Many beginners who are following with interest our monthly e-courses, we ask small simple assemblies and materials. This receiver will fill the medium wave. It is simple to perform and, mounted on a circuit single sided, it does not require more care than an hour to hear his first sounds.
In these times, no one would lose more time to make a crystal radio because, as we are demanding now, we do not just hear the helmet with a low emission of single local transmitter. To receive a speaker in several medium-wave transmitters, just two integrated circuits.
A bit of history
The crystal radio was a radio receiver consists of a single inductor and a variable capacitor, necessary to achieve agreement on any transmitter broadcasting on medium wave range. The signal picked up by the tuning circuit is then detected by a piece of lead sulfide ore and silver called "Galena" (which is why they say "a crystal radio" "Well, you and Galena, eight days! "answered the adjutant of the gag Fernand Raynaud).
This mineral, today, is no longer used because it can very advantageously replaced (in all respects and in particular with regard to research on the stone of a PN junction with a needle: no pie if I think my great grandfather ...) by a germanium diode (OA70, for example). Since the amplitude of the signal detected by a crystal radio was very low, to hear you had to use a helmet. To increase substantially, if I may say, this amplitude, it was also installed on the roof of the house, a long copper wire (as antenna) and connect the mass to a good ground (which was often costs a cast iron radiator central heating or water pipe lead).
Our achievement
After this explanation, we will propose to implement a receiver OM (Medium Wave, not Old Man which means in the language of the amateur radio operator) simple, but still more sensitive than a crystal set and running with a high speaker (headphones, it takes the lead!).
The wiring diagram of the receiver OM
Figure 1: Diagram of the receiver medium wave. To operate, it is supplied in 12 or 13 V to charge power pack 230 V 
Figure 2: Pin Assignments of IC μA703 and BB112 varicap viewed below and the integrated circuit TBA820M seen from above. As shown in Figure 1, to achieve this receiver two integrated circuits are sufficient. The first, a μA703 (IC1) is an RF amplifier capable of amplifying 40 dB, 100 times in tension, any radio signal up to 150 MHz. The second, a TBA820M (IC2) is a final stage of LF power capable of attacking an 8 ohm load (speaker) with a power of 2 W.
Let's start the description of the wiring diagram MF1: it is a self shielded with a primary winding provided by two variable capacitance diodes, DV1 and DV2, on the transmitter you want to receive. The signal goes from the tuning circuit by induction in the secondary winding of MF1 to be removed, then be applied to the input pins 3 and 5 of integrated circuit IC1 can be amplified. The RF signal is amplified 100 times, is taken from the output pin 7 of IC1 to be detected by the germanium diode DG1 AA117 (a bit more modern than the old OA70!). At the output of this diode, an audio signal is available: it is applied to the volume potentiometer R6. The self JAF1, connected to pin 7 of IC1, serves to bring to the integrated circuit positive voltage of 12 V, while preventing the RF signal amplified from draining to the positive supply. The slider of potentiometer R6 is sampled signal BF applied directly to pin 3 of the second integrated circuit IC2 to be amplified in power. On the output pin 5 of the second integrated circuit, we take the audio signal power to enforce, through the electrolytic C16, across a small loudspeaker of 8 ohms.
Returning now to the primary winding of the choke MF1 to explain how any agreement on the medium wave range with potentiometer R1 agree. By turning the potentiometer wiper R1 Agreement from one extreme to another, we apply the varactor DV2 and DV1 through R3, a voltage variable from 0.9 to 12 V. As you know, the varactor diodes are having special feature to modify their internal capacity in terms of the variation of the voltage applied across them. When you turn the wiper potentiometer R1 to R2, so as to apply to these diodes a voltage of 0.9 V, we obtain maximum capacity is 500 pF. If it is turned the other way, so that they apply the maximum voltage of 12 V, we obtain a value of approximately 15 pF. The varactor DV2 DV1 and being in series, the total capacity is divided and so we can see that these diodes behave as a variable capacitor, all the slats are closed, has a capacity of 250 pF, and all the slats open a capacity of 7.5 pF.
To power the receiver requires a voltage of 12 V can be taken from a small power supply unit.
To avoid the disastrous effects of a reversal of polarity, we inserted a silicon diode DS1 in the positive.
Figure 3: Photograph of a prototype receiver OM. Using a good antenna and good ground, you can even listen to foreign radio stations distant, especially at night when the spread is favorable. 
Figure 4a: Schematic implementation of the components of OM receptor. The metal frame of the two potentiometers must be grounded by a piece of copper wire weld between the housing and the stud neighbor. Notice the small pin-keyed side marker to insert the μA703 in the right direction: it must be turned to C6. 
