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Please help

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mahek3054

New Member
I am a senior student in engineering and I am doing my project for my graduation. I am having a problem using one of the chips manufactured my National Semiconductor. I am building a exercise bike for handicapped children which gives output in terms of lights according to the speed they bike. I am almost done but I am stuck with circuit of LM3914. I followed the circuit given in the specs for the 0V to 5V bar graph meter, in there I just changed the value of the R2 to 8.2k with the help of the equation of Ref Out. However, the circuit does not work as I want it to work. Since my input is going to be from 1V to 10V, I am looking to see if each Led lights up to 1V. so the first one will come on at 1V and the second one at 2V and so one until the last led will turn on at 10V. Right now the circuit works only till 4V after that each volt is turning on two Leds instead of one. I am not sure if I am missing anything else or if I am using the wrong circuit. I will really appreciate if any one could give me any advice on this situation. My power supply for the whole circuit is 10V.

Thanks......
 

k7elp60

Active Member
LM3914 problem

One thing I see is that your are trying to get a reference voltage out of pin 7 of 10 volts with a ten volt supply. That won't work. You need about
1.8 volts of head room for the reference to work. If you can increase your supply voltage to 12 that should solve that problem. Another possible problem is circuit oscillations. I have found by putting a bypass
capacitor from the anode buss of all the LED's to pin 2 solves this problem.
A 2.2uf tantalum or a 10uf electrolytic works fine.
If you cannot increase your supply voltage to 12 volts, you can go back to R2 of 3.83k and then use a voltage divider consisting of 2ea 10k resistors from the input voltage to ground. The junction now going to pin 5(signal).
It now becomes a 0 to 5V indicator, but scaled to 0 to 10 volts.
Hope this helps. :D
 

Roff

Well-Known Member
You can't have Rhi (Vref) equal to your supply voltage. See the graph below. You will have to either raise your supply voltage to 12 volts, or put an attenuator on your input. For instance, you could attenuate the input by 0.8 (with a resistive voltage divider), and set Vref=Rhi=8 volts.
I'm not sure if this explains why you get two LEDs on at the same time. I'm surprised that the lower ones switched at the correct voltages, unless you had Vcc higher than 10 volts. If you did - are you using a digitally controlled supply as your input signal? Here is a quote from the datasheet:
When in the dot mode, there is a small amount of overlap or“fade” (about 1 mV) between segments. This assures that at no time will all LEDs be “OFF”, and thus any ambiguous display is avoided
If you are setting your input steps to exactly 1 volt, and you are in dot mode, this is a possible cause, although it would be extremely unlikely.

Edit:
After posting this, I saw that k7elp60 came up with the same solutions. Great minds think alike. :lol:
 

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k7elp60

Active Member
Thanks Ron H for the compliment.
:D
 

mahek3054

New Member
thanks

Thank you guys, I am going to apply your suggestion today. will post to tell what happend. I am trying not to change my voltage supply to 12V since I have to again test all my previous circuits to make sure they work with 12V too. anyhowz if worst comes worst I might change.

Thanks again.
 
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