Category Archives: Spy Circuits

Buy or build your circuits

Building circuits takes some time and buying is sometimes cheaper than you get the parts for. There was a time where I liked to build circuits because it was fun and was a learning opportunity. But now with everything you can buy out there, you can be well on your way to doing some more interesting things.

I like to purchase a lot of my electronics projects from Amazon. You can get deep into their integrated controllable circuit systems. Those are fun to use and can help you teach your kids a lot about electronics. Snapcircuits is a great learning aid for kids if you do it right.

Small FM Transmitter

If you ever wanted to build a very small FM transmitter, then this is the circuit for you. With only 16 components, this is a very compact FM transmitter. I like small circuits because they give you a chance to build something quickly. Sometimes if you pick a project too large, then you can’t get it done.

If you would like to pick up the parts before building, here is a list of the parts for this small FM transmitter.

Two 100K resistors
1 10k resistor
1 470 ohm resistor
2 470pF capacitors
2 4.7µF, 16V, electrolytic capacitors
2 4.7pF capacitors
1 4-40pF trimmer cap (this is optional, but if you are putting in an order to Digikey for electronic parts, just go ahead and get this as well.)
1µH  inductor
2 2N2222, NPN transistors
1  Electret Microphone
1 9 Volt, Alkaline battery, or you can use a 9 volt power source.

I like to create these projects on a breadboard first. If you can, purchase a good breadboard kit. Here are the full instructions to build it:

400 Meter Range Miniature FM Transmitter Circuit

Here is a small two transistor FM transmitter that can transmit up to 1/4 of a mile. That is a good distance and should not cause many problems interfering with other radio communications. There are a wide variety of applications for a small device like this. You can build one to listen to nature sounds. You can put this in a waterproof housing and put a 9 volt battery in it. However, after a while, that is going to drain your battery down. For a listening device that kicks on when sound hits a certain threshold, you need a small sensor that can pick that up. Almost like a photo sensor.

Here are the list of parts:

Parts List
R1,R4,R6 = 10K C1,C2 = 0.1uF Q1,Q2 = 2N3904
R2 = 1M C3 = 0.01uF L1 = 0.1uH
R3 = 100K C4 = 4-40pF
R5 = 100 ohm C5 = 4.7pF
R7 = 1K

I would recommend buying your parts at Digikey or Mouser. Both are good parts suppliers. It all maters how low of a price you can get.

(source –

3 Volt Miniature FM Transmitter Circuit

This 3 Volt Miniature fm transmitter can only transmit for short distances, but it is small. Having a small discrete FM transmitter can be useful. If you don’t want to consume too much power or have your transmitter too visible, then the smaller the better.

I prefer working on small projects like this one because when you create small electronic projects, then you can understand how the more complex ones work. Here is the parts list:

1 – 4.7k resistor

1 330 ohm resistor

1 0.001uf capacitor

1 10-40 pf variable cap

1 2n3904

and then you will also need an electret mike, some magnetic wire for the antenna, and then a 3 volt battery. (source – I would buy the magnetic wire at Radio Shack but order most of the capacitors from Digikey or Mouser.  Variable capacitors are handy to have as well.

Single Transistor FM Transmitter

This sensitive FM transmitter is also a fairly strong transmitter for its size. If  you are wanting to pick up low sounds, converstations, or wildlife sounds, then you may enjoy building this FM transmitter. You can build this device or you can also purchase small fm listening devices like this for not that much money.

For parts, you will need:

2 10k reisistors
1 100k resistor
1 4.7k resistor
1 330 ohm resistor
2 0.1uf capacitors
1 0.001uf capacitor
1 10-40pf capacitor
1 4.7 pf capacitor
2 2N3904 transistors
some magnetic wire and something small like a pencil for your coil

(source –

Minature 9 Volt FM Transmitter Circuit

This small FM transmitter circuit uses a 9 volt battery for its power source.

With 16 parts, this is actually one of the larger number of parts for FM transmitters listed on my site. However, that’s still a relatively few number of components, so it is not that difficult to build. (source – ).  You can also buy an FM transmitter if you would like. If you don’t want to buy one, then I suggest going through the list of parts and sourcing them from a place like DigiKey or Mouser electronics.

200 MW FM Transmitter Circuit

You ever have one of those times when you are sitting in the living room trying to hear music coming from the kitchen or vice versa but you can’t hear it. Then you go to turn up the music in the kitchen and it becomes too loud for those in the kitchen but it sounds good in the living room.

What you need to build is an FM transmitter to transmit that music throughout your house. Sound difficult? It might be the first time to build it, but with these instructions and a few parts, you will be well on your way.

(source –

High Quality FM Transmitter Circuit

This FM transmitter circuit is useful for transmitting music around your house. It has adjustable power and does not take many parts. The circuit has two parts, a transistor circuit and an amplifier circuit. There is also an included circuit for a power meter that you can hook up.

I think this circuit does a good job at keeping the circuit noise low while also giving you options on power output.

(source – )

1.5 Volt FM Transmitter Circuit

This 1.5 volt transmitter circuit is a very basic FM transmitter circuit. You can build it with a minimum of components.

These are fun projects to build with your kids so that they can see how fm transmitters work. FM transmitters have many similarities. A place for input of the signal and then different options for rebroadcast of the signal.  The nice thing about this transmitter is that it doesn’t take up too much space.

(source –