See Fritzing screenshots here. http://i.imgur.com/1tpt697.png
The idea is taken from Sparkfun’s tutorialhttps://learn.sparkfun.com/tutorials/do … okup-guide , but only uses 2 cascading LM3916’s.
Have I read the data sheet correctly? I have all the parts, but dont want to damage anything before soldering.
What is the green wire doing ? I think I understand what you meant the blue wire to do.
The green wire is set to every LM3916’s pin 5. Sorry, i know its kinda hard to see. But pin 5 on the LM3916 is signal, so they should all be tied togeather.
The Blue wire is going from LM3916 pin 1 to the led bar graph pin 1. This allows 10 leds when the chip only has 9 pins.
Cascading a LM3916 is likely to not produce the results you are looking for unless that is non-uniform steps. For cascading, the LM3915 is a better choice because it has uniform 3dB steps.
But there is a reason why the device data sheets do not cascade devices the way that the SparkFun page suggests. To illustrate this consider the voltage divider of a single LM3915.
If Vhi connected to some reference voltage, Vref, then the voltage at the input of comparator at pin 11 is Vref * 16K/(16K+6.63K) = 0.707Vref or 3db down.
Now cascade two and compute the same voltage divider output for the top LM3915:
Vref * 38.63K/(38.63K+6.63K)
Which is very different (-1.3dB) and probably not what you wanted. With four units cascaded, it gets even worse. I suggest that you extend the opamp based method of cascading as recommended in the device data sheet. But four units is probably not worth the effort because it will be impossible to get anything useful out of the lowest unit. It is working at -90 to -120dB which is likely to be in the noise not to mention the offset issues.
Then would the current wiring setup work with the LM3915? or would i need an opamp somewhere?
UhClem, if cascading them won’t work, then why did Sparkfun post a tutorial about it and a pic that it looks like it’s working?
codlink:
UhClem, if cascading them won’t work, then why did Sparkfun post a tutorial about it and a pic that it looks like it’s working?
It will work only if the altered transistion levels meet your requirements.
I believe that’s what the pots and resistors do. Change the values to get different transition levels…
codlink:
I believe that’s what the pots and resistors do. Change the values to get different transition levels…
The resistors are there to set the LED current.
On page 7 of the datasheet, you can add a resistor (R1) to set the voltages. R2 is the current set resistor.
Schematic attached.
