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I'll describe here how to build a simple 915MHz audio transmitter in SMD technique. All the RF signal generation is being taken care of by an integrated circuit, almost no adjustments are needed on the RF side.

It has several power settings, and the range obtained is from 120m to 400m LOS.

An UHF receiver is needed to receive and demodulate the signal.

Step 1: Schematic

Here is the basic schematic for the transmitter. You can also download a higher definition picture here: Schematic

To the left we see the adjustable gain MEMS microphone whose output signal goes to an operational amplifier. The microphone gives out about 0.93V DC with the superposed audio AC voltage. The DC passes through the OPAMP unaltered, the AC is amplified. Gain and frequency response may need to be adjusted for a particular situation, there is no automatic gain control in this circuit and overdriving the varactor diode will lead to distortion.

The audio signal is then applied to a varactor diode. The varactor diode changes its capacitance according to its reverse voltage and is therefore suitable to affect the crystal frequency of the transmitter IC.

The IC uses the principle of crystal frequency pulling for its FSK modulation scheme. Our little trick allows us to FM modulate the carrier frequency.

The IC TH72035 contains all the circuitry to generate the radio-frequency, only the antenna matching components and power may need to be adjusted. The frequency is very stable. Refer to its datasheet for in-depth information TH72035 datasheet

The circuit in this form will work without any problems at 3V to 3.6V.

The same transmitter circuit with additional voice activation and a DC/DC converter for use with batteries down to 0.9V can be obtained on ... link will follow

Step 2: Assembly

First, download the gerber files in ZIP format from here: GERBER. The link also contains an EAGLE board file in case you want to create your own GERBER files.

You need to send these to a PCB manufacturer (unless you have the equipment at home to make your own PCB)

I can recommend OSHPARK.COM.

The board is a simple 2-layer board, 0.8mm thick. The thickness could be different depending on your preferences.

1. Before starting the assembly the board may need to be filed down to remove the excess on the edges. The minimum width is 6.5mm. If the reed switch is not used the board can be cut at the white line.

We start with the top layer.

2. Solder paste is deposited on each pad where a component will be placed. Remove excess paste to avoid solder bridges.

3. After that the components are placed on the PCB.

4. Once this is done the whole board is heated (oven, hotplate) to about 250 deg Celsius. The solder paste melts and connects the components to the board. As soon as the solder paste has melted remove the PCB from the hot environment to avoid damage to the components.

Let it cool down and inspect the solder joints.

5. Now manually assemble the parts on the bottom side.

Step 3: Battery and Transmit Power

Connect the battery.

The power of the transmitter chip can be adjusted with the PSEL resistor. The battery characteristics need to match the transmitter power, i.e. don't use the highest power setting with the smallest battery. Be sure not to exceed the battery maximum current draw.

The pictured unit here was adjusted to the lowest power setting, current consumption is only a few mA wich is why small button cells can be used that fit into a 6.5mm inner diameter pen.

Step 4: Antenna, Range and Mechanical Considerations

The line-of-sight outdoor range is about 120m with the lowest power setting and up to 400m with the highest setting.

A better range could be achieved with better receiving equipment and/or directional antennas.

The antenna attached to the transmitter is about 1/4 wavelength long or a little shorter. It could also be formed into a spiral if space is limited.

The transmitter is very frequency stable and will not be de-tuned by touching the antenna. The antenna should nevertheless point away from the transmitter and not be near metal parts. This would negatively affect its performance.

Click here and watch the video to see the transmitter working! (Youtube)

Questions? Put them in the comment section or write me a PM at

tomtechtod at gmail com

Have fun! :-)

<p>Since the first transmitter you've published you're designs have become more sophisticated, I like that! :D</p><p>Would you mind to explain the differences between your transmitters? I do have some experience with low power microcontroller based designs, but I've never done anything with RF, so I'd really appreciate your thoughts as an expert. :)</p><p>Thanks for continuing to publish these 'ibles!</p>
<p>the difference is mainly stability, current consumption, range and antenna length. The first one I published is a very simple oscillator and ,if moved around, unstable design, the others are RF IC based. The higher you go in frequency the shorter you can make the antenna but the worse will also be penetration inside buildings... I'm no expert in either RF design nor spy bug design, just tinkering around and trying out stuff. I'll do an ultrasmall stereo bug next and have some other ideas after that. The 915MHz bug for example is not bad because antenna can be kept pretty short but the disadvantage is you need a special receiver, which makes the whole thing a little expensive if you had to buy one. I think best would be anything that could be received with a cellphone, that leaves us FM 76 to 108 MHz, bluetooth and Wifi.... </p>

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