This article introduces a pocket transmitter, which is very suitable for beginners. The circuit is simple and easy to manufacture, the cost is low, the output power is no more than 5-8mW, and the transmission range can be up to 100 meters in the housing area. Use an ordinary FM radio. Receive, showing its sensitivity and clarity, the most challenging part of circuit design is the ability to transmit with only a 3V power supply and a half-wave antenna. In addition, because the circuit requires very few parts, it can be placed in a matchbox (larger than the domestic matchbox). As a bug, it is ignorant and unintelligible, but it is not limited to this. In terms of use, it can be placed in the baby room, gate or corridor to monitor the actual situation, and can also be used as a night security device.
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The current consumption of the circuit is less than 5mA, and it can work continuously for 80 to 100 hours with two dry batteries.
The circuit is very stable under normal operation, and the frequency drift is very small. Test: After 8 hours of operation, it is not necessary to re-calibrate the receiver. The only effect on the output frequency is the condition of the battery. When the battery ages, the frequency changes slightly.
With this production, learn about FM transmission, you can understand its superiority, especially it produces a very high quality signal without noise, even with low power transmission, it is easy to get a good range.
Circuit working principle
From the circuit in the figure, there are two levels, a primary audio amplifier and a primary RF oscillator.
There is actually a FET hidden in the electret microphone. If you like it, it can be regarded as one level. The FET amplifies the capacitance change of the front diaphragm of the microphone. This is the reason why the resident microphone is very sensitive.
The audio amplifier stage is operated by its emitter transistor Q1 with a gain of about 20 to 50, and the amplified signal is sent to the base of the oscillation stage.
The oscillating stage Q2 operates at a frequency of about 88 MHz, which is regulated by an oscillating coil (5 turns total) and a 47pF capacitor. This frequency is also determined by the transistor, the 18pF return capacitor, and a few bias components, such as 470Ω emitter resistors. And 22K base resistance.
When the power is turned on, the 1nF base capacitor is gradually charged by the 22K resistor, and the 18pF is charged by the 470Ω resistor of the oscillating coil, but even faster, the 47pF capacitor is also charged (the voltage is only small at both ends), and the coil generates a magnetic field. .
When the base voltage gradually rises, the transistor turns on and effectively connects the internal resistance to both sides of 18pF. When the 1nF capacitor is charged to the operating voltage of this pole, several chaotic cycles occur, so we assume that the discussion is close to the operating voltage.
The base voltage continues to rise, and the 18nF capacitor attempts to prevent the emitter voltage from moving. When the energy in the capacitor is exhausted and the emitter level is not prevented from moving, the base-emitter voltage is lowered, the transistor is turned off, and the current flowing through the coil is also stopped. The magnetic field is broken.
The magnetic field decays, producing a voltage in the opposite direction. The collector voltage in turn rises from the original 2.9V to over 3V and is charged in the opposite direction of 47pF. This voltage also affects the charging of the 18pF capacitor and the 470Ω emitter resistance. The voltage drop causes the crystal tube to enter a deeper cutoff.
When the 18pF capacitor is charged, the firing voltage drops and falls to a certain transistor to start conducting, and the current flows into the coil, which is against the decaying magnetic field.
The voltage on the coil is reversed, forming a collector voltage drop. This change is transmitted to the emitter through the 18pF capacitor. As a result, the crystal tube enters a deeper conduction, shorts the 18pF capacitor, and the cycle begins to repeat.
Therefore, Q2 forms an oscillation here, generating an AC signal of 88 MHz. The amplified audio signal is broken through the 0.1uF capacitor! The base of Q2 changes the oscillation frequency to produce the desired FM signal.
Production process
Before loading, it is best to put the pre-prepared printing plate and two batteries in the empty pear box to see how much space is available.
Although the vacancies are limited, it is still necessary to leave a small position for a single row of matches. These matches can be glued to the cardboard to cover the circuit, making people think that it is just a box of matches, not aware of it. A bug.
Now put all the parts on the work table, sort out the values ​​one by one, and then sort the sorts in order. This is very organized to avoid soldering the wrong parts. For the tin wire, it is best to use a special 0.6lmm resin (rosin) tin wire. Because of its thin body, it is welded quickly and easy to tin.
