How to Design Schematic Circuit of FM Wireless Transmitter.

 These circuits have always been popular for both utility and entertainment

purposes.

The FM transmitter described here is built with few components tosave space

and cost. The circuit has only one transistor and is powered by two or four AA

cells.

The broadcasting frequency of the unit, in the range between 88 and 108

MHz (FM band), is determined by Ll and the variable capacitor CV (a 2O or

30 pF trimmer capacitor). L1 consists of four turns of No. 22wire, wound in a

single close-spaced layer on a 112-inch form. The coil is center tapped where

it is wired to the antenna terminal.

Any piece of stitf wire will serve as the antenna and will give good coverage.

For better operation, antenna length should be between 5and 15 inches.

Try out your unit at any convenient dead spot on the the FM band. Adjust CV

to the best signal. The signals can be tuned in distances up to 150 feet.

A schematic diagram of the FM transmitter isshown in Figure 1. Only one

transistor as used as a high-frequency oscillator. Any small RF NPN transistor can be used for the task.

Components placement on a printed-circuit board is shown in Figure 2.Capacitors are critical components and should be the types indicated in the

parts list. Don't use metal film capacitors instead of the ceramic capacitors

which are called for in the parts list.

You can also use a terminal strip as chassis as shown in Figure3, but keep

all the component terminals and wiring short to avoid instability.

The circuit can be housed in d small plastic box along with the power supply.

Avoid the use of metal boxes to house the circuit.

Take care with position of the polarized pieces, such as the electret microphone, electrolytic capacitor and power supply.

Parts List - Wireless FM Transmitter

Q1 - 8F494 RF silicon transistor

MIC - two-terminal electret microphone

L1 - coil - see text

CV - trimmer capacitor (20 to 50 pF)

R1, 3 - 4,00 ohm, 114W,5% resistor

R2 - 10,000 ohm, 114W,5% resistor

R4 - 47 ohm,1l4W,5% resistor

C1 - 10 uF, 6 WVDC electrolyticapacitor

C2 - 0.01 uF ceramic capacitor

Cg - 4.7 pF ceramic capacitor

C4 - 0.1 uF ceramic capacitor

S1 - SPST slide or toggle switch

81 - 3V or 6V - two or four AA cells

A - antenna - see text