Feature: RF
A simple build Construction of this AM/SW1 receiver is very simple and quick. Its PCB layout and breadboard are shown in Figures 3 and 4. All resistors should be mounted
Figure 1: Principle of operation of the simple radio receiver
(5.5MHz ceramic filter) through coil L2 – five turns of 36SWG wire wound on L3 (SW1 oscillator coil), and to the base of transistor Q2 (BF494) and the first IF amplifier as it amplifies the modulated IF signal. Te amplified, modulated IF signal is coupled to the base of transistor Q3, the second IF amplifier, via an X2455kHz ceramic filter which further amplifies the modulated IF signal. Te twice-amplified IF signal is coupled
by X3 (third 455kHZ ceramic filter) to the detector circuit, which contains diode D1 (OA79 or 1N4148), and will detect so that only the audio frequency (modulated signal) remains.
Te audio signal is taken from the
variable end of volume control VR1 and fed to the input pin 3 of AF amplifier IC LM396 via a coupling capacitor C2 (1uF/25V) that blocks (isolates) DC voltage from the detector circuit. A detected AF signal from diode D1 is
connected to input pin 3 of the LM386 IC. Pins 1 and 8 are connected to an electrolytic capacitor C1 (10uF/25V). Ground is allocated to pin 4, and the power supply to pin 6. Output is pin 5, via an electrolytic capacitor (470uF/25V). The power amplifier’s output drives the speaker coil of 8Ω to produce an audible output.
vertically, and their correct polarity observed, especially for the electrolytic capacitors. Checking and re-checking would save you from any initial disappointment and any followup troubleshooting. Similarly, be careful with detector diode
D1’s polarity when mounting it. Transistors Q1, Q2 and Q3 are BF494,
available in either round metallic casing or block-type plastic casing. Te three 455kHz ceramic filters (X1, X2
and X3) and oscillator coil (which looks similar to an IFT) should be identified carefully and placed in their proper positions, being careful not to swap them. Don’t bend the two round pins 455 ceramic filters and oscillator coils. Cut off excess pin lengths before soldering, and clamp. To fix the antenna coil assembly on the
PCB, first loosely mount the antenna coil holder on the component side. Next, slide in the ferrite, fix the coil and solder its terminals, then slide in the ferrite rod and tighten the holder.
Figure 2: The circuit diagram of the simple and cheap 455kHz radio receiver
www.electronicsworld.co.uk June 2021 37
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46
