Intro to Electronics 110 - Audio Amplifier Project

AUDIO AMPLIFIER PROJECT

In this project, you will learn how to master a device by studying all the parts and building it with a partner.

Our test subject: Audio amplifier is a good example of a generic

or common circuit. It’s a device that takes a stereo (two channels)

headphone/line signal and convert it to a 1W amplified mono

(one channel) sound. It will work with phones, mp3 players,

computers and any other audio device that has 3.5mm stereo

outputs. It turns on with a toggle switch, runs from a 9V battery,

has an LED indicator that’s ON when it’s powered and another

LED indicator that’s ON when the battery is low. The IC is the

Texas Instrument’s LM386.

Participation: Remember to keep a positive and constructive attitude throughout the project.

Consider that it shows, for example, your employability and your innovating skills.

Theory: We will study its design and every step of the circuit. A test will be given on this after the whole project is done.

Practice: You will be with a partner to build one amplifier. Requirements are;

1) You provide the 3.5mm plug, new 9V battery and a project box

2) The whole circuit is fully functional

3) All solder joints are properly done

4) All components are insulated from each other and secured in place

5) Is sturdy and works safely

6) In a box with all the interface components in a proper place

PART 1: COMPONENTS

Interface components: Parts that interact with the world (usually with humans or objects).

Inputs: Parts that influence the circuit

General examples: computer mouse, Blu-ray disc drive, touch screen, push-button

Inputs of the Audio Amp: Power switch, Volume control, 3.5mm plug

Outputs: Parts that show or does something made by the circuit

General examples: printer, video screen, USB charging port, electric motor

Outputs of the Audio Amp: Power LED, Low Battery LED, Speaker

AUDIO PLUG: 3.5MM, STEREO, MALE Plugs in a phone or other audio device.

TOGGLE SWITCH: SPDT, On-On

Will be used as On-Off switch if only terminals 1 and 2 (or 2

and 3) are used. See https://goo.gl/5Y1fbO for more info

POTENTIOMETER: 10KOHM, 1/10W, LOG Variable resistor that adjusts the volume.

SPEAKER: 8 OHM, 0.8 WATTS (MAX 1W) Outputs sound

9V NEGATIVE TERMINAL: SNAP-ON, MALE Connects on negative terminal of 9V battery.

9V POSITIVE TERMINAL: SNAP-ON, FEMALE Connects on positive terminal of 9V battery.

LM386 IC AMP: 1W, MONO, 8DIP, 9-12V The brain of the circuit. Amplifies the incoming low audio signal up to a 1W speaker level.

IC SOCKET: 8DIP, 2.54MM PITCH Will be soldered on a PCB and will hold the LM386. By having a socket, we don’t solder on the IC and don’t heat it up and makes it easy to replace the IC if it goes bad, which happens.

ZENER DIODES: - 3.3V, 500MW

- 8.7V, 500MW

Zener diodes let the current through in one direction like normal diodes but only if the minimum voltage is meet. In our circuit, it

is used in reverse to light up the Low Battery LED indicator.

RESISTORS: - 10 OHM, 1/4W, 1%

- 470 OHM, 1/4W, 5%

- 470 OHM, 1/2W, 5%

- 2X 1 KOHM, 1/4W, 1%

- 2X 10 KOHM, 1/4W, 1%

Resistors can: A) Lower the voltage B) Block the current until it is pushed harder: - Will make the current go another way if there is an

easier way (a lower resistance). - The current always go to the path with less resistance. - The lower resistance path acts like a magnet that attracts the current to it. - For the current, a resistor acts like a wooden road block. It’s like “Only go through if you really need to”. In case of emergency, you can go through if you really want to.

CAPACITOR: 0.1UF, 25V, CERAMIC

Capacitors (or caps) temporarily hold a charge and discharge

continually. In our circuit, they are used to reduce audio noise to keep a clean audio signal. They prevent interference or “bad

mood” between signals that could create audio noise. Metaphorically, they act like springs or suspensions by absorbing the shock caused by the bumps on the roads, keeping the ride smooth. Caps smooth out the current.

