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In order to get to know the field-effect transistor better and to know its features, we propose to assemble several designs in which it not only "solo", but also "acts" as part of a "duet" and "trio" with bipolar transistors.

DC voltmeter attachment

To measure the DC voltages in various circuits of amateur radio designs, you usually use an avometer operating in voltmeter mode. And of course, you know that this device consumes current, having a relatively low input resistance, which means that it is a load for a controlled circuit. That is why the measurement results may differ from the true voltage values. How to be?

First of all, it must be remembered that a pointer avometer often has a relatively low input resistance, for example, Ts-20 - about 6 kOhm / V, Ts20-05 - 20 kOhm / V, and it can only be used to control voltage in relatively low-resistance circuits, through which a significant current flows compared to the measuring circuit.

To control high-resistance circuits, it is necessary to increase the relative input resistance of the avometer to at least hundreds of kilo-ohms per volt. The proposed prefix will help here (Fig. 1). It uses a field-effect transistor with an n-type channel KP303D, which ultimately makes it possible to increase the input resistance of the voltmeter to 10 MΩ at all measurement limits.

The transistor is connected according to the common drain circuit (source follower). In order for it to work in the linear section of the characteristic, the desired bias voltage at the gate is created by the resistor R7 included in the source circuit. The indicator RA1 is connected to the source - the Ts-20 avometer, operating in the DC current measurement mode at the limit of 0,3 mA. To compensate for the initial voltage across the resistor R7, the second output of the indicator is connected to a variable resistor R9, which allows you to set the indicator needle to zero division of the scale before starting measurements.

At the input of the set-top box, a voltage divider is included, made up of resistors R1-R5. The measured voltage is applied to sockets X1 and X2 in the polarity indicated on the diagram. Depending on the expected maximum value of the measured voltage, switch SA1 is set to one position or another. In this case, the voltage on the movable contact of the SA1.1 section of the switch should not exceed 1 V - this is the voltage corresponding to the deviation of the indicator arrow to the final division of the scale.

To protect the transistor from possible overloads when an excessive voltage is accidentally applied, a limiting resistor R6 is included in the gate circuit. And in order to exclude the influence of various AC voltage pickups on the high-resistance input circuits of the set-top box, a capacitor C1 is connected between the gate and the common wire.

The set-top box is powered by a 3336 battery or three 343 or 373 cells connected in series. The current consumption does not exceed 7 mA. The power switch is the SA1.2 section of the measurement sub-range switch.

Fixed resistors can be MLT with a power of at least 0,25 W. Each of the resistors R1-R5 of the divider is desirable to be composed of two resistors connected in series, the resistance of one of them is equal to 80 ... 85% of the resistance of the additional resistor. Resistor R1, for example, can be made up of resistors with a resistance of 2,7 MΩ and 620 kΩ. This will allow in the future to more accurately select the appropriate resistances of the resistors of the input voltage divider. Setting up the attachment will be much easier.

The variable resistor R9 can be SP-1 or another. Switch SA1 - biscuit for five positions and two directions (type 5P2N), capacitor - any type. Field-effect transistor of the KP303 series or another, with the type of channel indicated on the diagram, the initial drain current (at a voltage of 4,5 V) of at least 5 mA and the slope of the characteristic of at least 2 mA / V. These requirements are explained by the use of an indicator with a relatively coarse scale - 0,3 mA. If you use the measurement subrange of 0,1 mA (100 μA), which is available for Ts20-05, then you can use the KP103Zh - KP103L transistor by changing the polarity of connecting the power source and the RA1 indicator.

The selected parts of the attachment are placed in a suitable housing. It can also be a self-made case, made, for example, from thin sheet aluminum (Fig. 2).

Establishing the set-top box is reduced to the selection of the resistor R7. An avometer operating at a DC current measurement limit of 3 mA is connected to terminals X4 and X0,3, and the set-top box switch is set to the "1,5 V" position. With a variable resistor R9, the arrow of the avometer indicator is brought to the zero division of the scale. Then a 1,5 V direct current source is connected to the sockets of the set-top box.

If the indicator needle deviates beyond the final division of the scale, the resistor R7 should be slightly less resistance. It is necessary to choose such a resistor so that the indicator needle deviates exactly to the end mark of the scale. Each time you replace the resistor, you should temporarily disconnect the element from the input jacks and set the indicator needle to zero on the scale with resistor R9. The selection of a resistor can be considered complete if, when the element is connected, the indicator arrow is set exactly at the final division, and when disconnected, it returns to zero.

