# Simple Project

E

#### electricked

Jan 1, 1970
0
Hi all,

I've been reading some electronics theory and I've finally decided to do a
simple project. My project consists of having a microphone and 4 red LEDs.
The 4 LEDs act as a volume meter. So when someone talks in the mic, the leds
light up depending on the volume. If the volume is low, then no LEDs light
up. If it's very high, then all 4 LEDs light up. If it's somewhere in
between only 2 LEDs light up.

I'm thinking on loading the circuit from a 9V battery. So my initial
thoughts were to use a transistor and build the circuit as follows:

R=105ohms
___
-------------|___|----------------------
| | | | |
--- | | | |
- VCC 9V | | | |
| | | | |
| | | | |
| V V V V
| - - - -
| | | | |
|/ | | | |
-----------| | | | |
| _ |> | | | |
|-/ \ | | | | | |
(Mic)| | | | | |
|-\_/ | | | | | |
-----------------------------------------------------
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

First of all, how does a microphone work? Do I need to apply voltage to it
in order for it to work, or does it produce current on it's own when noise
is detected?

What I was thinking is having the microphone switch a transistor if someone
is talking and then depending on the loudness of the volume then the
transistor will let current flow from C to E accordingly. At least that's my
understanding of transistor so far. I'm thinking, if the base current is
lower, then the current in main circuit will be less, if current at base is
higher, then current in main circuit will be higher. Having this figured
out, I was thinking the transistor will drop 0.6 volts so now I have
9-0.6=8.4V when the transistor is fully open. So next I was thinking I'd let
the max current be 80mA since 4 LEDs times 20mA each in parallel will be
80mA max current required for them to light up fully when the transistor is
fully open. So I'll need a resistance value to limit the current in the main
circuit. So I have R=V/I=8.4/0.08=105OHMs. So that's my resistance. Next
thing, I figured say the transistor is half way open it will allow 40mA to
pass and only two LEDs will need to be lighted up. But if I connect all 4
LEDs in parallel then isn't the 40mA going to split into 4 10mA for each LED
connected in parallel? So this is not the effect I want. This will light up
all LEDs but with less brightness. What if I connect the LEDs in series?
Then I'll have to change the resistance since only 20mA max have to be
allowed at fully open transistor. But then if the LEDs are in series and I
have 20mA then the brightness depends on the voltage, right? So each LED
will drop about 1.6V is it?

So I'm stuck here. If I put the LEDs in parallel will that get me the effect
I want? Basically, if the transistor is half open only two LEDs will light
up at their full brightness and the last two in series won't light up at
all. Is that going to work? How do I figure the value for the resistor in
that case?

All help is welcome. Sorry for the long-winded post. I'm just trying to put
my thoughts into writing so it makes sense.

Thanks!

--Viktor

A

#### Andrew Howard

Jan 1, 1970
0
...The 4 LEDs act as a volume meter...

I think this is commonly called a VU Meter.
I'm thinking on loading the circuit from a 9V battery. So my initial
thoughts were to use a transistor and build the circuit as follows:

R=105ohms
___
-------------|___|----------------------
| | | | |
--- | | | |
- VCC 9V | | | |
| | | | |
| | | | |
| V V V V
| - - - -
| | | | |
|/ | | | |
-----------| | | | |
| _ |> | | | |
|-/ \ | | | | | |
(Mic)| | | | | |
|-\_/ | | | | | |
-----------------------------------------------------

I think in this circuit, all of the LEDs will glow at the same time,
regardless of the input. I don't actually know of a circuit but perhaps
doing a google search on "VU Meter circuit" (or something) might turn up
something.

First of all, how does a microphone work? Do I need to apply voltage to it
in order for it to work, or does it produce current on it's own when noise
is detected?

It depends on the type of mic, electrets use a voltage, but I don't think
dynamic ones do.

Andrew Howard

P

#### Peter Bennett

Jan 1, 1970
0
Hi all,

I've been reading some electronics theory and I've finally decided to do a
simple project. My project consists of having a microphone and 4 red LEDs.
The 4 LEDs act as a volume meter. So when someone talks in the mic, the leds
light up depending on the volume. If the volume is low, then no LEDs light
up. If it's very high, then all 4 LEDs light up. If it's somewhere in
between only 2 LEDs light up.

