AM Radio For Beginners (ART472E)

The 3-transistor radio we describe uses common components, many of which can be availed from other out-of-use AM radios. The circuit is powered by 4 batteries and with a small antenna and good ground connection can pick up several local stations. Note the connection of the inverted batteries, as we use PNP transistors in this circuit.

The operation is simple to understand. It is a simple direct detection and amplification receiver. The signals selected by the tuning circuit are detected by D1, extracting the modulation that receives a first amplification by Q1.

Passing through P1, which is the volume control, the signal is taken to a stage with good gain Darlington that excites the small speaker. The circuit has a tone control via P2.

Assembly

In figure 1 we have the complete circuit of the receiver.

Figure 1 - Receiver diagram

A recommended assembly is a contact array or printed circuit board, but there is the simplified terminal strip option, shown in Figure 2.

Figure 2 - Terminal strip assembly

The coil consists of 30 + 70 turns of enamelled wire 28 or 30 on a ferrite rod and the variable capacitor CV can be harnessed from an out-of-use AM radio.

On assembly, observe the positions of the transistors and the polarity of the diode and the electrolytic capacitor, as well as the battery holder. The resistors are 1/8 W with any tolerance and the 4 or 8 ohm speaker with 5 to 10 cm.

In figure 3 we show where the connection of a variable of small radic plastic must be made.

Figure 3 - Variable connection

To test, simply turn on the radio and try to pick up a station by setting P1 and CV.