AM Receiver
Description:
This is a compact three transistor, regenerative receiver with fixed feedback. It is similar in principle to the ZN414 radio IC which is now no longer available. The design is simple and sensitivity and selectivity of the receiver are good.
Notes:
All general purpose transistors should work in this circuit, I used three BC109C transistors in my prototype.The tuned circuit is designed for medium wave. I used a ferrite rod and tuning capacitor from an old radio which tuned from approximately 550 - 1600kHz. Q1 and Q2 form a compund transistor pair featuring high gain and very high input impedance. This is necessary so as not to unduly load the tank circuit.
The 120k resistor provides regenerative feedback,between Q2 output and the tank circuit input and its value affects the overall performance of the whole circuit. Too much feedback and the circuit will become unstable producing a "howling sound". Insufficient feedback and the receiver becomes "deaf". If the circuit oscillates,then R1's value may be decreased; try 68k. If there is a lack of sensitivity, then try increasing R1 to around 150k. R1 could also be replaced by a fixed resisor say 33k and a preset resistor of 100k. This will give adjustment of sensitivity and selectivity of the receiver.
Transistor Q3 has a dual purpose; it performs demodulation of the RF carrier whilst at the same time, amplifying the audio signal. Audio level varies on the strength of the received station but I had typically 10-40 mV. This will directly drive high impedance headphones or can be fed into a suitable amplifier.
Construction:
All connections should be short, a veroboard or tagstrip layout are suitable. The tuning capacitor has fixed and moving plates. The moving plates should be connected to the "cold" end of the tank circuit, this is the base of Q1, and the fixed plates to the "hot end" of the coil, the juction of R1 and C1. If connections on the capacitor are reversed, then moving your hand near the capacitor will cause unwanted stability and oscillation.
Finally here are some voltagee checks from my breadboard prototype.This should help in determining a working circuit:-
All measurements made with a fresh 9volt battery and three BC109C transistors with respect to the battery negative terminal.
Q1 (b) 1.31V
Q2 (b) 0.71V
Q2 (c) 1.34V
Q3 (b) 0.62V
Q3 (c) 3.87V
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ZN414 Portable Receiver
Circuit : Andy Collinson
Email:
Description:
An AM portable radio receiver made from the ZN414 IC. The ZN414 ic has now been replaced by the MK484 which is identical in performance and pinout.
Notes:
Designed around the popular ZN414 IC this receiver covers the medium wave band band from approximately 550 to 1600 KHz with the values shown. The coil and tuning capacitor may be taken from an old MW radio to save time. The ZN414 IC, has now been replaced by the MK484. The integrated circuit is a 3 pin, tuned radio frequency circuit, and incorporates several RF stages, automatic gain control and an AM detector. It is easily overloaded and the operating voltage of th IC is somewhat critical to achieve good results.
In this circuit a small voltage regulator is built around the BC108B transistor, four 1N4148 diodes, the 2k7 and 10k preset resistor and the 820R resistor. The 10k pot acts as a selectivity control for the whole receiver, controlling the operating voltage for the ZN414 (or MK484). If you live in an area that is permeated with strong radio signals, then the voltage may need to be decreased. I found optimum performance with a supply of around 1.2 volts.
The audio amplifier is built about an inverting 741 op-amp amplifying circuit. Extra current boost is provided using the BC109C / BC179 complementary transistor pair to drive an 8 ohm loudspeaker. The voltage gain of the complete audio amplifier is around 15. The audio output of the complete receiver is really quite good and free from distortion. I may provide a sound sample later. Click here to see a picture of my prototype. I used a small wooden enclosure and the complete tuning assembly from an old radio.
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Short Wave Receiver Using the ZN414
Notes:
The original data sheet for the ZN414 states that the maximum working frequency is around 4 MHz. That may be
true, but SW broadcasts are so powerful that this receiver will work well with signals up to around 6 or 7 mhz.
The 10k resistor controls the operating voltage for the ic which is critical for good performance.
The tuned circuit consists of a variable capacitor and fixed air spaced coil. For the inductor, I wound 10 -20 turns
of wire on an empty tube of around 1.5 inches diameter. The turns were spaced so that the overall length was
around 3 inches. The variable capacitor tuned 0 - 300 pF but there is plenty of scope for experiment here. One
final point, you will need an external antenna to receive broadcasts. I have an outside wire that is about 7 meters
long and this was quite effective. The antenna can be connected at either end of the coil or via a series capacitor
value between 10pF and 100 pF.
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AM Transmitter
Notes:
Please read the disclaimer on this site before making any transmitter circuit. It is illegal to operate a radio transmitter without a license in most countries. This ircuit is deliberately limited in power output but will provide amplitude modulation (AM) of voice over the medium wave band.
The circuit is in two halfs, an audio amplifier and an RF oscillator. The oscillator is built around Q1 and associated components. The tank circuit L1 and VC1 is tunable from about 500kHz to 1600KHz. These components can be used from an old MW radio, if available. Q1 needs regenerative feedback to oscillate and this is achieved by connecting the base and collector of Q1 to opposite ends of the tank circuit. The 1nF capacitor C7, couples signals from the base to the top of L1, and C2, 100pF ensures that the oscillation is passed from collector, to the emitter, and via the internal base emitter resistance of the transistor, back to the base again. Resistor R2 has an important role in this circuit. It ensures that the oscillation will not be shunted to ground via the very low internal emitter resistance, re of Q1, and also increases the input impedance so that the modulation signal will not be shunted. Oscillation frequency is adjusted with VC1.
Q2 is wired as a common emitter amplifier, C5 decoupling the emitter resistor and realising full gain of this stage. The microphone is an electret condenser mic and the amount of AM modulation is adjusted with the 4.7k preset resistor P1.
An antenna is not needed, but 30cm of wire may be used at the collector to increase transmitter range.
Submitted by Zac Soden
Description: AM Radio Receiver
This is an extremely simple AM radio receiver and amplifier circuit which is capable of driving a small, 8 ohm speaker. Construction is simple and could be performed on a breadboard with decent results. C1 is used for tuning. VR1 is used to control volume.
Part / Description / Qty / Notes
R1 / 100K ohm resistor / 1 / All resistors 5% tolerance
R2 / 6.8K ohm resistor / 1
R3 / 1.2K ohm resistor / 1
R4 / 1M ohm resistor / 1
R5 / 22K ohm resistor / 1
R6 / 20K ohm resistor / 1
R7 / 1K ohm resistor / 1
R8 / 1 ohm resistor / 1
C1 / 160pF variable capacitor / 1
C2 / 0.01uF ceramic capacitor / 1
C3, C6 / 0.1uF ceramic capacitor / 2
C4 / 0.039uF ceramic capacitor / 1
C5, C7 / 2.2uF bipolar electrolytic capacitor / 2
C8 / 0.22uF ceramic capacitor / 1
VR1 / 10K potentiometer / 1 / Substitute: >1K<100K
L1 / Broadcast band ferrite rod antenna coil / 1
Q1 / ZN414 AM radio receiver IC / 1 / Substitute: MK484
IC1 / LM1875 audio amplifier IC / 1
SP1 / 4-8 ohm speaker / 1 / Must be low wattage
9V / 9V battery / 1