Nudge needed please on this

Hi,
Please could some one point out the meaning of the bits I have marked in
red circles on the attached vu-sch.jpg

Do The (X marks) mean external connections or..... ?
And what about the bit above the 10 led's (in red circle) ?.


These forums are great for us people starting out in the world of
electronics.

Thanks in advance

The X's are external connections. The one that you circled at the top means you tie that point to +12v which is your X4 connection. You may also want to put a 2 uf tantalum cap from +12v to ground, with ground being your X5 connection.

Thank you for the fast reply.
Sometimes I get stumped on silly things like this,mostly when there are floating
connections in a schematic.

Other times would be , when not recognizing a component symble ect.
I am so glad to have found these forums ,where we can ask, and learn from
the more experienced people like yourself.

Could I just ask you about your suggestion for a capacitor across 12v to ground, i:e its purpose ect ?.

I think it may be with regard to interference ect , but not sure.

Best Regards.

It would function as a bypass cap for the IC. It supplies quick current during transient periods since you actually have to wait a brief fraction of a second to suck current through the relatively long line to the battery (and you have to fight the battery's internal resistance as well).

In this application, if this was all you were doing, I would say it would be optional. You might have the top LED of your bar not flick on quite as bright during brief loud parts of the music (or it might not flick on at all if the loud part was brief enough). I dunno about a tantalum though, I'm not terribly fond of 'em. Maybe a 1uF or 2.2uF ceramic should do the fast transient bit just as well. If you do go tantalum, make sure you derate the voltage by ~50% (for a 12V line, I'd want at least an 18V rated tantalum).

Hi,
thanks for the additional info on the cap.
Can't seem to locate any uf ceramic caps though on a quick web search.

Seems that there are an abundance of tantalum ones though.
Will keep looking for the ceramic ones (maplin electronics) here in the uk
have gone to the dogs lately, so will look at places like digikey or farnell.

Thanks & regards.

Cobra148 - that username rings a bell .........

Thats not you is it Bob ?

The data sheet recommends a 2.2uf tantalum or 10uf aluminum electrolytic. This is required if power leads to the LED's are longer than 6 inch. I would just use it even if leads are shorter.

https://www.electro-tech-online.com/custompdfs/2008/07/DS005104.pdf

Thanks Mikebits,
just found good cheap bundles of 10uf aluminum electrolytic's on ebay.

Just wondering though if the cap should be placed at the main board end,
or the LED PCB end.

I am making the circuit components on 1 board, with 12" long wires from pcb
terminals to a 10 LED Support pcb board.

I made this PCB & layout with 10uf electrolytic from line in
do I need another one on the +12v ?.

LEDs Support boards (1.jpg) attached
MY PCB MAIN BOARD ( 2.jpg) attached

Before any one asks, I used pcb terminals because hundreds of them fell out
of the back of a truck near me LOL.
I know header's would be the choice, but hey I got to use them terminals
some where.

Thanks also to mdwebster for your help & suggestions.
And to picbits , no I am not Bob, hope I didnt take his username my name
is stuart.

I would say, the main board end, as it is the chip you worry about. Just one suggestion, All the green space going around the PWB, you should make it all ground plane. Big grounds, good...little ground, bad... In other words tie all your floating green copper to ground. Make fatter traces to your grounds as well.
Your layout does not seem to quite match your original schematic. Can you post your schematic that matches your layout?

If all 10 LEDs stay on for more than a few seconds then the LM3915 will overheat and be damaged in your new circuit because the supply voltage (12V) is too high. It should be 6V or have a huge power resistor feeding the LEDs to share the heat.

EDIT: Its pin 9 is set up to make a dot of light, not a bar of LEDs so there will not be much heat.

But the Mickey-Mouse rectifier has a huge error if the transistor does not have the high supply voltage.
The datasheet for the LM3915 shows a much better Half-Wave Peak Detector circuit that has a very small error and works fine from a low supply voltage.

It looks like the C and E of the transistor are backwards.

Well spotted Mikebits, yes I did sway to another schematic on the vu search.
This schematic looked a little clearer to me, so I went ahead and drew up a
board trace (sorry should have mentioned it).
Thanks for the tip on the ground plane problem, will ammend that to the pcb.

