I just opened an old, non-working stereo system and found a lot of useful components. I designed a Bluetooth speaker system with these components (along with some other parts found in other old equipment).

Hardware components

FM module
Audio Amplifier IC 6283 or any other similar IC
Loudspeakers

Step 1: FM Module (with USB/AUX/REMOTE Functions)

This is the component whose output acts as the input to the audio amplifier circuit. So basically, this is the central component of this system. It is powered by 5V DC and its various pins have the following functions:

First 5 pins (SDVcc, DET, SDD, SDCLK, SDCMD) are for SD card connection.

The next bunch of 4 pins is for USB connection (GND, DP, DM, UVDD).

Purchase a female USB socket for as low as 5 INR and solder its terminals to these pins accordingly (as shown in image 1).

You can insert flash drives in this.The next 3 pins are for AUX connection. AUXL and AUXR are for left and right channels respectively. AGND is for ground.

Purchase a 3.5 mm jack female socket and solder its terminals to these pins accordingly as shown in image 2.AUXL, AUXR and AGND are for the outputs of this module.

L is the left channel output and R is the right channel output.

The next 4 pins (NEXT, MODE, PREV, PLAY) are for switches i.e. push buttons.

I checked these and these pins were pulled up. So, connect one side of the push button to the ground and the other to one of these pins. Do this for all the 4 pins. When you push the key for MODE, you'll be able to choose between the mode of operation,

i.e. radio, AUX or USB. It will be displayed on the LED display.

Step 2: Audio Amplifier Circuit

Luckily, the name of the IC was visible and it was CD6283cs.

This is an audio amplifier IC, and it amplifies the audio signals with the least noise when connected with some circuitry.

These components are capacitors as well as resistors.

You can find the pinout for this IC very easily and solder these components accordingly.

You can look up the pinout here.

One thing to mention is: pin 5 and 7 of this IC are audio input pins.

Connect L of the FM module to pin 5 and R to pin 7 (or the other way around).

Connect GND of the FM module to pin 6 of this IC. But since I am using a single 20W speaker, I connected the L and R together and fed it to pin 5, with pin 7 left unconnected. Power this circuit with regulated 12V DC.

Step 3: Assembling Loudspeakers

Loudspeakers : They are called so because they drive high (tweeters), low (woofers) and mid-range frequencies, and are enclosed within the same structure. The complete circuit looks something like image above.

The Bluetooth part is nothing but a cheap shopping material.

This is a Bluetooth audio adapter that connects to an AUX port of any stereo system.

Just power it using USB port, attach the AUX cable to this adapter and the other side to the speaker system we just made.

And you are good to go! You have a Bluetooth speaker! The Bluetooth audio module was twice the price.

So, I bought this already made 'product'.

You can use this with any other sound system without the need to do all of that above. Or you can make your own sound system with very inexpensive components and also have fun.