LM3914.. another Help!

Hello, I'm new in this forum. I've read this thread and I've learnt a lot from it. However, I tried constructing the exact circuit as in the LM3914 datasheet with a pot.

Attached is my schematic..

My problem is, when I turn the wiper of the variable resistor, the LEDs won't light up. Nothing happens. Is my schematic correct?

LED specs : Forward voltage= 1.6V, Forward current =20mA, reverse voltage =2.0V.

I'm a bit puzzled because when I tested with a simple circuit, the LEDs dont follow the common package polarity. The "flag" (cathode) needs to be connected to the +ve of the source and the anode to the -ve bfore it lights up. But i construct the circuit according to the schematic, the cathode going to the pin of the LM3914. Do I have the wrong understanding about the polarity of LEDs?

The 1.21k at pin 7 will only allow 10.3mA to my LEDs.. so I guess that's not enough to light it up rite? Is that why I'm not seeing any light? Help!

minnie_m0use said:

Hello, I'm new in this forum. I've read this thread and I've learnt a lot from it. However, I tried constructing the exact circuit as in the LM3914 datasheet with a pot.

Attached is my schematic..

My problem is, when I turn the wiper of the variable resistor, the LEDs won't light up. Nothing happens. Is my schematic correct?

LED specs : Forward voltage= 1.6V, Forward current =20mA, reverse voltage =2.0V.

I'm a bit puzzled because when I tested with a simple circuit, the LEDs dont follow the common package polarity. The "flag" (cathode) needs to be connected to the +ve of the source and the anode to the -ve bfore it lights up. But i construct the circuit according to the schematic, the cathode going to the pin of the LM3914. Do I have the wrong understanding about the polarity of LEDs?

The 1.21k at pin 7 will only allow 10.3mA to my LEDs.. so I guess that's not enough to light it up rite? Is that why I'm not seeing any light? Help!

Click to expand...

It's been a long time since I used an LM3914 in anything but that schematic looks good to me (it's consistent with the datasheet anyway). Therefore it should probably work if everything's in order.

Obvious things to check:

Battery OK? Check it with a meter to be sure!
General wiring and connections: most things that don't work at all are simply not wired up correctly even if you think they are, so check that closely and meticulously.
LED polarity: the cathode (-ve end of the LED, that is, the end attached to the LM3914) is signified by the short lead as well as the flat on the body of the LED.

10mA is less than the maximum permissable current for most typical LEDs, but should still give significant brightness depending on the exact characteristics of the LED - but most will be near full brightness at this value. So I don't think this is your problem.

Let us know if you get it to work (or not!).

-CF

minnie_m0use said:

Hello, I'm new in this forum. I've read this thread and I've learnt a lot from it. However, I tried constructing the exact circuit as in the LM3914 datasheet with a pot.

Attached is my schematic..

My problem is, when I turn the wiper of the variable resistor, the LEDs won't light up. Nothing happens. Is my schematic correct?

LED specs : Forward voltage= 1.6V, Forward current =20mA, reverse voltage =2.0V.

I'm a bit puzzled because when I tested with a simple circuit, the LEDs dont follow the common package polarity. The "flag" (cathode) needs to be connected to the +ve of the source and the anode to the -ve bfore it lights up. But i construct the circuit according to the schematic, the cathode going to the pin of the LM3914. Do I have the wrong understanding about the polarity of LEDs?

The 1.21k at pin 7 will only allow 10.3mA to my LEDs.. so I guess that's not enough to light it up rite? Is that why I'm not seeing any light? Help!

Click to expand...

Hi,
The power lines are incorrect, look at this dwg.

Thank you EricGibbs for correcting me.. i believe the wiring is now ok seeing that the LEDs light up when i hook it with the 9V battery.

CarlosFandango, yes when i finally wired it correctly according to the schematic EricGibbs posted, it's already very bright!

But there's another problem though.. sure, the LEDs light up, but ALL of them light up at once even when I adjust the pot wiper (bar mode). When i leave pin 9 open (dot mode) only the 10th LED lights up. I also tried adjusting the pot slowly, but to no success. I'm thinking that it must be the signal input not working properly. Maybe the voltage distinction levels are too small for me to see? Does anybody have any experience on this?

*I tried my best to inspect the circuit wiring, and i find it pretty accurate. maybe wrong wiring somewhere, but I'll get back to you after I check it again and again.. I haven't soldered the wires to the pot legs so the output may not be visually stable tho.

minnie_m0use said:

Thank you EricGibbs for correcting me.. i believe the wiring is now ok seeing that the LEDs light up when i hook it with the 9V battery.

CarlosFandango, yes when i finally wired it correctly according to the schematic EricGibbs posted, it's already very bright!

But there's another problem though.. sure, the LEDs light up, but ALL of them light up at once even when I adjust the pot wiper (bar mode). When i leave pin 9 open (dot mode) only the 10th LED lights up. I also tried adjusting the pot slowly, but to no success. I'm thinking that it must be the signal input not working properly. Maybe the voltage distinction levels are too small for me to see? Does anybody have any experience on this?

