There are times when the right time at hand may not be necessary for you radio. This can happen, for example, during repair of the equipment or any experiments with the circuit. This is especially true if you do not live in the city and to the nearest shop of radio components - many kilometers. In such cases, you may need information presented in this article.
There are various options for use as one of components (electronic components) other at a slightly different connecting them when they do not operate in a standard mode for itself.
For example, a known circuit of transistors and diodes as Zener or stabistorov (Diode in the "forward" is turned on can be operated as a small zener voltage value, that is - Zener diode). Alternatively, the transistors and diodes can be used instead of thermistors schemes. In this case, the effect of the temperature dependence of p-n junctions.
In this article, we consider the case of application of the transistor instead varicap - element capable of changing its capacity depending on the voltage applied to it.
In this case, the transistor is turned on as shown in Fig.1. As the transistor used in the experiments common and widespread KT315, but of course you can try to use any other low-power transistors.
To test the efficiency of such a "varicap" In practice, this option was switching transistor tested in a real circuit (Figure 2), as the oscillator frequency tuning element metal detector. In the diagram, a diagram of the generator on the elements of a digital logic "NAND". It is usual, classical tunable oscillator circuit.
The output frequency is given to it oscillatory circuit at the input, consisting of a search coil and a variable capacity condenser in parallel. By varying the capacitance of the capacitor, it is possible to change the tuning frequency.
Leaving this scheme without any changes, add to it the items highlighted by the dashed line.
If you use a varicap (highlighted in blue), then the rotation of the engine variable resistor voltage on it bude change that will lead to a change in its capacitance. The oscillator frequency will vary in a range sufficient to tune the detector.
By the way, in such schemes as the varicap works well Zener type KS156.
Then, instead of the varicap, we include KT315 transistor (shown in red). We get the same effect - when changing the voltage on it, the tuning frequency will also vary within sufficient.
In some cases, for a correct operation of the circuit, it may require the selection of elements of "binding" - a resistor R (100 ohms) and capacitor C (200 pF). The capacitor C (0.01 uF) between the slider of the variable resistor and the common wire is needed to filter the voltage control varicap (or in its role as a transistor). The overall operation of the circuit it has no effect and may have a capacity ranging from 0.01 to 0.1 uF.
Power to the varicap must be supplied from a stable voltage source in the range of 5 to 12 volts, as indicated in the diagram. This voltage can be taken from the power supply digital logic chips themselves if it lies within those limits and stably. The most suitable nominal variable resistor can pick up experimentally. It may be in the range of tens to hundreds of ohms.
In this way can be replaced with the setting of "mechanical" (rotation of the rotor of the variable capacitor) to some sort of "e". Variable capacitor capacitance in this case can be replaced by a small trimming, for preliminary "rough" adjustment to the desired frequency range.