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3.4. Simple Radio Receiver with LM386 IC Listening the programme over the headphones has its advantages, but the true radio receiver is certainly the one that with the loudspeaker. In this project the simplest such receiver is being described, consisting of input circuit, diode detector and audio amplifier with LM386 IC, that allows the loudspeaker reproduction to be performed.
3.5. Radio Receiver with Increased Sensitivity Audio Amplifier If you cannot obtain the signal reception that is loud enough with
receiver from the previous project, the sensitivity of the LF part of the
device must be increased. The simplest way to manage this is adding a
transistor pre-amplifier, as shown on pic.3.21. Please note that all the
components except C1, C and L are located on the PCB, which allows you to
use the input circuit from pics.3.15 and 3.20. The receiver is supplied from
the 4.5 V battery but, if you intend to use an adaptor, the supply voltage
can be bigger, its maximum value being from 12 V (for the LM386) till 18 V
(for the LM386-4). 3.6. Universal Audio Amplifier In majority of the receivers described so far, as well as in many those that will be described further, an audio amplifier is being used. This makes it useful to build an universal amplifier that would be used during testing of all receivers. That can be the circuit from pic.3.21 (from C4 inclusive to the right), but it can be something else, too, e.g. the one from the pic.3.22. Two IC’s are used in it: operational amplifier TLO71 as the pre-amplifier, and LM386 as the power amplifier. The TLO741 is exactly the same as the more famous 741, the only difference laying in fact that TLO71 has the FET on its input, and 741 - the bipolar transistor. Since inverting input is being used in the operational amplifier, its voltage amplification is given by Au=RP/R1, where RP stands for the potentiometer resistance from the sliding contact to its left end. When the slider is at its rightmost position, it is then RP=470 kOhms, yielding Au=470, and when it’s at its leftmost position the RP=0, therefore Au=0. As you can see, moving the slider changes the amplification from 0 to 470. That’s what theory says, the practice is, of course, somewhat different. When capacitors are being connected between the input and ground, maximum amplification gets lower than 470 but is still more than enough for our needs. * All the receivers that have been described so far, as well as all those
to come, can be tested with this universal amplifier. That is done by taking
the signal from the Detector output (through two wires as short as possible
or through the microphone cable that can be significantly longer) to the
amplifier input (between left end of C1 and Ground). If the antenna, input
circuit (and other stages, if exist) are OK, you will be able to hear the
tuned station program from the loudspeaker. The sound volume can be adjusted
by moving the slider of the potentiometer P.
PCB and component layout are given on pic.3.23. * KS marks the short-circuit wire. It’s a piece of wire which ends are taken
through the holes on the plate and soldered. It can be omitted, but then the
left end of potentiometer is to be soldered in such a way to be connected to
the bottom end of R1. * To connect the potentiometer with the PCB, as well as the male, a shielded
cable should be used. The shield (outside conductor) is connected to the
Ground. If cables are shorter than app. 10 cm, the ordinary wires can be
used instead. * The amplifier, the loudspeaker and the battery (or adapter) are put in the
common box, on whose front plate the potentiometer, the switch and some
simple plug (the chinch will do) are mounted. If the device is powered from
the adapter, it is useful to also mount a LED. With these parts you have
made yourself the so-called “active loudspeaker”, which is a very useful
gadget for every electronics lover. * Just before you start mounting and soldering the components, clean well
the PCB copper conductors with a piece of wet sponge dipped in some cleaning
powder. Components should be mounted and soldered in the following order:
short-circuits, resistors, IC’s, capacitors (all except C4), connecting
wires and microphone cables (if used). The last component to be mounted is
the C4 capacitor, but first you have to check whether the amplifier operates
correctly. Connect the loudspeaker, and the wire marked as “B” on pic.3.23-c connect to
the minus battery pole. The other end of the wire marked as “A” should be
connected to one probe of the multimeter, which is to be set to measure DC
current. Contact the plus battery pole with second instrument probe, and it
will measure the idle current of the amplifier, which should be app. 5 mA.
If this is OK, touch the pin No.2 of the LM386 with finger, and if
everything is fine you will hear the 50 Hz hum from the loudspeaker. The
power amplifier is OK. You can now solder C4. Touch the left end of the wire
that is connected to the C1 with your finger, the hum should be heard again
from the loudspeaker, its loudness depending upon the position of the slider
of the potentiometer P. (If you don’t possess an instrument, connect the
battery on the wires “A” and “B” and do the touching described. If you hear
the hum, the amplifier is OK).
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