This small experimental radio, very simple, captures well the local stations of the medium wave band and even weaker stations with the use of a good antenna. Playback is on a small speaker and power can be supplied by two or four small batteries.
Although medium-wave transmissions are expected to close soon, there are still many stations operating in this range, especially in small towns.
The receiver we describe has both didactic and recreational purposes, and can use scrap components.
In it we have an excellent gain input stage with field effect transistor, applying the signal to a Darlington stage of amplification.
As the consumption is high, we recommend the use of medium batteries or a small power supply of at least 300 mA.
The source can be from 6 to 12 V and good filtering so that snoring does not occur.
This is the ideal assembly for a course of initiation to electronics.
In figure 1 we have the complete diagram of the receiver of two transistors for the band of medium waves.
As it is a didactic assembly we suggest the use of a terminal bridge, although a matrix version of contacts can also be adopted.
The arrangement of the components for the terminal strip case is shown in figure 2.
The FET may be the BF245 or equivalent with the same array of terminals.
On assembly, observe the positions of the transistors and the polarity of the diode.
The coil is wrapped in a ferrite stick that can be harnessed from an old out-of-use transistor radio.
This coil consists of 80 to 100 turns of enamelled wire 28 or 30 AWG or even hard phone wire.
The variable capacitor can be harnessed from an out-of-use radio, as well as the small speaker and even some other components.
The resistors are 1/8 W with any tolerance and the capacitors conform to the bill of materials.
To test, connect an antenna (piece of wire from 2 to 5 meters) and ground connection, adjusting CV and P1 for best reception.
Material list
Q1 - BF245 - junction field effect transistor (JFET)
Q2 - BC548 - NPN power transistor
Q3 - TIP31 - NPN power transistor
D1 - 1N34 or equivalent - germanium diode
L1- Coil - see text
CV - variable capacitor - see text
S1 – On-Off switch
B1 - 6 to 12 V - Medium batteries or power supply
FTE - 4 or 8 ohms - speaker 10 cm or larger
P1 - 4M7 ohms - potentiometer
R1, R3 - 1k ohms - resistors - brown, black, red
R2 - 22 k ohms - resistor - red, red, orange
R4 - 220 k ohms - resistors - red, red, yellow
R5 - 47 ohms - resistor - yellow, violet, black
C1, C4 - 100 nF - ceramic capacitors
C2-470 pF - ceramic capacitor
C3 - 10 nF - ceramic capacitor
C5 - 100 uF - electrolytic capacitor for 16 V or more
Several:
Termina stripl or protoboard, power supply or batteries, heat sink for power transistor, wires, solder, etc.
