I was going to add this to an older thread titled "Cool Voltmeter" where we talked about a voltmeter kit I saw at Allelectronic.com, but the thread is gone :cry:

So, I'll start a new one. I needed supplies for another project so I thought I'd tack on this digital voltmeter. http://www.allelectronics.com/cgi-bi...TER,_20V_.html

My plan is to swap out the red LED's for green one's and see if I can place the guage where the stock clock is. Mine was dead when I bought the rig.



In the old thread Midiwall or WARTD mentioned there were two kinds of LED displays, common cathod... (Something like that) and the opposite

Either way I'm hoping someone can point me in the right direction for green LED displays.

Here's some more info on the current ones.









Erich

 

Bump for the morning folks

Erich

 

I found some 2 digit, 7 segment green LEDs at Jameco Electronics, but I'm not sure how to tell if they are the correct buss polarity for my sensor. http://www.jameco.com/webapp/wcs/sto...67&pa=334967PS

Anyone know how to tell?

Erich

 

nice cant wait to see what it looks like i need to make something cool to replace the clock thats broke

 

Common cathode and common anode, which are just fancy ways of saying that all the negatives are tied together and the positives are individually switched to turn on segments or all the positives are tied together and the negatives are switched. With light bulbs, it wouldn't matter which you had, but when you are replacing LED's, the circuit will only function one way, so you have to replace the bank with one of the same polarity.

I think Marky Mark originally mentioned it.

 

Wow, I wasn't far off!

Okay, so here's the next question...

How do I tell if the current red LED's are Common Cathode or Common Anode? What method of diagnosis do I try. Just looking on Jameco, it looks like CA (Common Anode) is the only style they carry in Green, so I don't have much of a choice there.

They do have CC (Common Cathode) in red so hopefully this will work

Erich

 

just a wild guess but if you have continuity between all negative pins i bet it'd be safe to say you've got common cathodes. i could be way off base though.

 

check out dakotadigital.com... they may have what you want without having to rebuild it.. then you can just sell this one...

 

You are going to need either a schematic or a volt meter. From there, you will need to find the pins that are common and go from there.

 

Lucky for me I have both I downloaded the specs for both CC and CA on Jameco.

Now I'm wondering how to get that honking chip off the board so I can get to the other set of pins on the LED. I tried gingerly pulling, but I don't want to break it.

Is there a special technique?

Erich

 

I got the chip off, so I can test the LED's for CC or CA. I've found that the positive wire is common, so I just need to check with you guys if the positive is the CATHODE or ANODE? :pat:

I'll Google too.

Erich

 

I may just be talking to myself here , but as near as I can tell I need Common Anode (CA) LED's. The Anode is the positive side of a battery, the Cathode the negative.

Now, I was trying to make things more difficult by getting all caught up in the fact that electricity is negative charged electrons but they come from the positive side of the battery, so I'll just shut up and buy the CA LED's. If for some reason they don't work, at least I'll know what to try next

I'm going to try radio shack just in case I can buy pass shipping, but I'm not holding my breath.

Erich

 

im following, just have no input

 

(coming out of hiding... musta been that "Marky Mark" reference... )

The schematic will probably show the LED segments as --->|--- symbols. Either the ">" side or the "|" side will all be tied together. Whichever side that is will be referred to as the "common" side. If it's the ">" side, then that's the Anode, thus the LEDs are "Common Anode". If it's the "|" side, then... Yeup, "Common Cathode".

When you replace the displays, you'll need to match a couple of things - Common Anode or Cathode, Size, Pinouts, Voltage and Current. Most all of it will just "fall into place" as the LED digits have pretty well standardized over the years.

Be careful of current - different LED colors will draw different amounts of current for the same brightness. Red tends to draw the most these days, so if you go to green, you may need to add a current limiting resistor into the circuit lest you blow the displays. Maybe not, but they'll be really bright if you don't.


From the looks of the pic of the displays, you'll want two "2 digit 7-segment LED displays with decimal point". Pretty easy to find from Jameco and other folks.

