The ultra-simple radio transmitter shown in figure 1 sends its signals to an AM receiver located at distances that can reach a few meters, even through walls and other obstacles.
The coil consists of 50 + 50 turns of enameled wire (AWG 32, for example) in a ferrite rod of 10 to 15 cm in length.
The variable tuning capacitor (as well as the ferrite core) can be obtained from an out-of-use AM radio. As a microphone, we use a small speaker that can also be harnessed from the same out-of-use radio.
The transmitter will be mounted on a terminal strip for easier beginners, since it is not a critical circuit, and then installed in a wooden or plastic box. Figure 2 shows the assembly.
The antenna consists of a piece of ordinary wire from 1 to 5 meters.
To use the unit, place an AM radio tuned out of station (free frequency) near you. Then adjust the transmitter's variable capacitor until it picks up its signal.
Go talking in front of the microphone until you get the most intense signal. Reach will depend greatly on the existence of local interference and receiver sensitivity.
Material
Q1 - BC548 - NPN general purpose transistor
L1 - Coil - see text
CV - variable capacitor - see text
R1 - 2.2k ohm x 1/8 W - resistor - red, red, red
C1 - 10 nF - polyester or ceramic capacitor
C2 - 100 nF - ceramic capacitor
SPKR - Small Common Speaker
B1 - 6 V - 4 small cells (AA)
A - antenna - see text
Miscellaneous:
Ferrite stick, enamelled wire, 4-cell holder, mounting box, wires, solder, etc.
