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Some help with circuit... Ground and negative pole...? Answered

Hello, just a small help.

Please look on a scheme. I wonder what should I do with ground. I just leave these wires conected to nothing? And where is here a negative pole? It's not nessesary? This is VU meter.



Best Answer 8 years ago

A Ground is a common reference point for a circuit, which is why it's also termed Common. Electricity needs a return path, or nothing flows.

This circuit, like most, expects all the ground symbols to connect together. Whether you plan to power this with a battery of power supply, the negative "pin" of the supply voltage also needs to connect to the ground.

They left the negative pole of the battery out of the drawing, 'cause it's just assumed the negative supply pin is connected to ground.

For schematic noobs, this is confusing. They'd rather see a wiring diagram than a schematic. Wiring diagrams explicitly show each-and-every connection. Trust me--the more experienced you get with electronics, the more you will prefer the schematic... Schematics are actually simpler.

Notice how Pin 4 (RLO) is also connected to ground? (over at the V- pin.) It's the ground (shield) of the mono audio signal. The ground connection of a standard audio cable is usually the sleeve of a TS plug, or it's the outer collar of an RCA plug.

You don't actually have to connect the audio ground to RLO and V-, only to the common ground. Since all three will share the same ground they are connected by definition.

When you connect the audio ground to the common ground on your project, your bargraph will then have a true "earth ground"--if the audio device is AC powered. If it's an iPod or something, your common ground will be "floating." That's not a problem at low voltages.

The ????? part is a potentiometer--you might call that a "volume control." POTs are variable resistors. They have three pins--one connection for the "wiper" that sweeps across the resistor, and a connection at each end (that's three total.) The wiper connection is ALWAYS the center pin. The other two connections are identical--they will only control the direction of the POT (i.e., will twisting to the right turn the effect up, or down...)

That 10K POT connected to REF pins is the brightness adjustment for the LEDs.

Here's the Datasheet (PDF.)


8 years ago

Jayefuu, almost, something like that...

So, i connect ground to negative? You confused me... :S

And, the part with "????" I connect to, for example, to left side of resistor, and it's right side i connect to left, also reight goes to negative... Huh :S

PIN 2 is the NEGATIVE connection for your battery.  The OTHER ground pin 4 is also connected to battery negative.   If you have a SHIELDED cable on your audio input (pin 5) then the shield should ALSO be connected to ground.  The end of resistor R2 also connects to battery negative. Pin 3 and pin 9 go to battery positive... and also to the LED lineup.   A good method of building circuits like this is just to use a SOLID metal surface to mount parts on.  An UN-etched pc board would work fine.  Or just a thin sheet of copper or brass a few inches square.  Just glue the parts down... then SOLDER pins 2 and 4 to the metal sheet.  Thats your GROUND.   Negative of the battery goes there also.  The reason those points are shown just hanging there is  to simplify the drawing.  If schematics showed EVERY GROUND connection, the schematic would be very cluttered. As a wise and experienced technician, you should KNOW that those points are CONNECTED TO EACH OTHER and to battery negative. 

Ground is technically a term meant to describe an earth ground, and it is for safety. That's a story for another fireside chat.

But in the vernacular of the layman, one can sometimes think of "ground" as "com" or in a single voltage supply, the negative power supply terminal, or in a bipolar supply, the 0V connection.

The connection shown should be connected back to the supply's output side  "com" connection.

You don't need a negative voltage for this circuit, the LM3915 IC needs a supply between 3V and 25V. As it says on the diagram, connect pin 3 to the positive terminal of your battery/power supply and the ground to the other.

Is this the instructable you're trying to follow?

That's correct. "Ground" here means "the zero-volt connection relative to which other voltages are measured", not necessarily the chassis and not an "earth" ground. So V+ gets conencted to the + side of your power source and ground gets connected to the - side; the potential (voltage) difference between them should be between 12 and 28 volts.

The ground symbol is used for this in order to make circuit diagrams more readable. If it's intended to be a chassis ground or safety ground, the circuit diagram will say so. If it's intended to be an earth ground, there's another symbol used for that, which looks something like

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