3.1.4 Other Components
Where Cx denotes the so-called parasite capacitance. It comprises the capacitance of the trimmer capacitor (its average value) that is connected parallel to the variable capacitor C, input capacitance of the next stage of the receiver (where the signal from the input circuit is being lead), antenna capacitance, coil capacitance and capacitance of the connections between the components of the input circuitry. The amount of this capacitance is not known in advance, therefore must be assumed. Taking that value, the coil inductance is calculated and the appropriate coil is made, together with the input circuit. The error being made with the assumption of the capacitance Cx is then compensated with the abovementioned trimmer capacitor. In all our projects this capacitor had minimum capacity Cmin=12 pF. We assumed Cx=15 pF, and therefore:
We made this coil, conented it with other components from Pic.3.1 and, after some experimenting and measurement, came upon the conclusion that its inductance should be somewhat smaller. We uncoiled a few reels, re-checked the bandwidth, then uncoiled some more, re-checked again, and after several tries came up with the solution. With variable capacitor that will be described in the following chapter, the abovementioned bandwidth is achieved with the coil of inductance L=330 ìH (microhenries).
The coil body i.e. the body where the coil is being reeled is a cylindrically shaped isolation material. For this purpose we have been using the carton core of the household aluminium foil package, its diameter being 3.2 cm. The number of bends required and wire diameter are calculated acc. to the formulas from Pic.3.5.
In order to use these expressions coil length must be assumed first. If this length later proves to be incorrect because the wire is too thick or thin, new length is adopted and the calculation is repeated. Let us assume that coil length is l=4 cm. The number of reels and coil diameter are:
Since there is no wire of such diameter, we adopt the closest existing value, d=0.3 mm. In that case the length l will be somewhat bigger, and so will be the number of reels. After a few iterations in calculus and later inductance check of the finished coil, we came upon the n=144 reels of lacquered copper wire (the mark for such wire is CuL), its diameter being d=0.3 mm.
This coil is shown on Pic.3.6. As you can see, two holes are made in coil body (with a bodkin) and through them the wire origin is being threaded twice. After that 90 reels are made, then a leg, then another 55 reels and finally the wire end is again threaded twice, through the other two holes. The leg is made by multiple twisting the wire. It is then cut, and from these new ends about 5 mm of isolation is removed, after which they are tinned, twisted around each other and finally, soldered (the easiest way to remove the isolation is by burning it with lighter, then carefully scrapping it with the pocket knife or similar). Two small pieces of wood are then glued onto coil's ends. When the coil is being mounted into the box, they are pasted onto its top panel, as shown in the rightmost part of Pic.3.6-b.
If you are using a coil of different diameter, you should keep in mind that the necessary inductance for the coil which measures more than 3.2 cm in diameter will be obtained with number of reels less than 144 and vice versa, if the coil body is less than 3.2 cm you will need more than 144 reels.
d. Variable capacitor C is hard to find in stores, therby we have been using in all our receivers the one that we took from a disused commercial pocket size MW radio receiver, the one shown on Pic.3.7. On Pic.3.7-a you can see it together with the reel with numbers that represent the frequencies, divided by 100, on which that receiver was able to be tuned at. On Pic.3.7-b you can see the front, side and rear views of this capacitor. Electrical diagram is given on Pic.3.7-c. As one may notice, there are actually two variable capacitors under the same cover, Co and Ca, and two trimmer capacitors connected parallel to them, Cto and Cta. The dashed line shows symbolically that the rotating plates of the variable capacitors are connected on a common shaft, so that by turning the reel their capacitances are being changed simultaneously. For our use, all four capacitors are parallel connected, by joining the legs O and A. The trimmers are set to minimal capacitance. In such way the variable capacitor is attained with capacitance ranging from Cmin=12 pF til Cmax=218 pF.
In commercial radios that can receive both stations from AM and FM ranges, variable capacitor shown on Pic.3.8 is being used. Four variable and four trimmer capacitors are placed under the same cover. If you wish to use capacitor like this in the receiver from Pic.3.1 (and in most of the receivers described in this book), you should then connect in parallel Cto, Co, Ca and Cta, after which you shall obtain a variable capacitor ranging from Cmin=16 pF til Cmax=286 pF. Other capacitors from this block are not being used.
