This small AM-MW range radio uses four transistors with a complementary amplifier for the output, providing excellent power for a small speaker. The radio is powered by 2 or 4 batteries and has good sensitivity.
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.
This is the ideal assembly for a course of initiation to electronics.
The circuit uses an amplifier of 3 transistors in complementary configuration with excellent quality of sound and power.
Larger speakers (15 to 20 cm) can even be used in a small speaker for good desk-radio sound.
In figure 1 we have the complete diagram of the receiver of four transistors for the MW band.
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.
We can also have a printed circuit board assembly with the pattern shown in Figure 3.
The coil is wrapped in a ferrite stick that can be harnessed from an old out-of-use transistor radio.
L1 consists of 20 turns of enamelled wire 28 to 30 and L2 of 30 + 70 turns of the same wire, as shown in the figure.
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.
P1, at this receiver, is the volume control.
Material list
Q1, Q2, Q3 - BC548 - general purpose NPN transistors
Q4 - BC558 - PNP general purpose transistor
D1 - 1N34 or equivalent - germanium diode
L1, L2 - Coils - see text
CV - variable capacitor - see text
S1 – On-Off switch
B1 - 3 or 6 V - 2 or 4 small batteries
FTE - 4 or 8 ohms - small speaker
P1 - 47 k ohms - potentiometer
R1 - 1 M ohms - resistor - brown, black, green
R2 - 1 k ohms - resistor - brown, black, red
R3 - 10 k ohms - resistor - brown, black, orange
R4 - 150k ohms - resistor - brown, green, yellow
R5 - 1k8 ohms - brown, gray, red
C1 - 100 nF - ceramic capacitor
C2-470 pF - ceramic capacitor
C3, C4 - 10 uf - electrolytic capacitors for 6 V or more
C5 - 100 pF - ceramic capacitor
C6 - 220 uF - electrolytic capacitor for 6 V or more
C7 - 100 uF - electrolytic capacitor for 6 V or more
Several:
Terminal strip, mounting box, battery holder, wires, solder, etc.