After that, you should check the indicator readings on other subranges. For the sub-range "6 V", four 1,5 V elements connected in series can be connected to the input of the set-top box. If you turn on another "Krona" in series with such a battery, you can check the readings of the device on the sub-range "15 V", etc.

The prefix may have other measurement subranges. In this case, you will have to recalculate the resistance of the voltage divider resistors. But their total resistance in any case should remain the same - about 10 MΩ.

The calculation of the resistance of the divider resistors is carried out according to the following formulas: R5=Rtotal Uin /Umeas; R4=Rtotal Uin /Umeas -R5; R3=Rtotal Uin/Umeas -(R4+R5); R2=Rtotal Uin / Umeas -(R3+R4+R5); R1=Rtotal -(R2+R3+R4+R5), where R1-R5 are the resistances of the divider resistors, MΩ; Rtot - the total resistance of the divider, equal to 10 MΩ; Uin - input voltage corresponding to the total deflection of the indicator pointer, 1 V; Umeas - the selected measurement subrange.

These formulas allow you to calculate the divider for any of its total resistance, which is the input resistance of the voltmeter, as well as for any resulting input voltage required to fully deflect the arrow of the indicator of this avometer.

AC voltmeter attachment

It is designed to increase the input resistance of the Ts20 avometer when measuring AC voltage. The prefix somewhat resembles the previous one according to the scheme (Fig. 3), but unlike it, there is no filter capacitor here and instead of a constant resistor, a tuning R7 is included in the source circuit of the transistor. From its engine, an alternating voltage is supplied through the capacitor C1 to the rectifier on the diodes VD1 and VD2, connected according to the voltage doubling circuit. The rectified voltage is supplied further through the clamps HZ, X4 to the indicator RA1 (avometer Ts20 in the mode of measuring direct current up to 0,3 mA).

Resistors R1-R5 of the input divider have the same ratings as in the previous prefix. The range of measured voltages is limited to 60 V, but if desired, it can be increased by introducing additional resistors.

The transistor must be with the same parameters as for the previous set-top box. Trimmer resistor - SP-1 or another. Capacitor C1 - K50-6, but you can use K50-3 or another for a rated voltage of at least 6 V. Diodes - series D2, D9 with any letter index. The power supply is a 3336 battery or 1,5V cells in series.

The prefix can be mounted in the same case that was taken for the previous one, but the R7 resistor can be installed inside the case.

When setting up the set-top box, the SA1 switch should be set to the "1,5 V" position and apply an alternating voltage of 1 V (effective value) to the input (sockets X2, X1,5). The trimming resistor slider is set to the position at which the avometer indicator needle deviates to the final division of the scale.

The reading of the measurement results is carried out on the scale of variable voltages of the avometer.

Receiver for tape recorder

Do you want your tape recorder to receive transmissions from, say, the Mayak radio station? It's easy to do. Indeed, in any tape recorder there are several inputs designed for various sources of sound information. The most sensitive microphone input. If you even connect a detector receiver to it, you can not only listen to, but also record interesting programs on magnetic tape.

A diagram of a simple radio set-top box for a tape recorder is shown in fig. 4. An oscillatory circuit, tuned to the frequency of the desired radio station, is formed by an inductor L1 and a variable capacitor C1. By changing the capacitance of the capacitor, the frequency of the circuit is changed. As soon as it coincides with the frequency of the radio station, the largest signal amplitude will appear on the circuit.

Further, the signal selected by the circuit is fed to the source follower, assembled on a field-effect transistor VT1. The use of such a cascade, which has a high input impedance, made it possible to connect a detector assembled on diodes VD1 and VD2 according to the multiplication circuit to the entire circuit, and thus do without an amplifying radio-frequency cascade.

At the load of the detector (resistor R3), an AF signal is emitted, which is fed through connector X2 to the input of the tape recorder.

The prefix is ​​designed to receive radio stations in a small area, say, the medium wave range. The sensitivity of the set-top box is small, so for its normal operation you will need an external antenna in the form of a meter-long piece of wire inserted into the X1 socket with a stripped end. True, a powerful local radio station will be received even without such a wire, since the L1 coil wound on a ferrite rod already becomes a magnetic antenna that captures the magnetic component of radio waves.

If the signal is weak even with an external antenna, the easiest way to increase its level at the connector sockets is to increase the supply voltage to 4,5 V. This slightly increases (up to 0,8 mA) the current consumed by the set-top box compared to the current (0,6 mA ) at the voltage indicated on the diagram.

