Tuesday, December 31, 2013

FM Radio Station Notes Part 24


TUTORIAL: How Radio Waves are Produced

As a reference to going even smaller and more simple, this schematic shows possibly the most simple one transistor FM radio voice transmitter design, using a common 2N2222 transistor and only three capacitors and two resistors. There's only 7 electronic parts total to this assembly plus batteries and the antenna. A power on/off SPST switch should be added to the schematic.

Parts List
R1 Resistor 4.7K
R2 Resistor 220 ohms
C1 Capacitor .01uf
C2 Capacitor 33pf
C3 Capacitor 6.8pf
L1 Coil
Electret Microphone
2 - 1.5 Volt Battery

A slightly modified version is at this web site:

The video also includes a schematic diagram for a very simple matching two stage FM radio receiver. This battery driven radio has only two transistors, amounting to 6 electronic parts plus batteries, switch and antenna.

Parts List
R1 Resistor 12K
C1 Capacitor 220n
T1, T2 Transistor BF199
C Variable Capacitor
3 Volt Battery
SPST Switch

BF199 transistor data sheet

The simple FM transmitter circuit shown above is a spinoff of this project. It uses only one resistor, one capacitor, one transistor and one coil. The article is found all over the web. It has no microphone but the coil is so microphonic that it will pick up noises in the room via vibrations on a table. The circuit does not have any section that actively tunes the frequency. The transistor turns on via the 47k resistor and this puts a pulse through the 15 turn winding. The magnetic flux from this winding passes through the 6 turn winding and into the base of the transistor via the 22n capacitor. This pulse is amplified by the transistor and the circuit is kept active. The frequency is determined by the 6 turn coil. By moving the turns together, the frequency will decrease.  The circuit transmits at 90MHz which is at the low end of the 88 to 108 MHz FM band. It has a very poor range and consumes 16mA.





This Google search link will find a lot more simple FM radio transmitter schematics and projects.

Here's another link to a project claiming also to be the simplest FM transmitter ever made.

This circuit has audio input and needs only 2 resistors and 3 capacitors plus a coil and transistor.

"For input from low power output devices, such as mobile, computer’s sound card, the value of the unlabeled capacitor is 0.1 or 104. For higher wattage outputs, the value is 0.01 or 103. An optional microphone pre-amp can be also added in this circuit to enable it transmit voice directly."

One idea to create the small transmitter antenna is to use a small wheel bicycle used spoke from a children's bike available from a bike shop probably for free.

How to Change a Dark Detector into an FM Transmitter

A dark sensor can be converted to a simple FM transmitter using a similar schematic layout.

The dark sensor will light the LED as the output indicator. The transmitter substituted a microphone for the LDR.

This FM transmitter is very sensitive and it has transmitting range of 30 meters with a 9-volt battery. Reducing battery power will reduce transmitter range.

Comments given at the link about the transmitter
The circuit may work but it relies on a “Q-factor” from the coil and capacitor in the tank circuit to produce a high voltage.

This high voltage gives the circuit a good range. Firstly the coil and capacitor should be near each other. The coil should not have long leads. and a 22n capacitor should be across the supply to give the circuit better performance. The value of C2 is too high. It should be 10p. The coil should be 5 turns. The electret mic should not be connected directly to the base of the transistor.