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VHF WIRELESS DATA TRANSMITTER     

 

No doubt about it. I need to hire a real hand model someday.

Note: I don't recommend soldering directly to buton cells. The cells you see

above were soldered successfully, but when I tried it on another project

there was a loud "POP" and the button cell was nowhere to be seen -it

had exploded. So, don't try this - instead buy or make a battery holder.

NOTE: This project is only an example of what can be done. The legalities of building and operating radio transmitters are different in different parts of the world. Be sure to check your local laws before building or operating a transmitter like the one shown here.

Download:

When the SEND button in pressed, power is applied to the mirco controller and to the transmitter. The transmitter repeatedly sends an on command and then an off command to control the LED on the compaion receiver, thus by pressing the button for a short time the LED on teh receiver is turned on. Pressing and holding the button a little longer turns the LED on the receiver off.

In more detail, when the SEND button is pressed the micro controller holds its output low, and in this state, the transistor is biased off and thus cannot oscillate. After a hardware delay on the microcontroller to allow the 4 Mhz crystal to start up and stabilize, the microcontroller comes out of reset, sets up PORTB as an output port, and sends a runin sequence, a 8 bit hedder that includes the address for the companion receiver, and then one byte that contains the value $A5. If the SEND button continues to be closed, a short delay is incurred and the transmitter sends another byte, also preceeded by the runin code and hedder. This second bye contains the value $5A. Another short delay takes place then the cycle starts again as a runin code, hedder and byte with a value of $A5 is sent. This continues as long as the button is held down. The value, $A5 is interpreted by the receiver as a command to turn on the LED on is output port. If the receiver receives any other value, the LED is turned off.

As a result of the foregoing, the transmit sequence can be represented as follows:

While power is applied to circuit (while send button is donw)
       Send LED on Command
       Wait a short time
       Send LED off Command
       Wait a short time
       (repeat this cycle as long as power is applied to tbe circuit)

Encoder code

The encoder code was written as straight line with only one subroutine call and one loop and no interrupts. This was to make it as easy to adapt to different applications as possible. Upon reviewing the code, one will see many obvious opportunities to minimize code size (time is available to use more subroutines, if there is stack space to support it). and to minimize register usage. This code is functional, not pretty.

Assembly source for the encoder firmware is here.

REMEMBER TO SELECT THE EXTERNAL CRYSTAL OSCILLATOR WHEN PROGRAMMING.

RF Stage

The transistor is a gated 55 MHz oscillator, achieving 100% modulation, and probably a lot of chirp and key clicks while at it. A crystal controlled oscillator could not be driven this way because the startup time for the crystal would be too long to allow the millisecond-wide pulses to be transmitted. In this cirucit, I cut corners by using a single decoupling capacitor for both the micro controller and the RF stage. This is sloppy practice, but since I am only buildind on of these, I got away with it. Be sure that whatever decoupling capacitor you use, that it is a good quality monolithic ceramic capacitor. Some of the ceramic disc capacitors just don't cut it at these frequencies.

The antenna really isn't an antenna as much as a capacitive coupler of the near field RF. At 55 MHz an efficent antena would appear electrically to a little over a meter long. The antenna can be folded and bent to fit inside whaever enclosure you want to use. If you only want this thing to work for a few cm, the wire antenna can be omitted entirely on both the transmitter and the receiver. The wire antenna is really only half of the signal coupling mechanism. The "ground" part of the circuit - those parts connected to the transmitter's gound are also part of the signal coupling mechanism. If the transmitter is in your hand or close to a conductive table top or sitting on you keyboard -anything to improve its coupling to a larger ground structure, the range will be much greater than if it is left floating in space. The same goes for the receiver.

Construction

I used a pieces of pre-drilled phenolic board with annuluses (dougnuts) around each hole rather than a PC board. The RF stage is getting into the range of fequencies in which things get a little squirley without a ground plane, so be patient as you adjust the coil. In your layout, you might need to add or remove a turn to get it to tune tit o where you want it to be. Notice that in the photograph at the top of this page the coil is quite spread out - it took a little less inductance from the coil on this board because the layout is much tighter than on the last one I build for this fequency.

The RF section is an osciallator, and it will probably osciallate with a very sloppy layout, but if you lay the components out so that their physical arrangement corresponds to the placement on the schematic and keep component leads short, it should be easier to get this working at the frequency you want it to.

This is a good place for a digression relating to frequency measurement. If you have a spectrum analyzer, a tunable field strength meter, or a grid dip meter its not hard to determine what frequency your transmitter is operating at. On the other hand, if you don't have one of these, all is not lost. You can get a pretty good idea by using television sets and radio receivers. Here is a chart showing U.S. television channel assignments through channel 69, which I found on the Federal Communications Commission's web site. This covers the range 54 through 806 MHz, with gaps below 470 MHz that can be made up with communications receivers.

Variations

This transmitter was built using 3 volt batteries (top-left of circuit board in the photograph), but the range is a whole lot better if the transmitter is run from higher voltages. Most of my testing was carried out using a 9 volt transistor radio battery and a 7805 regulator for the ATtiny12. When it got down to building something to last, I opted to do away with the 7805 in order to minimize parts count. If you want more range, use a 9V battery or even one of those small 12v camera batteries, but remember to regulate the power supply to the microcontroller to a lower voltage, like 5V.

Another thing you might want to try to increase range is to drive a tap on the inductor and attach the antenna to the undecouple end of the coil. Tapped coils are a bit of a hassle, but doing this would raise the signal level at the antenna. Also, the emitter resisitor can be dropped by a factor of 10, which should also give a slight increase in range.

A crystal controlled version would offer better frequency stability. Not importand because of the receiver (This TRF receiver is not very selective) but because of possible interference to other services. If you do this, then the micro controller will have to turn on the transmitter, wait 50 or more milliseconds for the crystal to settle, then modulate an output stage with the data pin. The output stage could be a fequency multiplier to minimize the number of stages necessary if you are running at such a high frequency that you can't get a crystal to osillatre the the appropriate overtone reliably.

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Updated March, 2004.