RANGKAIAN FM WIRELESS MICROPHONE

FM Wireless Microphone circuit diagram

The range of frequencies for the FM broadcast band is 90MHz (MHz = Megahertz or 90 million cycles per second). Because the FM microphone has a variable tuned circuit, it can be tuned to a quiet spot on your local FM broadcast band for the best reception. When the small microphone element is struck by sound, it converts the audio to a change in current through resistor R1 (see schematic diagram).

This electrical change is amplified and eventually frequency modulates the transmitter. The transmission range of the FM microphone is approximately 100 feet, depending on the efficiency of the antenna (properly tuned or not) and the quality of the FM radio receiver.

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RANGKAIAN WIRELESS TERMOMETER DIGITAL

Wireless Digital Thermometer

This simple circuit is designed to report the temperature via any standard FM broadcast band radio at regular intervals. It uses a simple low bit rate tone signalling scheme which can be interpreted easily by both humans and machines. Such a device can be used stand-alone or as part of a PC-based weather station or HVAC system.

My prototype was constructed with an ATMEL AT90S1200 fitted to a Dontronics SimmStick model DT-104. To save design time, I modified a low cost "wireless microphone" kit to act as the transmitter.

Commercial wireless thermometers are usually sold with only one receiver/display unit, and thus are limited to monitoring at only one location. My design allows any inexpensive FM radio to be used to monitor the temperature. Any number of monitoring locations can easily be set up.

Also, by plugging the output of the FM radio into the sound card on a PC and running a program which I am developing, temperature data can be logged for later display/analysis.

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