Stable Radio

A radiobeam is a simple radio transmitter with a microphone that picks up the slightest sound around itself and transmits them through a radio wave. Thus, leaving such a device in a suitable place, you can listen to everything that is done around it. You can find a lot of applications for a radio recorder - for example, to use it as a baby monitor, wireless headphones, or just to eavesdrop on someone else's conversations (such use is not entirely legal). If you assemble two radio transmitters, you can organize a wireless connection, for example, between neighboring houses. On the Internet there are a large number of schemes of bugs, from the most simple to expensive and complex, built on the chip. The following diagram, in my opinion, is the most optimal: for its construction, rare or expensive parts are not required, but it provides a good signal transmission distance (up to 500 m) and high stability of operation.


Stable radio chip
The circuit includes three stages.Transistor VT1 built a sound amplifier microphone, on the transistor VT2 carried a generator of the carrier signal, and the transistor VT3 is a high-frequency amplifier, it is responsible for a good transmission range. The following transistors are used: VT1 - КТ3130Б, VT2 - КТ368А, VT3 - КТ3126Б. To convert sound, an electret microphone is used in an electrical signal. You can find this in any store of radio components, or drop it out of the unnecessary headset of the phone, because the necessary microphones are used there. Coil L1 is wound with copper wire on a mandrel with a diameter of 3 mm, 6 turns, the wire can be taken with a section of 0.3-0.4 mm. As a mandrel, you can use a conventional drill. L2 - choke, inductance 100 µH, it is best to use ready-made. As an antenna, you can use a normal piece of wire, but if you need to reach the maximum transmission distance, you need to use a wire that is one quarter the wavelength at which the transmitter operates.

Assembly of the radio chip

The scheme is assembled on a printed circuit board with dimensions of 70 x 30 mm, for its manufacture the LUT method is used. Mirroring the board before printing is not necessary.Below are a few photos of the process.
Stable Radio
Stable Radio
Stable radio chip
After the copper is etched, the holes are drilled, the tracks are tinned, it is possible to solder the parts. Particular attention should be paid to the microphone, or rather, its polarity. One of his contacts is short-circuited with his body - it is this contact that is minus, it is soldered to the minus of the circuit. The microphone needs to be sealed quickly, otherwise there is a risk of overheating it. The power wires can be soldered directly to the connector for the crown battery for convenience. Build a bug is not difficult, if you do everything gradually and do not rush to check the correctness of sealing every detail.
Stable Radio
Stable radio chip

Setup and test run

For the first start, you need a milliammeter to monitor the current consumption, it is included in the gap of one of the supply wires.We connect the crown and look at the readings of the device - the current must lie within 10-30 mA. If it is normal, you can turn on the radio and try to find the signal of the transmitter under test. Most often, the signal already lies within 80-110 MHz. If it "crawls" beyond this range and the receiver can not catch it, you should adjust the L1 coil by squeezing and loosening its coils. The transmission frequency depends on the inductance of this coil. When the signal is found, the coil can be filled with dielectric varnish or epoxy, so that the setting is not lost. It is very important to ensure that the frequency of the broadcast of the assembled transmitter does not overlap with the frequency of any radio station. If they intersect, it means you need to change the frequency by tuning the coil L1. No tweaking is required anymore; you just need to adjust the broadcast frequency. Now you can test it for the distance of the signal transmission, moving away from the pocket receiver further and further from the transmitter itself. A properly assembled and tuned transmitter can provide a transmission range of up to 500 m in open terrain. It should be noted that the reception distance also depends on the sensitivity of the receiver.Successful assembly.

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