An LED Bar Graph VU Meter

VUmeter

Bar graphs VU meters can be easily made with a simple integrated circuit. There are several of them with different characteristics, but they all present the same basic functionality: one or more LED in a row are used to visualize, more or less precisely, the voltage amplitude presented to the input. That voltage can be either a direct current or an alternate current and, in particular, it could be the output of an audio amplifier.

But, what is inside these integrated circuits? How do they make possible this kind of behavior?

Here is the design of a simple gadget that shows how a bar graph VU meter works. Building and using this device is certainly a fun way to learn the principles used to design the bar graph integrated circuits.

bar_graph_vu_meter

The basic element of the circuit is the one made with the components Q1, R1, R10, D1, and D10.

This block of components then replicates several times to increase the number of LEDs used in the bar graph. In this particular case, the same circuit is replicated 8 more times, for a total of 9 LEDs in the bar.

You can also see that each block, or stage, receives as the input the output from the previous stage.

Diodes form D10 to D17 are used to provide a different threshold to each stage. In fact, let’s say that the first stage is triggered when the voltage on the anode of D10 reaches about 3V. In order to trigger the second stage we will need 3V on that stage, but that means that the voltage at the first stage has to go up to 3+0.6V, or 3.6V. The extra 0.6V is the forward voltage of diode D10.

Similarly, to reach each further stage, we will need an input voltage 0.6V higher for each stage we want to light up.

In the end, to light up the last stage we will need an input voltage of at least 3 + 8 times 0.6V, or 7.8 volts.

Once the threshold is reached in a stage, the corresponding transistor switches on and starts conducting a current that is only limited by the LED and the series resistor which, in this case, is 330 ohms. With the values in the circuit, the LED current will be about 20 mA.

So, when we apply a voltage to the input terminals, depending how high the voltage is we will see a number of consecutive LEDs lighting up, while the remaining will stay off because their respective stages have not been triggered yet or, in other words, the voltage at those stages hasn’t yet reached the threshold imposed by the diodes.

Note also that resistors R10 to R18 are not all of the same value. This is because by the time the voltage reaches the threshold in the last stage with transistor Q9, the voltage on the previous transistors is higher and higher while we move to the left of the circuit, since we need to add back the 0.6V that the diodes are dropping. Therefore, to avoid damage to the transistors on the left, we need to increase the base resistor when moving from the right to the left of the circuit.

Another thing to notice is the trimpot located at the connectors for the input signal. The circuit as it is, is capable of handling signals up to the value of the power supply, which is 9V.

However, we can adjust the trimpot to handle higher signals, just by moving the trimpot cursor toward the end that is connected to ground, in order to get only a fraction of the actual input signal.

Conversely, if you had a very small input signal, that could not trigger even the first stage of this circuit. In that case, you could still add a very simple amplifier between the source of the signal and the input of this circuit, which would help increase the level of the signal to the required value.

Finally, you see that the bar graph meter is powered with a 9V power supply. I used such value so you can use a 9V battery, if you like to try the circuit.

However, if you wanted to use this circuit as part of a more complex system having a higher value of the power supply, you could just modify resistors from R1 to R9 and use the power supply of that system.

For example, if you planned to use 12V instead of 9, you would use resistors of 470 ohm rather than 330, and everything would work just fine.

Remember, however, that 9V is the minimum voltage you can use to correctly power the bar graph circuit. You can only increase the power supply voltage to a higher value and increase accordingly the resistors R1 to R9. That is because we need a power supply that exceeds the voltage at the base of the leftmost transistor Q1, which can be as high as 7.8 volts, as we said before.

The circuit can be easily assembled on a per-board, like in the case in the frnt picture of this post. There, I used a 3 prongs connector to provide the power supply and the input for the external signal.

Once the circuit is assembled, set the trimpot with the cursor toward the ground side, then power up the device and put a signal to its input. The signal should be the highest possible with the amplifier to which you are attaching the VU meter. Then, adjust the trimpot until all the LEDs are lighted up. Now the circuit is tuned and you can input to it any signal that changes between 0V and the max you used for the tuning.

For further information, and to see the VU meter in action, take a look at this video that I posted on my YouTube channel.

Happy experiments!!!

How Resistors Work

resistors_series

What is a resistor? Why would I want to use it? Where can I find it?

I’m sure you have asked these questions and many others to yourself several times. Here, I hope to give you at least some of the answers. But keep in mind that there is much more behind this and I could keep writing pages and pages on the subject barely scratching the surface of it.

So, why am I doing this? Because, for the most part, the information I will provide here are enough for day to day use of the resistors in simple electronic circuits used to for learning and for early experimentation. If you need to know more, then you are already on the road for becoming a true electric or electronic engineer.

A resistor is an electric device which only reason to exist is to reduce the flow of the current in a circuit. It obtains this effect by dissipating the extra energy of the current into heat. Yes, heat! There was never in the engineering history a device that wasted more energy than a resistor (percent wise). But then you’d ask: why in the world we want to use it? Because, used in the appropriate way, it allows us to do a lot of things that wouldn’t otherwise be possible. Just think at this: there is no electronic circuit in the world that does not use resistors.

