Do you love electronics and want a fun, beginner-friendly project? Build a two transistor radio! This classic DIY project, featuring simple components, gained popularity years ago and still teaches radio technology effectively. Let’s explore how you can create your own two transistor radio easily.
What is a Reflex Circuit?
A reflex circuit cleverly reuses one transistor to amplify both RF and AF signals. This reduces component count while maintaining impressive performance for a simple receiver. The design shown in this project embodies that efficiency.
How the two transistor radio works
At the heart of the two transistor radio receiver is a coil (L1), made from 88 turns of enameled copper wire (32 SWG) on a 5/16-inch ferrite rod about 4 inches long. A tap is made at 8 turns to connect part of the signal back into the circuit. Together with variable capacitor VC1 and capacitor C1 (50nF), this forms the tuned circuit that selects the desired frequency.
Although VC1 is rated at 350pF, its exact value isn’t too critical — most transistor-type tuning capacitors will work.
Signal Path
RF signals enter the circuit through the coil and appear at a high impedance point. The 8-turn tap provides a good match for the base of the first transistor (Q1 – BC108), which amplifies the RF signal. This is then passed to the second transistor (Q2 – another BC108), configured as an emitter follower.
From here, the signal reaches the earpiece through a germanium diode (D1 – OA91), which rectifies and detects the audio signal. After being smoothed by C1, it’s fed back through the 8-turn tap to be amplified again by Q1 — but this time, as an audio signal. This clever reuse of Q1 for both RF and audio amplification gives the reflex circuit its name.
Two Transistor Radio – Components List
- Q1, Q2: BC108 transistors
- D1: OA91 germanium diode
- C1: 50nF capacitor
- VC1: 350pF variable capacitor
- R1: 3k3 resistor
- L1: 88 turns + 8-turn tap on ferrite rod (32 SWG wire)
- Power: 9V battery
- Earpiece: High-impedance type (>250 ohms)
⚠️ Note: Most modern 8-ohm earpieces will not work in this circuit. You must use high-impedance headphones or earpieces — ideally no less than 250 ohms.
Final Thoughts
This two-transistor reflex radio is a perfect example of early ingenuity in circuit design. It’s simple, uses very few components, and still manages to receive AM broadcast signals with clarity. Whether you’re an experienced builder or a curious beginner, experimenting with this design offers a deep dive into analog radio principles — and a rewarding hands-on experience.
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