You need considerably more electronics than you've shown.

First, you will have to amplify the signal from the microphone to get
a more useful level, then use a comparator to drive each LED, with the
comparators comparing the audio level to different preset values to
make each LED come on at the desired level.

National makes a few nice parts that will do the level comparison/led
drive part of this - see
http://www.national.com/pf/LM/LM3915.html#Datasheet for details.

The 3915 is logarithmic, suitable for audio level meters. The 3914 is
linear, and suitable for use as a voltmeter.

G

#### grahamk

Jan 1, 1970
0
Peter said:
You need considerably more electronics than you've shown.

First, you will have to amplify the signal from the microphone to get
a more useful level, then use a comparator to drive each LED, with the
comparators comparing the audio level to different preset values to
make each LED come on at the desired level.

National makes a few nice parts that will do the level comparison/led
drive part of this - see
http://www.national.com/pf/LM/LM3915.html#Datasheet for details.

The 3915 is logarithmic, suitable for audio level meters. The 3914 is
linear, and suitable for use as a voltmeter.

Have a look at
http://homepage.ntlworld.com/g.knott/elect141.htm
You will need an amplifier between the mic. and the input tho'

E

#### electricked

Jan 1, 1970
0
I understand. That's why I'm doing this project so I can gain understanding
of electronics and specific devices such as the microphone and so on.

How would I go about building this circuit without using an IC? If I'd like
to build my own comparator and so on...

What's the basic idea behind this project? How can I make it so depending on
the voltage coming in, I light only the lights that measure the voltage
correctly?

Thanks!

--Viktor

E

#### electricked

Jan 1, 1970
0
Thanks graham!

That's exactly what I need I just need less LEDs (4 to be precise).

What I want to do though is instead of using the LM3914, I want to build it
LEDs, amplifiers, etc. that you think are worth while?

Thanks!

--Viktor

Peter said:
You need considerably more electronics than you've shown.

First, you will have to amplify the signal from the microphone to get
a more useful level, then use a comparator to drive each LED, with the
comparators comparing the audio level to different preset values to
make each LED come on at the desired level.

National makes a few nice parts that will do the level comparison/led
drive part of this - see
http://www.national.com/pf/LM/LM3915.html#Datasheet for details.

The 3915 is logarithmic, suitable for audio level meters. The 3914 is
linear, and suitable for use as a voltmeter.

Have a look at
http://homepage.ntlworld.com/g.knott/elect141.htm
You will need an amplifier between the mic. and the input tho'

B

#### Bill Bowden

Jan 1, 1970
0
electricked said:
Hi all,

I've been reading some electronics theory and I've finally decided to do a
simple project. My project consists of having a microphone and 4 red LEDs.
The 4 LEDs act as a volume meter. So when someone talks in the mic, the leds
light up depending on the volume. If the volume is low, then no LEDs light
up. If it's very high, then all 4 LEDs light up. If it's somewhere in
between only 2 LEDs light up.

You might try something like this:

http://ourworld.compuserve.com/homepages/Bill_Bowden/page8.htm#db.gif

It uses a LM324 quad op-amp (4 amplifiers in one cheap IC)
and covers 60 to 70 dB with 3 lights, which is close to normal
conversation. A small speaker is used as the microphone.

-Bill

T

#### Terry Pinnell

Jan 1, 1970
0
electricked said:
I understand. That's why I'm doing this project so I can gain understanding
of electronics and specific devices such as the microphone and so on.

How would I go about building this circuit without using an IC? If I'd like
to build my own comparator and so on...

What's the basic idea behind this project? How can I make it so depending on
the voltage coming in, I light only the lights that measure the voltage
correctly?

Thanks!

--Viktor
Is that a reply to Peter's post? (It would be easier to follow if you
followed the general convention of *bottom* posting, as I am doing
here.) If so, IMO he has already given you the the key aspects to
pursue. To summarise, you need to study and experiment with these
topics:

1. Microphones; there are various types.

2. Amplifying their typically small signals, or restricted to the
specific type of mic you are working with.

3. Converting that amplified AC to a DC signal, smoothed sufficiently
to allow appropriate display of the 4 LEDs.

4. Using opamp or comparator ICs. A single chip can handle 4 of them.
In practice I expect you'll need to set the LED indication levels you
want by trial and error, using 4 presets.

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