The big suspension springs for bigger trucks would be the 220 uF cap.

IMPORTANT: To avoid explosions, only replace caps by; - SAME Capacitance (uF) - Volts are EQUAL or Higher Aluminum Caps Polarity (also to avoid explosions): POSITIVE: Long lead NEGATIVE: White Bar

CAPACITOR: 0.047UF, 50V, CERAMIC

CAPACITOR: 470PF, 2KV, CERAMIC

CAPACITORS: - 10UF, 35V, ALLUMINUM

- 100UF, 35V, ALLUMINUM

- 220UF, 16V, ALLUMINUM

About the LM386

This is a popular IC that is used to amplify low audio signals. It uses a set of transistors to amplify the sound and this

particular version can output 1W sounds. Other versions outputs at 0.325W and 0.7W.

Vs: Power input between 9V and 12V AC to make the IC turn ON.

GND: Connects to negative of the battery or ground of the power supply. In other words, it’s the common ground.

-Input: Reference (comparator) for 0V or ground. Sets whatever is on that pin as 0V and then compares to +Input.

+Input: Receives positive voltage from audio device. The audio signal to amplify.

Vout: Positive voltage to the speaker. The amplified audio signal.

Gain: Connect resistors/capacitors between pin 1 and 8 to adjust the gain (pre-volume).

Bypass: Whatever the IC flushes out. The leftover current.

Full Datasheet at: https://goo.gl/mB7lhA

PART 2: THE AUDIO AMPLIFIER CIRCUIT

SCHEMATIC:

1 kOhm

470 Ohm

Bar on top

23 88

Bar on top

22 68

THE ACTION INSIDE THE CIRCUIT:

See color version at: https://goo.gl/czjaCZ

MAIN POWER: A) Current comes in and feeds 9V to the IC.

B) With the decoupling capacitor 100uF, the current is smoother because the cap will absorb any irregularities or

interference. Like car suspension, it will eliminate the bumps.

C) With a current limiting resistor, an LED indicates that our circuit is powered.

LOW BATTERY LED CIRCUIT: D) The Zener diode has less resistance than the 1 kOhm resistor so it goes towards that diode. This Zener diode lets

current through if the volts are 8.7 V or more. E) When voltage is under 8.7 V (battery is low), the first Zener diode blocks the current. The current has to go through

the 1 kOhm resistor which brings the voltage lower than 3.3 V so is blocked by the 3.3 V Zener diode and the current

will go through the "Low Battery" LED. Then, current cannot go through the 8.7 V Zener diode since it's backwards, so

the current has to go through the last 1 kOhm resistor.

F) After going through the first 1 kOhm resistor, the current is lower than 3.3V, forcing that current to go through the

"Low Battery" LED. If for some odd reason, the current after the first 1 kOhm resistor is equal or higher than 3.3V, the

current will go through that diode and not the LED, thus protecting that LED. That 3.3 V Zener diode that is connected

to ground establishes the 0 V (reference voltage) and helps the 8.7 V Zener diode to work better.

AUDIO INPUT: G) Positive voltage being the Left channel signal. The current crosses the 10 kOhm resistor but cannot cross over the

Right channel 10 kOhm resistor because the potentiometer will likely be lower than 10 kOhm so that's where it goes. H) Positive voltage being the Right channel signal. The current crosses the 10 kOhm resistor but cannot cross over the Left channel 10 kOhm resistor because the potentiometer will likely be lower than 10 kOhm so that's where it goes. *Without those two 10 kOhm resistors (in G and H), the R and L channels could short each other out, creating a short circuit (with no resistance), thus potentially breaking the audio device. If we don't want to use 10 kOhm resistors, then only one channel at a time (L or R) should be connected to the potentiometer. I) The Left and Right audio signals (Stereo: 2 channels) are now merged together as one (mono: 1 channel) and the volume (the signal voltage) will be adjusted by the 10 kOhm potentiometer. The IC will read this as the low level audio signal to amplify. J) The leftover current that has been blocked by the potentiometer will drain in the common ground.