When selecting parts, it is permissible to replace the KP103Zh transistor with any other from this series, and instead of the D9D diodes, use any of the D9 series or other high-frequency germanium diodes. Antenna socket and connector - any design; resistors - MLT-0,125; capacitor C2 - KP-180 or another small-sized variable capacitor with a capacitance change of 5 ... 7 pF or more; the rest of the capacitors - any small; power source - galvanic cell 316, switch - toggle switch.

The inductor is wound approximately in the middle of a rod with a diameter of 8 and a length of 70 ... 90 mm from 600NN ferrite. For the SV range, 170 turns will be needed, and for the DV range - 250 turns of PEV-1 0,15 wire, laid turn to turn. Of course, not the entire indicated range will overlap with the KP-180 capacitor, therefore, to set the attachment to the desired area, you will have to select the exact number of turns by unwinding or winding them. The matter is simple.

The details of the attachment, except for the antenna socket and connector, are placed on a board made of insulating material (Fig. 5), having previously mounted mounting studs made of thick bare copper wire on it - the conclusions of the parts are soldered to them.

The ferrite core and the galvanic cell are attached to the board with rubber rings. The board is placed inside the case (Fig. 6) - it is held on the front wall with a switch fastening nut. A socket and a connector are attached to the corresponding side walls.

The receiver-prefix does not require adjustment. Just to be sure that the transistor is working, it is advisable to measure the voltage drop across the resistor R2 - depending on the transistor used, it can be from 0,5 to 1 V.

By connecting the prefix to the microphone input of the tape recorder and connecting the antenna to it, by rotating the knob of the variable capacitor, tune the prefix to the radio station. The AF signal level is controlled by the recording level indicator of the tape recorder. If the signal is significant and you have to reduce the amplification of the tape recorder, it is advisable to use another input - for recording from a pickup or a radio broadcasting network. If the signal level is so strong that distortions appear, you should weaken the connection of the circuit with the antenna by replacing the capacitor C1 with a capacitor with a capacity of 10 ... ).

Electronic timer

The proposed electronic device is designed to count the time. This can be, for example, the duration of developing a film or fixing it, cooking a particular dish on a stove, a sports performance, etc. In all such cases, it is enough to set a predetermined countdown interval with the timer knob, for example, two minutes, and turn on the device. Once this time has elapsed, a beep will sound.

The instrument is relatively portable and contains few parts (Fig. 7). The set time reference device is assembled on a field-effect transistor VT1, and a sound signaling device - on a transistor VT2. The timer is controlled by the switch SA1.1. In the initial position, the switch handle must be in such a state that, as shown in the diagram, the group of contacts SA1.1 is closed and SA1.2 is open.

To turn on the device and the countdown, move the switch handle to another position, at which contacts SA1.1 open and SA1.2 close. Now the device will be supplied with a supply voltage and the countdown of the time set by the variable resistor R3 will begin. It depends on the capacitance of the capacitor C1 and the total resistance of the resistors R2 and R3. When the slider of the resistor R3 is in the lower position according to the diagram, the total resistance is minimal and equal to the resistance of the resistor R2. In the upper position of the slider, the total resistance is equal to the sum of the resistances of both resistors. In each case, the capacitor will slowly charge, and the voltage at the source of the field-effect transistor, operating in the source follower mode, will also slowly increase. As soon as this voltage reaches a certain value, the transistor VT2 opens (after all, its base is connected to the source through resistor R5) and the generator turns on. A sound will be heard in the BA1 head connected to the generator transformer T1.

With the minimum resistance of the resistor R3, the sound will appear 1 ... 1,5 minutes after the power is turned on, and at the maximum - after 10 ... 15 minutes. If you set the engine to other positions, the time of the appearance of the sound signal will change accordingly. The tone of the signal depends on the capacitance of the capacitor C2.

As soon as a signal appears, the switch handle is moved to its original position. In this case, closing contacts SA1.1 connect resistor R1 in parallel with capacitor C1 and the capacitor is discharged, and opening contacts SA1.1 turn off the power from the device.

The field-effect transistor can be used with a different letter index, but the KP303 series is mandatory (for example, KP303V, KP303E). Any transistor of the MP39-MP42 series works well in the generator, but it is advisable to select a transistor with a small current transfer coefficient (12 ... 20). Oxide capacitor C1 can be K50-6, K5012, K53-1 for a voltage of at least 6 V, capacitor C2 - MBM. Variable resistor - SP-1, constant - MLT-0,125. The transformer is the output from any small-sized transistor receiver (the diagram shows the numbering of the outputs of the unified TV output transformer). The dynamic head is also any power of 0,1-0,5 W (for example, 0,25GD-19). The switch is a TV2-1 toggle switch, but another toggle switch, for example, a two-section TP1-2, is also suitable. The power source is a 3336 battery.

