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Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
The simplest method is just to connect an I/O pin to a piece of wire and toggle it at a fixed frequency. The higher the frequency the more efficient the transmission will be, it is a lot more complicated than this but it gives the general idea.
Since the signal is a square wave the transmission will happen at a wide range of frequencies. Mainly at multiples of the square wvave frequency. It might be a good idea to add a filter to reduce the transmission of unwanted frequencies unless the power is low enough to not cause any problems.
Some oscillators are not stable when the supply voltage goes outside a certain limit and you can let the microcontroller modulate the voltage to give different frequencies for FM transmission. Just be careful if the microcontroller runs off the same oscillator.
With a reasonably sensitive receiver and proper error detection/correction the range is significant.
Registered Member #286
Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399
That is a nice find Bjorn. I should try that with my junker labtop sometime. Transmitting with a squarewave is a big no no. A butterworth low pass filter should be used on the output of the squarewave crystal.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
A PC can transmit in several ways.
1. If you have analog video out you just put up the pixels needed to give the signal you want. This can give enough bandwidth to transmit music across a room. On some graphics card it is possible to more or less completely remove any deadtime and you can generate signals up to and above 400MHz without any significant distortion. Since there are 3 colour channels you can generate 3 independent signals.
2. Modulate the CPU usage and the switchmode regulators will change frequency and the signal can be received at a significant distance. Since the frequency will drift it is difficult receive it reliably. Since the signal is very broadband it is possible to receive it on almost any radio, FM or AM, depending on the motherboard.
3. Using the line toggle method using a printer port, serial port or PS/2 port.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I tried this today with a PIC16F88. With the internal PWM set to 1MHz I was able to receive the signal on a standard AM radio. I got clear reception in my brother's room which is across the house, so this works pretty well!
Registered Member #518
Joined: Tue Feb 13 2007, 05:20AM
Location: New York
Posts: 168
for reception you can probably use a simple audio amp chip, and use some kind of encoding layer that attaches a header to each packet with a CRC code. Also do an ACK reply and you can probably get good error free communication between two PICs or AVRs. It would possibly help if you make a small impedance transformer and drove it with a NPN switching transistor.
It just came to mind, that for receiving you can just use a mosfet with a high hfe, they are very sensitive to electrostatic spikes, and can even be used to detect the charge in clouds! ... you will just need to add a high value pull down resistor so that the gate isn't floating above ground, and youll have a nice range for detection.
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Simplest possible radio transmitter for microcontrollers
