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.
Click on the image to view circuit
The author has firstly met with the LM386 circuit over twenty years ago, and has ever since been using it very successfully in various devices. The fact that this IC has survived at the market for such a long time is a considerable proof of its quality, however, its most significant advantage remaining extremely low price. The readers can find more information on this circuit in book 4 of PE. Purchasing this IC, one must have in mind that it is being manufactured in several versions, marked as LM386, LM386N-1, LM386N-3 and LM386N-4, that differ themselves by the supply voltage values and the output power. In case the supply voltage being no greater than 12 V, any of these can be used in this receiver.
Electronic diagram of the direct radio receiver that has a LM386 IC in its LF stage is given on Pic.3.18. The resistor in the detection circuit is the log type 470 kOhms potentiometer. The LF voltage is being led over its moving end and the coupling capacitor C2 onto the inverting input (leg No.2) of the LM386. Other, non inverting input (leg No.3) is connected to the ground. The output is on the leg No.5. On this leg the load (i.e. 8 Ohm impedance loudspeaker) is being placed, connected over the C6 capacitor to the ground. With smaller battery power supply voltages a 4 Ohm impedance loudspeaker can also be used. If there's nothing in between the legs 1 and 8, the voltage amplification of the IC is Au=20, in which case the capacitor C7 can be omitted. If, however, a 10 ìF electrolytic capacitor is connected between legs 1 and 8 (+ end on leg 1) as shown on Pic.3.18, the amplification is Au=200. Adding a resistor in line with the abovementioned capacitor, any amount of amplification between 20 and 200 can be achieved. This resistor is shown in dashed line on Pic.3.18 and is being marked with "*" sign.
The PCB and components layout for the receiver shown on Pic.3.18 are given on Pic.3.19. Miniature loudspeakers from the pocket-size radio receivers should be avoided, since their efficiency and sound quality are poor, especially in the low frequencies area. On Pic.3.19 a 1W loudspeaker is being shown, whose membrane is 8 cm in diameter. That isn't such a bad solution, but even better would be using the loudspeaker with greater power and membrane diameter (During the testing the 3 W , 8 Ohm loudspeaker has also been used, and the sound quality was much better than with the one that is shown on the picture). As you can see, the cables connecting the loudspeaker with the PCB are firmly twisted around each other. This is a must, especially for the cables being longer than a dozen centimetres. The same has to be done with cables that connect PCB with the battery and the main switch Z.
Significantly louder reception, with bandwith being narrower in the area of the lower bound frequency, can be accomplished with the coil that has smaller inductivity and the coupling capacitor CA with greater capacitance. For example, if you are interested in radio stations that emits the signal in the MW area from 750 kHz to 1700 kHz, you could use the coil that has been previously described, by using only its' part between the legs 1 and 2, as shown on Pic.3.20. For the antenna that is about 6 m long use the CA=33 pF, and the reception is going to get better. Try increasing the CA capacitance (47 pF, 100 pF, 200 pF), the reception is going to get louder and louder, but the reception bandwidth will be getting smaller and smaller, some stations will not be heard any more, others will start to "mix" etc. it is up to you to find an optimum that suits you best. If using very short antenna, you should omit CA, and connect the antenna directly to the oscillatory circuit (at pt. 1).
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).
The optimal value for R3 resistor (obtaining the greatest amplification, minimal distortion etc.) depends upon the kind of transistor that is being used. The easiest way to finding it is by experimenting, with a few MOhms’ trimmer resistor, in the way it has already been described in chapter 3.2.
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).
If the Idle current is zero or is significantly bigger than 5 mA, something is wrong. If it’s zero, check out whether you have connected your instrument properly and whether it is functioning. Remove the instrument and connect the battery directly to the wires A and B. Touch the pin No.2 of LM386 with your finger, if you hear the hum - your instrument is malfunctioning. However, if the instrument is OK, you should check the DC voltages. Set your multimeter to measure DC voltages, connect one probe (the one marked with the ground sign) to the amplifier ground (minus battery pole) and with second probe touch the plus battery pole first, and then pin No.6 of LM386. These two voltages should be equal to the battery (supply) voltage. Contact then pin No.7 of the TLO71. This voltage should be slightly smaller than the supply voltage. Voltages at pin No.5 of LM386 and No.7 of the TLO71 should be app. equal to the one half of the supply (battery) voltage.
Try to locate the malfunction based on your measurements. Check out if there are some discontinued copper lines on the print, or if two adjacent lines are connected (by themselves or by a small piece of tin that you dropped from the iron during soldering), whether all junctions are the way they should be, if you happened to mount some electrolytic capacitors or IC’s upside-down, etc.
When you make sure that everything is fine, un-solder the potentiometer and the loudspeaker and put them on the front box panel. Also mount the plug and the switch, the LED (if used) etc. Tighten the plate with two small wood screws to the side pane, placing the LM386 on top (above the TLO71). Two distant-rings are to be mounted between the plate and the wood, making the distance between them a few (say, 5) mm. (These rings can be cut from some small plastic pipe). The screws are first to be put through the plate holes, then the rings and then screwed in the panel. After all this you can connect the loudspeaker, the switch, the battery, etc.