K) The 470 pF acts as a decoupling capacitor. It removes unwanted noise that could be made by the interference between the audio device and the 9V battery circuit. Again, caps will help smooth out the current. Like a car suspension, it smooths out the ride.

AMPLIFIED SIGNAL:

L) Pin 5 outputs the amplified audio signal up to 1 W. It is filtered through the 220 uF cap making a cleaner sound. Then goes to the positive speaker terminal. M) The current exits the speaker and drains to ground.

N) Decoupling cap and resistor. They eliminate the interference between the amplified signal and the rest of the circuit.

The high level current of the amplified signal could "rock the boat" of the circuit. If decoupled, it sounds better.

GAIN ADJUSTMENT: O) Gain multiplies the volume and is controlled by pins 1 and 8. If pins 1 and 8 are not connected (open), the volume is

very low. If a 10 uF cap is put between them, the volume is 10 times louder. The 470 Ohm resistor lowers the volume

slightly and will help avoid sound distortion. That resistor could also be replaced by a 10 kOhm potentiometer for full

gain adjustment.

BYPASS: P) Bypass is another term for decoupling: avoiding interference between currents and having a cleaner current with less

noise. In this case, smoothing out the current inside the IC. The 0.1 uF cap on pin 7 takes care of that.

PART 3: UNDERSTANDING THE PROJECT

1. Why is this audio amplifier our preferred test subject for learning about circuits?

2. What kind of skills are important to show during this project?

3. List all the materials your team needs to provide for this project.

4. What category of components interact with whatever is outside the circuit?

5. What is the difference between input and output components? Give two example of each.

6. What are the specifications of the audio plug?

7. What component adjusts the volume?

8. What is the main component of this amp circuit?

9. What pins on the LM386 are used as audio input and output?

10. Why do we use a socket and not just solder directly on the IC? Give two reasons.

11. What is the difference between a regular diode and a Zener diode?

12. What is the main use of a resistor in our circuit?

13. What is the main use of a capacitors in our circuit?

14. What voltage does the LM386 need in order to operate?

15. How can the gain be adjusted on this IC?

16. If you look at the schematic, what 3 active components need the 9V power to operate?

17. What is the role of the 8.7 V Zener diode in lighting up the “Low Battery” LED indicator?

18. What is the role of the 100 uF capacitor?

19. Why do we put 10K resistors on each of the incoming L and R signals form the audio device?

20. What becomes of the stereo sound in the end?

21. Why do we connect pin 2 (-input) to ground (GND)?

22. What is the difference between volume and gain?

23. How could you make the gain higher or lower?

24. Pin 7 on the IC is the bypass. What is the use of that?

25. In the “Low Battery” circuit, after the first 1K resistor, why does current go to the LED and not the Zener diode?

26. The ground (GND) is connected to many things, does it also connect to the negative terminal of the battery?

27. What is the role of the 470 pF cap?

28. What happens if we connect the amplified signal to the negative speaker terminal? Do your own research.

29. What is the advantage of having an LED indicator turning on only when the battery is low and not high?

PART 4: PROTOTYPING

Once you have the material, it is wise to know where you’re going before you assemble your final version.

STEP 1: INVENTORY

Find all the components you have and put a checkmark on your list.

STEP 2: PROTOTYPE ASSEMBLY

TIPS:

- Draw the circuit here first on that breadboard.

- Start with the IC in the middle of the breadboard and pin 1 on left side.

- Have your partner highlight the assembled parts of the circuit as you go.

It’ll make it a lot easier.

- Don’t push an insulated wire in too much, the insulation will get in. Bad!

AUDIO

INPUT

LED

CIRCUIT

IC

CIRCUIT

AUDIO

OUTPUT