There’s been a strategy in market of buying USB powered speakers for laptops. These speakers usually drives power from one available USB port and inputs audio signal from headphone port, thus consuming two ports of a notebook. Lets see how much output these kind of speakers can give.
A USB port delivers maximum 500mA current and at 5Volts, it comes to max 0.5×5=2.5Watts. So, if our circuit eats 0.5-1Watts power, only 1.5Watt is left for speakers output.
Now you might ask 1.5watt wouldn’t create much sound. But believe me, under good conditions, this 1.5Watts is much more than expected. And this time we are going to use 0.5Watt-4Ohm x 2nos speakers.
These speakers are flat type having magnet inside them, and available at wholesale electronics shop in around 30rs per piece. For speaker box, use old jumbo matches box like homelite box. And for circuit, you need is an audio amplifier circuit capable of giving 0.5Watt output at each channel.
Here we will use a general purpose stereo amplifier TDA2822M IC, which comes in 8pin DIP package and usually found in mini walk-mans, etc. This IC can give up to 450 mW/channel with 4-ohm loudspeaker at 5V supply which is near our requirement.
The datasheet of TDA2822M can be downloaded in PDF format here (PDF, 362KB)
The expense in making this circuit is no more than 25 rupees. Hence the total cost becomes 30+30+25=85rupees, and if we add the cost of wires, jacks etc then it well fits under 100rupees. It’s a very cheap solution when USB speakers in market costs more than 300 rupees.