I found the datasheet for the display:

http://www.datasheetcatalog.com/data.../LN526RA.shtml

Looks like the "RA" on the end stands for "Red" "(common)Anode"


For WAY too much more information:

Dude, for the extra $$$ you'll pay from RatShack you could pay for shipping. RatShack will have lousy product choice and even lousier product. Really, buy from Jameco or someone else.

 

Great info Midiwall.

I stopped by Radio Shack during lunch on Friday -no surprise- they didn't even know what a 7 segment LED was

So I ordered two of these bad boys back at the office https://www.jameco.com/webapp/wcs/st...oductId=334967

I'm positive the board is CA and it should be a pretty simple swap. I just had no luck at finding a very good reference for the CC-CA definition. My college circuits test book was also useless

I hope they don't take too long to get here

Erich

 

It deals with how current flows through a diode (an LED in this case) and what you need to do to be able to select a certain segment to light in a multisegment display.

As the schematic symbol for a diode shows, current will only flow one way, from the Anode to the Cathode. When you work with a single LED, you can turn it off by breaking the circuit on either side, either stop supplying the current (break the Anode side), or stop giving it someplace to drain to (brak th Cathode side). Depending on how your circuit is, one of those will make more sense than the other.

When you take a bunch of LEDs and want individual control over them (like the 7 segments of these "digits"), you have the same choice. If your circuit is designed such that it's easier to break the drain (the Cathode side), then you want "Common Anode" since you can tie all of the Anodes together and supply current to just one point. When you complete the circuit on the other side of a certain segment (by closing the circuit from the Cathode to your drain) that segment will light.

If your circuit is designed to switch the current _supply_ on/off, then you'd use Common Cathode, which is where all the Cathodes are tied together and then connected them to a common drain point.

Looking at JUST the LED (single lamp or a panel) there's no real plus or minus to which side you're switching. The choice in a circuit is usually driven by the support chips, in this case, the voltmeter-on-a-chip.

Hope that helps!

 

ooo... I just noticed something. That meter wants to run from 5v. Don't just hook it up to the 12v in the truck, you'll blow it. It can monitor up to 20v, but it needs 5v to run from.

You MIGHT be able to get away with a simple voltage drop using a resistor, but you'd be best to run a regulator. There's nothing on the AllElectronics site about it having on-board regulation and it'll be sampling that 5v feed to figure the monitored voltage.

2nd thought... from your pics it looks like it may have on-board regulation. I'd probably try dropping the 12v with a resistor first and then if th reading seems unstable, move to a regulator.

 

Good catch! I planned on using a resistor but I wasn't totally sure that was the way to go. I haven't identified the total amperage used to figure needed resistor --> R=E/I

Tell me more about regulators. I've never worked with them.

I really like how informative this thread is getting

Erich

 

PAINFULLY easy to use.. What you're looking for is an LM7805:

http://www.national.com/pf/LM/LM78M05.html

Basically it's a part that has 3 pins. One is IN, one is OUT and the last is GROUND. You literally just hook up the truck's supply to the IN, feed the OUT to the panel meter, and then tie the GROUND to the same ground that you're feeding to the meter.

The circuit on that page above shows a couple of caps in there, but you technically won't need them. They're there to smooth out the high freq noise which is a side effect of how the regulator works. For the circuit we're talking about here, you won't care.

The LM7805 should be available at RatShack, but like I alluded to above, I REALLY hate the quality of their parts. Actually, you could probably get a couple as "samples" (i.e., "free") from the National site.

One thing... I think the AllElectronics site states the current drain of the meter at 60ma. That "should be" okay for the regulator to handle on it's own, but you might want to put a heat sink on it. Anything will do, like a 2" square piece of aluminum scrap. For ease of mounting, you'll want the part to be in a "TO-220" case:



You can also get them in "cans" but it's more trouble to get a heatsink on them.

 

AWESOME

I really appreciate all the help. I have scrap aluminum, so I think I can get it all set up. I'll check out National's site for samples.

Thanks again

Erich