*I tried my best to inspect the circuit wiring, and i find it pretty accurate. maybe wrong wiring somewhere, but I'll get back to you after I check it again and again.. I haven't soldered the wires to the pot legs so the output may not be visually stable tho.

Click to expand...

hi,
Look a the image, its a configured for 0 thru +5V operation,same as your cct.

If your LEDs are VERY bright, check that you have the correct value of the 1K2 and 3K8 resistors.

Also check with a DVM the voltage on the wiper of the 10K pot, should be 0 thru +5Vdc.

Note: the dwg showing the 'star' connection of the 0V lines to pin2

Increase the 2.2uF to say 10uF.

As the original circuit wiring was 'reversed' to the LM3914 supply pins... the LM3914 maybe damaged.

Hi!

it works fine now.. after replacing the 2.2uF cap with a 100uF (thankfully I have 2 stocks of those!). so nothing's damage..yet! my room must be really 'noisy'..

another concern though, the LEDs light up accordingly when I turn the pot wiper. but, i don't often get the whole 10 LEDs light up.. the last 2 LEDs take some time to light up.. sometimes dont light up at all. I'd need to really turn the pot wiper even after it's already at its max.

I'm guessing this is because for the pot and the resistor values in series with it, I calculated the pot to drop exactly from 0-5V.

I used this equation to estimate the series resistor value with the pot..

R = [(V*RPot)/Vtop] - RPot

V = supply(9V), Rpot = pot value, Vtop= the max input signal

so i could use a little opinion here.. should I design the input signal(Vtop) lower or higher than my intended range of Vtop? Which values should I adjust so that I can get a 'safe' input so that I can easily get the whole 10 LEDs light up smoothly? (i prefer to keep my pot fixed at 10k tho)

btw, I measured R1=1.26k, R2=3.26k --- which gives me roughly 4.48V of max input signal that pin 5 will be able to interpret. which, don't exactly agree with my pot voltage drop...

hi,

I suspect the 9V battery level drops below 9V when driving the bar led's!
So most likely you dont have enough voltage at the top of the pot.

Reduce the 8K0 in series with the pot, to say 4K7

Certainly, it's the battery. why didnt i think of that? lol. I hooked the circuit with a dc psu at 9V and it works like a charm.

Another question..! -_-

I also build another circuit, which is "Bar Display With Alarm Flasher" (attached schematic).. it's in the datasheet but i use it with the same input feed, using a pot. No problem at all, but I'm just curious..

in the datasheet, it says there to use 5V for the powerlines. will the LM3914 be harmful if i use it for 9V? the thing that I observed is that.. at 9V, the flashing starts soon than i expect it.. that is, say the max turn of the wiper be 100% (highest volt across the pot), then at 70% i already have the LEDs flash. at 5V, fairly the same but a little more accurate. How can i make the LEDs flash only when I turn the pot wiper at about 90% on 9V supply? i'm not quite clear on how the circuit works.. so can anybody shed a brighter light?

some info for the bargraph with alarm flasher circuit that i build:
Supply = 9V,
Pot value = 5k, series resistor with it = 6.1k --> volt drop across pot ~4.1V (max)
R1 and R2 same as in the schematic.

sorry, i'm just a girl who has limited electronics knowledge.. but liking it! (",)

minnie_m0use said:

in the datasheet, it says there to use 5V for the powerlines. will the LM3914 be harmful if i use it for 9V? the thing that I observed is that.. at 9V, the flashing starts soon than i expect it.. that is, say the max turn of the wiper be 100% (highest volt across the pot), then at 70% i already have the LEDs flash. at 5V, fairly the same but a little more accurate. How can i make the LEDs flash only when I turn the pot wiper at about 90% on 9V supply? i'm not quite clear on how the circuit works.. so can anybody shed a brighter light?

some info for the bargraph with alarm flasher circuit that i build:
Supply = 9V,
Pot value = 5k, series resistor with it = 6.1k --> volt drop across pot ~4.1V (max)
R1 and R2 same as in the schematic.

sorry, i'm just a girl who has limited electronics knowledge.. but liking it! (",)

Click to expand...

hi,
When the #10 led is not lit the 100uF cap charges upto +5V via the 1K resistor. When the #10 is switched ON, the +V charged end of the cap is taken close to 0V, this makes the negative end of the cap -V [about5v] with respect to 0V.
This will flash the leds while the cap discharges.

Try increasing the 470R to 1K0, when using a 9V supply.

I have met many 'I'm just a girl' who are first class engineers.

So I tried.. adding the 1k resistor improves it just a little bit. maybe i just need to get over it and learn to say 'good enough'. hehehe..

may i know the reason why you suggest 1k though? do you still use the Vrefout formula in the datasheet to determine the max input signal?

minnie_m0use said:

So I tried.. adding the 1k resistor improves it just a little bit. maybe i just need to get over it and learn to say 'good enough'. hehehe..

may i know the reason why you suggest 1k though? do you still use the Vrefout formula in the datasheet to determine the max input signal?