In all input circuits (more about them soon to come), one end of the variable capacitor is always connected to the device ground. For capacitors shown on Pics.3.7 and 3.8 that is the middle leg, marked as G.
During the dismounting of the capacitor from the old radio, you should pay attention not to loose the screw for the reel attachment, and two screws for mounting the capacitor onto the PCB, since they are very hard to provide separately.
If the reciever is being put into the box whose front pannel is made of isolating material not thicker than 1 mm, then one 10 mm hole should be drilled on it, followed by two 3 mm holes, as shown on Pic.3.9-a. Having thicker front panel does represent a problem, the shaft of the capacitor being too short to mounting the reel. In that case you will have to make the auxiliary plate about 1 mm thick as shown on Pic.3.9-1 and then mount the capacitor on it , acc. to Pic.3.9-b. On the front panel a round eye should be made, its diameter being a little bit bigger than the reel. The auxiliary plate with the capacitor should then be tightened onto this front panel with two small screws Z1 and Z2, and the reel on the capacitor shaft with the screw Z3 (While tightening this screw you should hold the reel with your other hand, and not the capacitor housing). Finally, a button made of a thick plywood can be glued to the reel. This is not necessary, but gives the device a more sophisticated looks.
Different variable capacitors than the ones described here can also be used, for example, an air variable capacitor described in the first issue of Practical Electronics. The important thing for it is to have a big max/min capacitance ratio, at least 15, i.e. Cmax/Cmin>15. While connecting the capacitor, care should be taken to connect its rotor with the ground (as on Pic.3.1), labeled Z, and its stator with the point 1 of the coil.
e. The diode D, capacitor C2 and headphones' resistance comprise the AM signal detector, also called the serial diode detector. When the AM signal of the station the receiver is tuned at is brought on its input, NF signal is obtained on the output, its shape being the same as the envelope of the AM signal. An example of this is given on Pic.3.10. When voltage uul is present on the input of the detector, the voltage uizl appears on its output. It is useful to notify that on the output, besides the LF voltage (speak, music etc.), DC voltage Uo is also present.
The detection diode D must be of low-power GERMANIUM type, such as AA112, AA116, AA121, 1N21, 1N34, 1N54, 1N78 etc.
Product of the capacitance C and resistance R (on Pic.3.1. R is the headphones resistance) should be approx. equal to 50 ìs (microseconds). That means that if you're using the bigger resistor (which is advisable, since the detector then damps less the oscillatory circuit), the capacitor should then be smaller. For example, if R=500 kÙ then C=100 pF, and if R=10 kÙ, C=5 nF, etc.
f. The headphones are the electro acoustic convertor that transforms electrical signal into the sound. We have been using old fashioned electromagnetic headphones with 1.5 kÙ resistance that were serially connected, giving the total resistance of 3 kÙ. The receiver from Pic.3.1 will be working as better as headphones' resistance is bigger. if you're using the crystal headphones, parallel to them you should add a resistor of couple of hundreds of kiloOhms. There's a very simple way of testing the high resistance headphones: Hold one end of their cable between your fingers while rubbing the other over the surface of a big metal object, say, the radiator. If snapping can be heard from them they are, most likely, satisfactory.
All the components of the receiver from Pic.3.1 should be placed into some kind of a box. That can be any box made of an isolation material (plastics, wood etc.), big enough to receive all the components. As an example, a receiver is shown in scale 1:1 on Pic.3.11, being placed in a box made of plywood. The top, bottom and side panes are made of plywood that is 5 10 mm thick. The front and the rear side are being made of some thinner material, that allows for simple mounting of the variable capacitor. One can notice straightaway that the box is at least twice as big as it could be. That has been done for the sake of better visibility, and for the box to be big enough to accept the devices that will be described later in this book.
Component joining is being done by soldering. Performing soldering onto the hubs can represent a small problem. In order to complete this operation successfully, you should turn the box for the part of hub where cables are being soldered pointing upwards. Put a piece of the tinol wire through the eye on the hub, put the soldering iron top onto the eye from upwards and hold it like that a while, for the tinol to melt. Then add some more tinol, until the eye is completely covered with the solder. After that, push the wire end into the melted solder while holding it with pencers, until it hardens and cools itself a little. The pencers remove the heat and prevent its transfer onto the component that is being soldered.