Figure 4b: Drawing scale 1, the circuit board receiver OM. Component List
R1 = 22 kW
R2 = 1.8 kΩ
R3 = 100 kΩ
R4 = 100 Ω
R5 = 27 kΩ
R6 = 22 kW
R7 = 150 Ω
R8 = 22 Ω
R9 = 56 Ω
R10 = 1 Ω
C1 = 100 nF ceramic
C2 = 270 pF ceramic
C3 = 100 nF ceramic
C4 = 1 uF polyester
C5 = 1 uF polyester
C6 = 680 pF ceramic
C7 = 10 uF electrolytic
C8 = 1 nF polyester
C9 = 47 uF electrolytic
C10 = 100 uF electrolytic
C11 = 47 uF electrolytic
C12 = 100 uF electrolytic
C13 = 1 nF polyester
C14 = 100 uF electrolytic
C15 = 220 nF polyester
C16 = 470 uF electrolytic
Self JAF1 = 330 uH
MF1 = MF Pot 750 kHz (red)
DS1 = Diode 1N4007
DG1 = Diode AA117
DV1 = Diode varicap BB112
DV2 = Diode varicap BB112
IC1 = Integrated μA703
IC2 = Integrated TBA820M
HP = 8 Ω 1.5 W
The practical realization of the receiver
sort them first by value), then all capacitors (observe the polarity of the electrolyte, the longest leg is +, see Figure 4), then near MF1, insert the 2-pole terminal block for input of 12 V power supply and insert the LED DS1 protection (it is plastic), white ring mark-keyed to the right. Insert the second germanium diode DG1 (it is glass), close to R5, black ring mark-keyed to C8. To the left of MF1, solder the varactor DV2 and DV1: they are shaped like a transistor with plastic flat-keyed to reference guide to the top. Do not shorten their two legs.
You can then mount on the bottom of the printed circuit potentiometers, R1 for the agreement (it is a linear 22KA) and R6 for the volume (it is a logarithmic 22KB). Both are 22 kilohms, but does not invert. After inserting their 3-pin and soldered into corresponding holes of the PCB, solder another piece of copper wire between the metal body of each knob and pin neighbor noted "mass", if there's hum in the speaker .
To complete the installation of the receiver, you must solder two pins on the right side of the plate the coaxial cable coming out of the small loudspeaker: the cable core is welded on the central spike near C16 and the shield to that of low.
We have no housing provided for this collage, but you can choose a plastic or wood.
setting, antenna, earth and the first tests
Figure 5: As earth, we can use the spit of land protruding from a socket current "16 A". It will connect the ground pin of the receiver to this pin via a copper wire terminated at one end by an alligator clip. Before proceeding to the reception, you should screw slowly clockwise, the red nucleus of the self MF1. Thus you will succeed in exploring the range between 500 and 1800 kHz, well beyond the medium wave range from 530 to 1600 kHz.
To receive the greatest possible transmitters transmitting on OM, you must connect to the antenna input of the device (near pin C2), a long copper wire (the longest possible) and the other stud welding (about R3) a wire leading to a ground. The antenna wire may consist of a thin copper wire stretched between a window (bring a plastic insulator or glass or ceramic, etc.). And a stake, pole or tree (still with isolator).
The pin is connected to earth by a copper wire terminated by a crocodile clip to the ground pin (large pin projecting) a socket (Figure 5). Do not force the alligator clip, if possible, into a hole of 230 V ... it would shock you! You can also connect the ground wire to the metal casing of a household electrical appliance (dishwasher, washing machine, etc..) Necessarily connected to the earth.
You will see that day, by turning the agreement, you will succeed in capturing some stations because the ionized layer of the atmosphere is lower and it absorbs all frequencies OM or OC (Shortwave).
In the evening, however, when this low layer disappears, the waves reach the highest ionized layers, they are again reflected towards the Earth's surface and can be captured at large distances: also, do not be surprised to receive Stations Spanish, Italian, German, Polish, Greek, North African, etc.. If you want to know more about radio wave propagation through the ionized layers of the atmosphere, again, see your course published in ELM.
Before concluding this section, we must tell you that this receiver is very sensitive to electrical disturbances and so if you hear noise in the speaker, you can try to improve things, extinguish all lights Neon and even home computer: parasites should cease. Anecdotally, adding past when the weather did not exist in the absence of satellites, some were able to recognize an approaching thunderstorm, just by listening to the radio on OM strong electric shocks from Lightning: it was a warning sign of impending rain, a bit like the awakening of rheumatic pain in people prone.
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