Here is how resistors look like:

resistors

Back in 1827, a German physicist and mathematician named Georg Ohm, published a paper containing what it was later called Ohm’s law. It was basically a formula that correlated the current that flows in a wire with the voltage applied at its extremities. He found that increasing the voltage, the current also increased of a proportional amount, and he called the proportional constant “resistance”. The formula of the resistance was born (although he did not write it exactly this way):

V = I x R

A resistor, therefore, is fundamentally a piece of conductor that presents a certain resistance to the flow of the current. When we apply a voltage V at the ends of the conductor, an electric current I will flow, proportional to the amount of voltage by the constant R, the resistance of the conductor. In the SI system, the resistance is measured in Ohms, to honor the discoverer of the law, while the voltage is measured in Volts and the current in Amperes.

Today, resistors are made of different materials. These materials are substantially a mix of a good conductor and an insulator (a material that blocks the flow of current). Adjusting the mix of the two substances, it is possible to create resistors having a wide range of possible resistances, from tens to millions of an ohm.

There are two symbols that are normally used in schematics to represent a resistor. The most common in USA is the one with a zig-zag shape; the other is the one specified by the IEC (International Electrotechnical Commission):

resistor_notation

Let’s now talk about how resistors can be connected to each other to accomplish some simple tasks.

The first way to connect together two or more resistors is to put them in series. Two or more resistors are said to be connected in series if the same current I flows through all of them:

resistors_series

The voltage E applied to the whole circuit is split among all the resistors and the sum of all the resistor voltages equals the voltage E:

E = V1 + V2 + V3

while the total current in the circuit is:

I = V1/R1 = V2/R2 = V3/R3 = E/Req

Req is the equivalent resistance of the series:

Req = R1 + R2 + R3

resistor_equivalent

A series of resistors is normally used as a voltage divider like, for example, those that are used to polarize transistors, or other electronic components. Another usage example is to reduce the input voltage coming into a device, to bring it to a value more consistent to what the device needs. An example of that is the input of an amplifier that accepts a voltage no greater than 1V as its input. If the source of the signal that goes into the amplifier has a greater voltage, a couple of resistors in series can do the trick of lowering the voltage to a more adequate value.

Another way to connect resistors is to put them in parallel. Two or more resistors are said to be connected in parallel if the same voltage E is applied to all of them:

resistors_parallel.png

The current I that flows from the generator is split among the different resistors and the sum of all the resistor currents equal the current from the generator:

I = I1 + I2 + I3

The voltage in the circuit can be expressed by the following equation:

E = R1 * I1 = R2 * I2 = R3 * I3 = Req * I

Req is the equivalent resistance of the parallel:

1/Req = 1/R1 + 1/R2 + 1/R3

resistir_parall_equiv

Resistors in parallel have several uses, but the first two that come to my mind as the most usual are:

  1. You need a resistor of a particular value that is not available in the market. To solve the problem, you build the resistor with the value you need by putting in parallel two or more resistors of higher value, such that the Req equals the value of resistance that you need.
  2. You can view at all the devises and lamp that you plug in your house receptacles as resistors. All of them are connected in parallel, so each one of them can receive the same voltage regardless of how much current they need.

And, since we were talking about lamps, let’s also talk about power. We have said that the main function of a resistor is to restrict the flow of current by converting the excess power into heat. However, while we do so, we also need to avoid that a resistor becomes too hot, thus damaging its surroundings on the circuit board and maybe even catching on fire.

For this reason, each resistor has a power rating. The power rating the is the max amount of power that the resistor can dissipate without becoming hot enough to cause damage to itself or its surroundings. Normal ratings for resistors that are used in electronic circuit are 1/8, 1/4, 1/2, and 1 W

W is the symbol used in electrical engineering to identify the unit to measure the power, which is called Watt. And yes, that is the same power unit used for mechanics and for thermodynamics, if you were wondering.

The power dissipated by a resistor depends on the voltage applied to the resistor and the current that goes through it:

P = V * I = R * I2= V2 / R

When designing a circuit, you always have to make sure that you determine the max power that each resistor will dissipate, so you can specify the power rating of the resistors along with their value in Ohms.

Another thing that is worth mentioning is that resistors have also a voltage rating. However, even resistors with very low power rating, like 1/8W, have voltage ratings of at least 200V. Will you ever build an electronic circuit that is powered with such amounts of voltage? Probably not, that’s why you don’t usually have to worry about that. And, in fact, how many times have you heard somebody talking about voltage rating of resistors? Maybe never?

One last thing I would like to talk about is how to identify the value of a resistor. Resistors normally  used in electronic circuits are of two kinds:

  • through hole resistors
  • SMT resistors (Surface Mount Technology)

The value of the SMT resistors is always written in clear, with numbers and letters, for example 1k2, which means 1.2 kΩ (that is kilo Ohms).

The value of the “through hole” resistors is instead normally identified by a number of color bands painted on the resistor itself. Each color represents a digit in the value of the resistor, or a multiplier, depending on the position. The last colored band represents the tolerance, which tells you how precise is the value of the resistor itself.

Here is a table that shows you the color codes and the meaning of each color in relation with the position on the body of the resistor.

resistors_color_code

‘Till the next time…

(Watch also the video on YouTube:  https://www.youtube.com/watch?v=jV5nacrzsBw)