Wednesday, September 25, 2013

Stethoscope/Spy Bug Using Op-Amp

Welcome again. This post is about a hobby project of how to make a Stethoscope or a Spy bug using Operational Amplifier as known as Op-Amp.

I will write about Op-Amp later. But now will just talk about the project and how to build this.

When I was young(10 years from now) I came across Op-Amp for the first , the TL062. I had plan to make a piano or something, and found this chip very suitable for the job. For audio we can use other Op-Amps too.

For basic, Op-amp is such an amplifier which has a very high sensitivity and gain. If it gets a small amount of input voltage it can amplify it very largely. Well the amplification depends on its supply voltage too. If the gain is high enough and mathematically the output voltage is crossing the operating voltage, it will saturate and its output voltage will be same as its operating voltage.

Let's look at an Op-Amp.

Internal Diagram And Pin Configuration of LM 741, LM324 and TL072


What it does is takes input voltage and spites out output voltage which is actually a product of its gain and input voltage. So, it can easily amplify any signal.


To hear heartbeat or hear what people is saying all you need is a microphone, and a headphone to hear what the microphone is hearing. But the problem is  Output of a microphone is so small that you can't hear anything if you directly connect it to a headphone or speaker. So, we need an amplifier. And Op-Amp is a very nice choice here.

Let's look at the diagram.



In this diagram, we will need only one set of input and output. Here we will take the input from the microphone which is around 0.02V and we will need around 2V to power up the headphone. So we will need an amplifier with a gain around 100. so 0.02V*100=2V. Here gain for this circuit is 1+220K/2.2K=101. So, it will do. We need resistors with around 1% tolerance, 5% tolerance will do.

This Op-Amp is running in non-inverting mode.

First 9V supply and 5K resistor is for powering up the microphone, the next 0.1uF capacitor and 100K resistor is for Removing DC voltage, it's actually a High pass filter, it will filter out DC level and a good AC signal will enter the Op-Amp. Op-Amp needs 9V 0V -9V supply.

This circuit can be used to hear heartbeat or small noises.

Any way that is all. Just try it. You can use any other Op-Amp instead of LM324. Anyway happy experimenting!

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