The parts of the device, except for the dynamic head and the battery, are mounted on a board made of insulating material (Fig. 8). Mounting studs are preliminarily attached to the board, after which a variable resistor and a switch are installed. Next, they mount the remaining parts and, last of all, solder the terminals of the transistors.

The board is attached to the front panel of the case in such a way (Fig. 9) that the variable resistor and the toggle switch are fixed with nuts outside the panel. Under the diffuser of the dynamic head, a hole is cut out in the front panel and covered with a decorative cloth, and the head is attached to the panel from below. The bottom cover of the case is removable, the battery is fixed on it with a metal clamp.

Without closing the covers, set the variable resistor slider to the minimum resistance position, turn on the device and connect the voltmeter probes with a 3-5 V scale to the drain and source terminals of the field-effect transistor (the positive probe of the voltmeter is to the drain). The voltmeter needle should initially mark a small voltage (about 0,3 V), but over time it will gradually increase. After about 1,5 ... 2 minutes, a voltage should be established that is approximately equal to half the voltage of the power source. At this point (and possibly earlier) sound will appear in the dynamic head. If there is no sound, you will have to slightly reduce the resistance of the resistor R5. But, as a rule, this is practically not necessary, since the resistor R5 is selected based on the use of the VT2 transistor with the lowest transfer coefficient (about 12). The timbre of the sound will be somewhat high, and if you want to lower it, increase the capacitance of capacitor C2. Turn off the device - the sound will disappear.

Turn on the device again and note on the stopwatch (or the second hand of the clock) after what time the beep will sound. Check the constancy of the time delay. To do this, turn on the device several times in a row and each time mark the exposure time using the control stopwatch. As a rule, it does not differ by more than 5 s.

After that, set the variable resistor slider to another extreme position (when its resistance is maximum) and determine the longest time delay using the control stopwatch. Check the constancy of shutter speeds in this case as well. Of course, the difference between exposures will be somewhat larger here, but in percentage terms it should remain the same as with the minimum exposure.

If you want to change the shutter speed range, change the capacitance of capacitor C1 or, with the same capacitor, change the resistance of resistors R2 and R3. So, to reduce the shutter speed range, you must either reduce the capacitance of the capacitor, or reduce the resistance of the resistor R3. The minimum shutter speed in both cases depends on the resistance of the resistor R2, the maximum - on the resistance of the resistor R3.

Having finished checking and adjusting the device, close the bottom cover and proceed to calibrate the scale of the variable resistor. By setting its engine to different positions, turn on the device and count the shutter speed using the control stopwatch, and then put its value on the scale. Remember that the constancy of shutter speeds largely depends on the voltage of the power supply. Therefore, it is necessary to periodically check the battery, and if its voltage drops to 3,5 V, replace the battery with a new one. Check the battery voltage only during its operation under load, when the exposure countdown ends and an audible signal sounds.

Garland Fault Finder

When a garland suddenly goes out on a New Year tree or an illuminated panel of a lighting effects machine, it becomes difficult to replace a burned-out lamp, since it is difficult to find it in a garland. You have to either change the lamps one by one, or close their outputs until the fault is identified. This takes a lot of time.

A few minutes, and sometimes even seconds, will be needed to identify a defect using the proposed finder with a light indicator. A small plastic case for a fountain pen, which houses two galvanic cells 316 and a board with radio components - this is how it looks (Fig. 10). As soon as you bring the end of the case to a faulty garland lamp, the finder's LED will immediately flash.

Take a look at the device diagram (Fig. 11). Field-effect transistor VT1 in it acts as a sensor that "captures" even a very weak electric field strength. In the place of a burned-out lamp, it will be the largest, since one of its terminals has a phase wire of the lighting network, and the other has a zero wire. Therefore, when a field-effect transistor of the finder is next to such a lamp, the resistance of its drain-source section will increase so much that transistors VT2, VT3 will open. The HL1 LED will flash.

The field effect transistor can be any of the KP103 series, and the LED can be any of the AL307 series. Bipolar transistors can be any other low-power silicon structures indicated on the diagram and with the highest possible current transfer coefficient. Resistors - MLT-0,125.

When mounting a field effect transistor, it is placed horizontally on the board, and the gate terminal is bent so that it is above the transistor case. If during the operation of the finder its excessive sensitivity is revealed, the output of the shutter is shortened.

Author: B.Ivanov

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