Hi all, I've got an LM3914 wired up to an RF module receiver's RSSI output, so I can see the signal strength. I made this circuit before I put it on a PCB with the ol' breadboard and I had no problems with it. I simulated the RSSI output of the RF IC with a potentiometer and it worked beautifully.

Now though, if i calibrate the 10 LED bargraph to be right in the middle and I turn on and off the transmitter, just one LED of the 10 LED bargraph will barely flicker. The weird thing is this: When I use my voltmeter's probe to see what the input voltage is on pin 5, 3 of the LEDs on the bargraph light up, and if I turn the transmitter on and off, those LEDs will respond accordingly.

This leads me to believe I'm having some sort of oscillation problem or something. I would use a 2.2uf cap just like in the picture, but the LEDs are less than an inch from the LM3914. It only says to use it if the LEDs are more than 6" away. Should I decouple pin 5?

My supply voltage is 6V by the way.

I'd experiment with my circuit, but it's already on PCB, and I hate soldering and desoldering things on a PCB.

Attached Images

lm3914.jpg (12.4 KB, 27 views)

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I'm no electronics god, i just talk too much.

 

Recheck your solder joints and check for bridging or tiny little beads near the IC pins. It's usually the simplests things, that's what makes it frustrating sometimes...

 

Hi Zach,
The input bias current for the input pulls up the input if it doesn't have a resistance to ground of at most about 1M. It will display the amount of voltage that the input bias current is pulling up the input.
Your meter's lead at the input picks up mains hum which will be displayed.
The ratio of the two resistors' values program the 10th LED to light with an input voltage of 5.2V which is too close to the supply voltage for the circuit to work properly. Set the 10th LED turn-on voltage to be no higher than 1.5V below the supply voltage.

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Uncle $crooge

 

I've checked the circuit so many times for bridged joints, etc that it's rediculous.

Audio, I have a 1.5k POT for the two resistors. I guess that I will just have to fiddle around with those values. It's weird that it worked so good on breadboard.

Thanks for clearing up the voltmeter thing though, that would have confused me a lot!

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I've fiddled around with the values and poured over the datasheet to try and figure out how to calculate the right values. It seems a bit vague, or maybe it's just because its 12 AM.

I want the LED bargraph to be at zero when the voltage input is 1.5V, and highest when it is 2.7V. Could somebody explain to me just how to do this?

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Hi Zach,
In your circuit, you want to make Rlo at pin 4=1.5V, and make Rhi at pin 6=2.7V. When the input at pin 5 is 1.5V then the 1st LED will light and when the input is 2.7V then the 10th LED will light.
The LM3914 has a built-in voltage regulator so you might as well use it to make the 2.7V and 1.5V. The voltage regulator works exactly like an LM317 except the resistor values can be higher here. The brightness of the LEDs (their current) is determined by the current from the voltage regulator times 10.

1) For 15mA in each LED, use 1.5mA from the regulator. There is 1.25V between the output pin 7 and the ADJ pin 8 so use a resistor of 1.25V/1.5mA= 833 ohms. Use 820 between pin 7 and pin 8.
2) You want the regulator's output voltage to be 2.7V. The output is 1.25V higher than the ADJ pin, so you want 2.7V-1.25V= 1.45V across the resistor from the ADJ pin to ground. You know its current which is 1.25V/820= 15.2ma so the resistor is 1.45V/15.2mA= 951 ohms. Use 1k.
3) Connect Rhi at pin 6 to the regulated 2.7V at pin 7.
4) Make Rlo at pin 4=1.2V less than Rhi at pin 6 by connecting two 1N4148 diodes in series and between pin 4 and pin 6. Connect a 4.7k resistor from pin 4 to ground to add about 1.5V/4.7k= 0.32mA of current to the diodes and internal resistor ladder.

I would sketch it but the sketch would have hundreds of wires crossing and will look confusing.

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Uncle $crooge

 

audio, take a look and make sure that I've done this right. I had to change R1 to 1K because it was too much current for the LEDs, they were starting to dim. I'm not sure if I got them in the right direction, but I tried them both ways and both don't work.

The LED display is completely lit up.

Attached Images

lm3914.jpg (5.7 KB, 15 views)

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Hi Zach,
Your circuit looks fine. The LM3914 can easily supply up to 20mA and LEDs are rated for up to 30mA so the 820 ohm resistor should have made the LEDs about only 18mA and the 1k should make them 15mA.

The two diodes in the circuit are 1N4148 diodes, not LEDs. With a 6V supply, 2V LEDs and all 10 LEDs at 18mA the IC will get very warm but not too hot.

It isn't the LM3914 or the LEDs doing the dimming, it must be the supply voltage dropping with the nearly 200mA total current.

I use a 9V battery in series with a 1k resistor to see how bright LEDs are, if they work and which wire is which.

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Uncle $crooge

 

I think based on what I've found, we'll be able to solve this problem one and for all:

The display goes straight to the top UNLESS i connect my voltmeter to ground and pin 5 input on the LM3914, in which case it works exactly the way I want it to. WHAT GIVES!?

I'm sorry, on my last post, I meant diodes, not LEDs

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Like I said earlier, the LM391x needs a resistor of at most, 1M to ground at its input to supply its input bias current and a reference of 0VDC.

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Uncle $crooge