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help with Hard drive VU meter with LM3914

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kaffine

New Member
Hi, I am building a Hard drive activity meter based on the schematic at https://www.gideontech.com/guides/HD_VU/index03.shtml and I am having some troubles. I am pretty sure I have built the circuit correctly, and am testing it. The directions say to short pins 4&5 on the optoisolator socket, which I did, and then put 5V over the input. I attach the power, and the first 5 leds light up, without the last 5. Ideas?
 

crust

Member
Well, I looked at the circuit and shorting pins 4&5 definately does not seem like the proper thing to do. That will always put 0.95V at the reference pin. The circuit then divides this by 10 and each led represents on of those values. I didnt read the article, but I guess this is what they found from their filter by testing the circuit.

Try to take out the short between 4 and 5 and let us know what happens. That puts a higher voltage at the input than your circuit was supposed to handle. Though I thought the 3914 could handle that, maybe it is safe, but does not behave properly.

Perhaps you could measure the voltage at pins 8 and 5.
 

panic mode

Well-Known Member
There is not a single mention of shorting of optocoupler pins 4 and 5
in the link you posted. This is probably something you have read in
the forum on that website.
You can do this to test to circuit. When shorting the 4 and 5 pins,
220uF capacitor will charge through 470R and you will see change
on the led scale. When you remove short, capacitor will discharge
through 330R and you can watch the sweep of leds.
This led swwep to full scale and back to zero is meant only as
a final test before connecting it to a PC.
Think of it as simulation of run condition.
 
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