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mdnanwelch7(at)hotmail.co
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 Posted: Sun Nov 28, 2010 8:43 pm Post subject: Trim tab indicator |
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>the schematic on page 2 of the data sheet referenced by Mike Linse will work. http://www.national.com/ds/LM/LM3914.pdf
| Quote: | For the 0V to 5V signal source, make a 5 volt regulator like this: http://www.tkk.fi/Misc/Electronics/circuits/psu_5v.html
Connect aircraft power to the input of the 5 volt regulator.
Connect ground and the output of the 5 volt regulator to the trim servo pot orange and blue wires.
Connect the green wire from the servo pot to pin 5 of the LM3914N.
Joe
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Joe Gores
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Hi Joe,
Regarding circuit;
http://www.national.com/ds/LM/LM3914.pdf
I finally got around to building the circuit(s) you suggested. I got the +5V regulated one--no problem.
I'm getting ready to start on the LM3914, and I have a couple of questions.
In studying the schematic, the way they have the LEDs attached is "odd". To me, it looks like the LEDs
are sharing a common +12V power supply (to their respective anodes), and the LM3914 acts like a grounding
source for whichever LED should be lit. If this is the case, I guess it'll work just fine, it's just not how I thought the
LM3914 worked.
Ok, now for a quick question; Pins #1, & 10-18--duh, obvious. PIN#2 & 4--GND Pin#3 & 9--+12V supply, so far so
good. Now, you mentioned the green wire from the servo Pot goes to Pin#5. The Pins #6,7 & 8 I'm not quite sure
about. I don't know if they refer to a 5K pot, or just install the listed resistors, or what?? Any ideas?? Am I
supposed to install a 5K pot for calibrating the LEDs?
Thanks for your help, Joe.
Mike Welch
ps. here's a picture of the first half of the circuit, with the LM3914 just sitting there.
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user9253
Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan
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| Posted: Mon Nov 29, 2010 8:00 am Post subject: Re: Trim tab indicator |
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Mike,
Keep in mind that I have not built this circuit.
Since you are supplying the elevator trim pot with 5vdc and this circuit is designed for a 0 to 5 volt input signal, I would use the fixed resistor values as shown on the schematic. R1 controls the current through the LEDs. With a 12vdc supply, the LED current will be about 10ma. The problem with using a POT is that if adjusted for low ohms, high current could damage the LEDs. A prototyping breadboard makes it easy to experiment with different values of components, if necessary.
http://www.mouser.com/images/busboardprototypesystems/lrg/BPS-IMG-BB400.jpg
http://www.mouser.com/Tools-Supplies/Prototyping-Products/_/N-5gg5?Keyword=breadboard&FS=True
I would try the circuit as is. You might luck out and no calibration will be necessary. One LED should light with the trim at one extreme position and all 10 should light at the other extreme. If it does that, then no calibration is required. I suspect that R2 is the one to change for calibrating. The voltage on pin 6 sets the span. To change from a bar graph to only a single LED on, connect pin 9 to pin 11, not to the supply voltage. Have fun experimenting.
Page 110 & 111 of Forrest Mims' ENGINEER'S NOTEBOOK has schematics of the LM3914N. The book also has many other integrated circuit applications.
Joe
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Joe Gores |
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mikerv6a(at)ao-cs.com
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| Posted: Mon Nov 29, 2010 6:52 pm Post subject: Trim tab indicator |
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Hi Mike, here is another comment about your trim indicator:
Regarding your question about the LED connections, the
LM3914 LED connection is sometimes referred to as "common
anode", and it is widely used in LED display devices.
The LM3914 has two modes of operation: In "bar" mode, increasing
voltage at the input (pin 5) will light first a single LED, then
two LEDs, then three, then four, and so on, up to full scale in
which all ten LEDs are "on". In "dot" mode, only a single LED is
lit: The first one, or the second one, or the third one, etc.
Bar/Dot mode is controlled by pin 9, and pin 9 is connected to
the supply voltage to set Bar mode. If you want to use dot mode,
then just leave pin 9 unconnected.
There is a power limitation to consider if you use bar mode. If
your common-anode LED power supply voltage is 12-14 volts, and
voltage across the LED is typically about 2 volts, then the
remaining voltage, perhaps 10 to 12 volts, is imposed upon the
LM3914 drive circuits. If LED current is programmed for 10 milliamps,
then power in the LM3914 due to LED operation is about:
(ten LEDs) times (ten milliamps) times (twelve volts) = 1.2 Watts.
This is high enough power dissipation to cause significant temperature
rise in the LM3914.
If you choose to use bar mode, then you can reduce power dissipation
in the LM3914 by reducing LED anode voltage, perhaps using your +5V
power supply for both LM3914 power and LED power. Another way to
achieve this goal is by adding a resistor between each LM3914 drive
pin and its respective LED cathode. Resistors of about 560 or 620
Ohms would reduce the power in the LM3914 by about half.
The LM3914 will still function in bar or dot mode with this change,
just as it will if LED voltage is taken from your +5V.
Mike Linse
RV-6A builder
Corvallis, OR
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