# Specified frequency signal detector

I understand that band pass filters are used in some circuits to filter out the desired frequencies. I also understand that an antenna picks up radio waves in the air and converts it into electrical power for the circuit its attached to, but I am not sure exactly how it picks up on a specified frequency.

I was just wondering if someone would be able to explain to me (or draw) a circuit that contains an antenna and can be tuned to a specific frequency using either band-pass method or some other type of tuning circuit method which will then turn on a LED to signify the presence of that specific frequency.

P.S. Very new to this :(

• When you have learned how resonance allows you to receive a single frequency, come back and we'll help you transform that simple receiver into what you want. But you have to learn a few basic fundamentals first. – Dwayne Reid Aug 8 '15 at 20:58
• Hello, I know a little about resonant circuits (tuned circuits), in which a a capacitor, inductor and resistor are either in parallel or series and at any given L,C there is one specific resonant frequency. But in regards of this resonant frequency to an actual antenna and the electrical power it produces, I am still unsure. – Kira Aug 8 '15 at 21:15
• It also depends on your purpose. For example are simply trying to determine a particular carrier frequency strength or do you want to know if data is being transmitted? For the first you will ne detecting the carrier signal and for the second it will be the information signal. If you have access to an oscilloscope, buy a crystal radio kit and probe the signal to get an understanding of the wave form. Then once you understand how the waves interfere with each other you can modify the circuit to start filtering for the frequencies you are interested in. – BenG Aug 8 '15 at 22:18
• I actually do not need to know if data is being transmitted, I only want to create a simple circuit that is able to tell if there is the presence of a certain frequency signal, so it would not be as complicated as that. – Kira Aug 8 '15 at 22:46

As far as I understand, you are looking for a conceptional, introductory answer. There are two sides of it. First, antennas actually pick up waves with every frequency. However depending on their geometry and lengths, they pick up some frequencies more and others somewhat dampened. This is pretty similar to a bandpass filtering process, where only resonance frequency is able to pass through the circuit unaffected and the rest is diminished due to filter. For example the old radio antennas (long and straight shape) usually are better for receiving signals having wavelengths closer to their length, so longer the antenna higher frequency it would receive. You can learn more about this in a microwave class.

Second part is as you mentioned, we actually try to gather a wide range of frequencies in air and we use a bandpass filter to pick only one frequency among them.

However in real life applications, you need to amplify the signal before you filter it, because there is not much power transmitted to the circuit from an antenna. Therefore setup is (RECEIVE)->(AMPLIFY)->(FILTER) before using. And a buffer would be good before using the signal because of the impedances.

If you are actually going to do this setup, first get an antenna and make sure it can receive the frequency you are looking for without much dampening. Then use an amplifier, here is the topology: (use common emitter since its the simplest)

, just use a potentiometer and look for a good waveform. Then go for a band pass filter, and use a variable capacitor if you want to be able to do frequency tuning. A bandpass filter is just a RLC circuit, you easily can find the resonance frequency online if you need to.

Finally get an OP-AMP that would work in your frequency range because an OP-AMP is the easiest way to make a buffer. Just write op-amp buffer circuit in google and look for images (take the one without resistors)

then you can go ahead and connect it to a LED. Best of luck!