Click to expand...

hi eng.Minnie

The 470R to 1K change has nothing to do with Vref...

Its to reduce the -V being connected to the LM3914 when the #10 led lights.!

Try increasing the 3K8 to say 4K7, lets know what you get.

hehe.. i don't know which 3K8 u're talking bout. but i did try playing around with various resistor values at the -V end of the cap.. it has improvements for bigger resistors, but the effect is not that significant anyway, but still does what it says on the tin. i guess i need to stop playing with lm3914 and just be happy with the experience.

thank you for the guidance! i hope this thread will be helpful to others as it is to me..

minnie_m0use said:

hehe.. i don't know which 3K8 u're talking bout. but i did try playing around with various resistor values at the -V end of the cap.. it has improvements for bigger resistors, but the effect is not that significant anyway, but still does what it says on the tin. i guess i need to stop playing with lm3914 and just be happy with the experience.

thank you for the guidance! i hope this thread will be helpful to others as it is to me..

Click to expand...

hi,

Its the one marked on this dwg.

Instead of measuring the fluctuating voltage from the 9V battery, why not connect the pot to the regulated voltage reference at pin 7?

@ericgibbs : sorry, i thought u were talking about the alarm flasher circuit! yes, i tried it with a 4K7, and it actually takes more turns for the pot to light each LED one after another.. so i'm guessing the voltage level for each LED is improved with that..

@audioguru : that's a new idea to me.. but isnt the voltage across pin 7 is 1.25V? can you explain in a little more detail how that'd work? 1.25V is all i know, so thats the level i'm at....

another basic question..
based on the bar display with alarm flasher, would it be okay if I put more LEDs at the output of the IC? I mean, to put 2 or 2 LEDs in series at each pin branch which then would give me 2 or 3 arrays of LED meter. Would 9V be enough to power everything though? I'm guessing it can, but perhaps the LEDs would be a little dim..?

minnie_m0use said:

another basic question..
based on the bar display with alarm flasher, would it be okay if I put more LEDs at the output of the IC? I mean, to put 2 or 2 LEDs in series at each pin branch which then would give me 2 or 3 arrays of LED meter. Would 9V be enough to power everything though? I'm guessing it can, but perhaps the LEDs would be a little dim..?

Click to expand...

hi,
IF the battery maintained 9Volts while on load and the leds had a forward voltage drop of say, 2V then you could put 3 leds in series.

This would give a 6V drop across the leds, this will allow for the battery falling in voltage.
Each string of 3 leds will take the same current as 1 led..
The LM3914 has constant current led drivers.

OK.?

minnie_m0use said:

@audioguru : that's a new idea to me.. but isnt the voltage across pin 7 is 1.25V? can you explain in a little more detail how that'd work? 1.25V is all i know, so thats the level i'm at....

Click to expand...

The regulated reference voltage at pin 7 is adjustable.
There is 1.25V between pin 7 and pin 8. The resistor R1 is 1.2k so its current is 1.25V/1.2k= 1.04mA. The 1.04mA also flows through R2 so R2 has a voltage across it of 1.04mA x 3.8k= 3.95V. Then the voltage at pin 7 is 1.25V + 3.95V= 5.2V. Pin 6 is connected to pin 7 so the 10th LED lights when the input voltage is 5.2V.

another basic question..
based on the bar display with alarm flasher, would it be okay if I put more LEDs at the output of the IC? I mean, to put 2 or 2 LEDs in series at each pin branch which then would give me 2 or 3 arrays of LED meter. Would 9V be enough to power everything though? I'm guessing it can, but perhaps the LEDs would be a little dim..?

Click to expand...

The LED outputs have regulated current that is 10 times the current from pin 7. The current in pin 6 is 0.43mA so pin 7 has a total current of 1.04mA + 0.43mA= 1.47mA. So the LED current is 14.7mA.

With a 9V supply a few LEDs can be connected in series at each output and their total voltage must be 7.5V or less. The LED that causes the display to flash has a resistor in series with it so that output cannot have many LEDs in series.

so the pot configuration for measuring pin 7 would be the same if i were to use it on the alarm flasher cct right? Hmm.. say i want to have two of these bar displays (with alarm flasher).. but they're not cascaded, just separate from each other. is it possible to control it with only one pot? How would that be??

minnie_m0use said:

so the pot configuration for measuring pin 7 would be the same if i were to use it on the alarm flasher cct right?

Click to expand...

You can connect a pot to pin 7 and measure the voltage of the slider of the pot.
But the alarm flasher messes up the voltage at pin 7 to make the LEDs flash. The flasher circuit needs to have an external input voltage.

say i want to have two of these bar displays (with alarm flasher).. but they're not cascaded, just separate from each other. is it possible to control it with only one pot? How would that be??

Click to expand...

One pot with an external voltage across it can feed hundreds of flasher circuits because the input resistance at the input pin 5 is very high.