else tried to lenghthen the wire and gotten similar results to what I've been experiencing?

November 10, 2011
by hevans
(NerdKits Staff)

hevans's Avatar

Hi Axle,

I can't think of any good reason why those lengths of wire would be causing that problem. However looking back at how you solved the issue, it is possible that you merely had a bad connection on a specific spot in the breaboard (sometimes the little clamps in the holes do give out). When you used smaller wires you might have just wiggled it enough to make it work, and when you moved the 7805 you just solved the problem entirely. Thats just my hunch at the moment.

Humberto

Post a Reply

Please log in to post a reply.

Did you know that our kits have shipped to hobbyists in 40 countries? Learn more...