I have attached the correct schematic for this board & the pcb trace without
the placements in the way, for a clearer view, in case I have made errors that were hidden by the placements

audioguru, thanks for the extra info.
You have me slightly worried though about those dreaded transistor thingies.
They always catch me out, (base ,collector, emitter) god knows where they go sometimes.
I look the data sheets to check base leg ect, and still manage to get them
the wrong way round sometimes.

I would be very greatful if Mikebits & yourself could detect more errors if
any before I build this board.

The purpose for my building this board is for the car, so I can feed the
stereo or radio scanner through it.
So while at traffic lights people will look and say "what the fu**# is that" lol.
These 2 seperate vu scales (using 2 boards) will be on the dash.

About the heat thing, may want to use bar mode, so do I change to a
5v reg on the 12v line with a heatsink ?.
And would I get less LED brightness with 5 volts ?.

Thank you all so much for the help so far.

Hi Cobra,
The outputs of the LM3915 have regulated current so a current-limiting resistor is not needed for each LED and a voltage regulator is not needed.
The 680 ohm resistor sets each output at 17mA so 10 LEDs is 170mA.
When the car battery is being charged the "12V" is actually 14.0V.

If you use 2.0V red LEDs then each output of the LM3915 has 12V across it and 17mA through it. Then the power dissipation in the LM3915 is 2.04W when all 10 LEDs are lighted. Its max allowed dissipation is 1.36W at room temperature and less in a car in summer.

A 56 ohm/2W resistor in series with the supply to the LEDs will dissipate a max of 1.6W then the LM3915 will dissipate the remainder of only 0.42W. Then a 2.2uF (or 10uF) capacitor must bypass the LEDs side of the resistor to ground.

Hi audioguru.
Do you mean like this (attached) ?.

If I am right,then I presume it will be 2 components for each 10 LED support
board (if I go for 2 seperate channels) ?.

Thanks

AG, maybe you can answer this for me. Are not most stereo speaker outputs balanced? Can you ground one side of the speaker terminal as shown in first schematic, or is a Line out being used in which case should be ok.

Also, I think in a automotive environment, there is a lot of noise on the DC. Your circuit should have better DC filtering, a regulated voltage would also help.

The 68 ohm resistor on your layout is tiny, maybe only 1/8W. You need a huge 2W resistor.

Car radios usually have a bridged amplifier so one terminal of a speaker is not grounded. Then the signal into the first circuit could be connected to one wire of a speaker and the ground connection connected to the power supply ground, not to the speaker.

The transistor in the second circuit will pickup interference from the car electrical supply. Then a 5V voltage regulator will be a good idea. The current for the LM3915 plus the transistor is very low so a heatsink will not be needed. Each set of LEDs still will need a 68 ohm/2W resistor and 10uf capacitor.

Gee you guys are good.
I can see that without your help, I would have had stormy times ahead with this project.

And now I feel good to go with it, except for the second board transistor interference problem.Just need some clarity on that part. Do I now feed the both boards with 5v reg's ?.

One other thing was the transistor being the wrong way round ,on looking at
the last schematic I attached,do you think it is wrongly placed ?.

Can I use a wire wound 2 watt resistor, I seem to have trouble locating
a carbon type 2w, is the inductive part of the resistor important in this
project ?.

I thank both of you so much for staying with me on this, and as you
probably guessed I am only at the 1st rung of the ladder in electronics,
(Well standing on a brick towards the 1st rung to be more precise).

The thing you have both taught me is,that there is always room for
improvement , & I would have surely lost a few components if I wasnt
aware of the problems noted here.

Many thanks both.

The datasheets for the BC547 transistors that I have show which pin is which and the pins are in a straight line, not in a triangle.
With the leads pointing down and looking at the flat part of the case the pins from left to right are Collector, Base and Emitter. Most European transistors are like that.
American 2Nxxxx transistors are the opposite at Emitter, Base and Collector.

It does not matter which type of 2W resistor you use because the 10uF capacitor cancels its inductance.