Miniature Radio Receiver Without Integrated Circuits
| Polish version is here |
My fascination with electronics began in my early childhood, and radio technology was one of the first areas that truly captured my attention. As a natural next step in nurturing this passion, I decided to build my own radio receiver. Initially, I experimented with basic detector circuits consisting of nothing more than a diode, a resonant circuit, an antenna, a ground, and a high-impedance earpiece. Although these solutions had limited sensitivity and very little selectivity, working on them proved invaluable—it sparked my curiosity and motivated me to explore electronics even further.
Today's radio technology has reached a level that allows for the construction of miniature FM receivers with high-quality signals and additional features, such as decoding RDS information (eng. Radio Data System). Some of these receivers are genuinely tiny—Photo 1 shows the RDA5807 integrated radio receiver measuring just 11x11x2 mm (approximately 0.43x0.43x0.08 in).
These receivers, thanks to modern digital technologies, offer high selectivity, stable reception, and the possibility of automatic tuning and digital signal processing. Even so, building your own radio receiver remains a valuable experience for anyone interested in radio engineering and electronics.
Without Integrated Circuits?
Though modern radio receivers use advanced digital solutions, the fundamental principles behind radio signal reception remain unchanged, and constructing a simple receiver is an excellent way to grasp the basic phenomena in this field. That’s why I believe it’s worth building a simple radio receiver on your own, even if its capabilities are more limited than those of modern devices.
The main technical assumptions for my proposed design include:
- Receiving an amplitude-modulated (AM, Amplitude Modulation) signal – a straightforward design
- A reflex circuit – high sensitivity with minimal complexity
- Using a ferrite rod antenna – compact design
- Using low-cost, widely available components
- Listening via standard earbuds
A key aspect of this project is the use of a reflex circuit, where the same transistor stage amplifies both the high-frequency (RF) signal and the audio signal. This allowed for circuit optimization by reducing the number of necessary components (Figure 1), contributing to a more compact design and simplifying the mechanical work.
To build the receiver described here, you’ll need to gather the following components:
- T1, T2, T3: BC546
- R1: 15kΩ
- R2: 2kΩ
- R3: 150kΩ (30-300kΩ)
- C1: 150pF
- C2: 100nF
- C3: 100uF
- Cr: as described
- D1: AAO152
- Sl: ~16-32Ω
- L1, L2: on ferrite rod, as described
As you can see, the circuit uses NPN bipolar transistors of type BC546. Although they are generally intended for lower-frequency applications, they perform effectively here. The feedback resistor R3 should be chosen experimentally within the given range to get the clearest reception at the lowest noise level.
The detector is a crucial element of the receiver, as it extracts the audio signal from the high-frequency radio signal. Here, I used a germanium diode, AAO152, which has a lower forward voltage than the silicon diodes commonly used in modern electronics, allowing it to detect weaker signals.
The component in the receiver that determines its frequency is the resonant circuit, made up of coil L1 and capacitor Cr. In Poland, AM signal for I Program Polskiego Radia (by national public-service radio broadcasting organization) is broadcast at 225kHz from the Longwave transmitter Solec Kujawski. Knowing that the frequency f of a parallel resonant circuit is given by:
we can easily calculate the necessary inductance L once we select the capacitance C, or vice versa. For example, if f=225kHz and Cr=100pF, then L1=5mH. By knowing the AL value of your ferrite core, you can determine how many turns of wire you’ll need. If you don’t know this value, you can use the inductance meter function on a multimeter. Instead of using a fixed-value capacitor Cr, you could also opt for a variable one, allowing more precise tuning of the receiver or the ability to search for other stations, even overseas. In my case, coil L1 has 120 turns of 0.12 mm (about 0.0047 in) enameled wire.
As for coil L2, which couples the resonant circuit to the detector, its parameters are not critical—mine is 20 turns of 0.2 mm (about 0.0079 in) enameled wire.
Both coils should be wound in a single layer, turn by turn, in the same direction. Paper or electrical tape can be used as insulation between the windings and the core. I wound these coils on a miniature ferrite rod just over 4cm (about 1.57 in) in length and 0.6cm (about 0.24 in) in diameter (Photo 3). Of course, if you’re not concerned about miniaturization, you can use a larger core, which will generally produce better reception.
I assembled the entire circuit on a small piece of universal PCB, where there was also room for a jack to accommodate typical low-impedance headphones (16-32Ω), a switch, and a small 1.5V alkaline cell (Photo 4). The two earbuds (left and right) are connected in series.
Once the circuit is powered, you should hear a gentle hissing noise in the headphones, and tuning to the station should produce a loud and clear broadcast. If the noise level is excessive, try adjusting R3. Bear in mind that the ferrite antenna has a directional characteristic, so for best results, rotate it according to the direction of the broadcasting station. The whole device easily fits inside a matchbox (Photo 5).
To test the device’s functionality, I analyzed its performance under various environmental conditions. In open spaces, the signal was clear and stable, while in enclosed areas there was noticeable interference from electronic devices such as computers, cell phones, or LCD monitors. Of course, you can still use the receiver indoors; just keep it away from interference sources.
To demonstrate the sound quality you can expect from a properly built and tuned receiver, here is a recording of a broadcast:
I recorded the audio by placing a microphone against the earphone connected to the receiver.
Even building a receiver as simple as this one is a great way to become fascinated with and deepen your knowledge of radio engineering and analog electronics. In a world dominated by digital devices, it’s worthwhile to return to the basics, and there is immense satisfaction in picking up radio broadcasts on a device you’ve built yourself.
Further readings:
- Janeczek A., Wakacyjny miniodbiornik AM, Elektronika dla Wszystkich, 8, 1997, str. 62-63
- Ellingson S. W., Radio Systems Engineering, Cambridge University Press, 2016, str. 16-17
- Harman P. M., The Natural Philosophy of James Clerk Maxwell, Cambridge University Press, 1998, str. 6
Marek Ples