Simple Diode Radio For Low Impedance Headphones

Diode Radio For Low Impedance Headphones - using small amount of component , germanium diode semiconductor you can build small radio equipment that can pick up lot of AM/LW/SW frequency . This project will talk about this purpose. if you ever look at construction notes for building old detector type radios the type of headphones specified always have an impedance of 2 X 2000 Q. Nowadays the most commonly available headphones have an impedance of 2 x 32 Q, this rel- atively low value makes them unsuitable for such a design. However, with a bit of crafty  transformation these headphones can be used in just such a design. To adapt them, you will need a transformer taken from a mains adapter unit, the type that has  a switchable output voltage (3/4.5/6/9/12 V) without the rectifying diodes and capacitor. Using the different taps of this type of transformer it is possible to  optimise the imped- ance match.

For the diode radio (any germanium diode is suitable in this design) the key to success is correct impedance matching so that none of the received signal energy is  lost. The antenna coil on the 10 mm diameter by 100 mm long ferrite rod is made up of 60 turns with a tap point at every 10 turns; this is suit- able for medium wavereception. If a long external aerial is used it should be connected to a lower tap point to reduce its damping effect on the circuit. 
You can experiment with all the available tapping points to find the best reception. With such a simple radio design, the external aerial will have a big influence.

After construction done, connect ground to water pipe/make the grounding your own. The Key is the good grounding, so there is voltage between the air radio signal from the grounding, and this radio signal picked up by this circuit. This is such building a Crystal Radio

Component Lists :

Small PCB

Mini Box

Antenna Telescopic/Wire Cable AM/MW/LW/SW

Germanium Diode

VC1 500 pF

C2 10 nF

Headphone 2 X32 Ohm

Headphone Jack

Best One Transistor Radio

 

One Transistor Radio - Here is a simple circuit for a one transistor Audion type radio powered by a 1.5 V battery and Transistor BC548 It employs a set of standard low- impedance headphones with the headphone  socket wired so that the two sides are connected in series thus giving an impedance of 64 Q. The supply to the circuit also passes through the headphones so that  unplugging the headphones turns off the supply Using an Audion configuration means that the single transistor performs both demodulation and ampli- fication of the  signal. The sensitivity of this receiver is such that a 2 m length of wire is all that is needed as an antenna. The tap on the antenna coil is at l/5th of the total  winding on the ferrite rod.The antenna coil on the 10 mm diameter by 100 mm long ferrite rod is made up of 60 turns with a tap point at every 10 turns; this is suit- able for medium wave reception. If a long external aerial is used it should be connected to a lower tap point to reduce its damping effect on the circuit.  This circuit is suitable for reception of all  AM transmissions from longwave through to shortwave.

Parts List :

Transistor BC548

R1    100K

VC1     500pF

C2     10uF

C3     100nF

L1 10mm diameter Coil, 100 mm long Ferrite rod

60 turns with a tap point 10 turns

Headphone 2 X 32 Ohm

Battery 1.5 V

Mini Box

PCB 

Antenna Telescopic

Reference : Elektor Electronics - B.Kainka