It is enough to use a small soldering iron of 15 to 2Ow. Wipe the soldering iron tip with a sponge before use. The only thing that needs to be made is the coil. You need to use a 22# BS (Ф0.5mm) or 24#BS (Фm.71mm) enameled bag. Copper wire or tinned copper wire.
Wrap 5 turns on a 3mm diameter bobbin, such as a medium-sized screwdriver, and then separate the circle from the ring by about 5.5mm.
When the frequency is finally adjusted, the output frequency is changed by compressing or elongating the coil before and after. If your coil is made of enameled wire, peel off the patent leather on both ends of the wire and then apply a little tin.
The bottom plate can now be soldered in accordance with the part placement shown in Figure (3), starting with the resistor, followed by the capacitor, transistor, coil, and stencil. The resistor stands upright on the bottom plate but remains at a minimum. The pins of the crystal tube should be inserted into the bottom plate so that the height of the tube is not protruding.
The two batteries are welded together by a switch, and the electric poles are connected to the bottom plate by wires. Finally, a 10cm long copper wire is connected to the "A" point of the bottom plate. As an antenna, the entire manufacturing process is completed.
why?
Are you wondering why the circuit doesn't work? How many times after installation, I found that the circuit is not working properly?
Please don't blame yourself, or yell at the magazine that teaches you, many times because of the so-called "error."
All parts manufactured by the manufacturer have their values, but this value falls within the "difference" rather than the "normal" value printed on it. This difference is called the error, if the error is 5%. This means that the actual value of the part will be anywhere between 5% of its value and 5% of the above.
Errors are often used in resistors, capacitors, transistors, and other components such as microphones, coils, and integrated circuits.
However, there is another factor, called the limit, where each component is in the circuit, and there is a range of allowable values ​​for the case, as long as the value is still within the range or within these limits, the circuit Proper work, when selecting each component, is generally in the middle of this range.
Most circuits are not strictly limited. For example, selecting another higher or lower value from a specified component generally works well. If it is not, the circuit is not very strict or the selected value is not appropriate.
When you publish a line through the magazine, there will be trials by people of all walks of life to get the parts you need from all sources. Sometimes they use the specified value, sometimes they choose the next value. Also, some parts have 1-5% error, while others are up to 60% of the value. When these parameter differences and limits are mixed in any way, it is very common for you to encounter a circuit that does not work.
Take the microphone as an example. Under the 3V power supply, some words only need to use 100K load resistor (R1) to have very good sensitivity. Others may need 4.7K to obtain only acceptable sensitivity. You can't get from the appearance. Say the difference between the two, they look the same, but the electrical characteristics are far from each other.
It can also be applied to the transistor. The specification sheet may indicate that the characteristics of the two tubes are nearly the same. However, when they are connected to the circuit, one work is called and the other is out of order.
Please don't worry about failing to see the above paragraph. Just consider carefully the circuit to component requirements and do it step by step. It is completely successful.
After all the parts of the circuit are soldered, it is best to first check all the solder joints with the naked eye, whether there is any false soldering, or if the solder is used too much and cause short circuit with the proximity, and thoroughly check the calibration and test performance. The test procedure is to add a short antenna (5 to 10 cm long) to the D-point FM radio on the bottom of the backplane for the entire band to look for the signal.
It is best to keep the transmitter at a distance from the radio to prevent any harmonics or side waves from being picked up.
If the radio fails to detect the carrier, the frequency may be too low, the oscillation coil is slightly elongated, and try again. If a coiled copper wire is used to wind the coil, note that the figure and the circle should not touch each other. If a patented copper wire is used, it is necessary to know the continuity of the ring. It can be measured by the low blocking of the multimeter, or the current of the circuit should be about 2.5-3.5 mA.
Once the carrier is detected, place the bug on the side of a clock, check the sensitivity of the circuit, the radio should emit a clear and powerful "drip" sound, and the circuit should be more sensitive than your ear.
The microphone's load resistance (R1) determines the sensitivity, which can be reduced to 10K or to 47K, depending on the sensitivity required.
Make sure that the frequency of the transmission is completely far away from any FM radio station in your area, because the signal from the radio is powerful, and when you test the distance, it will cover the bug.
When the coil is compressed, the frequency is reduced; when it is stretched, the frequency is increased, which eliminates the need to trim the capacitor and saves the cost of the machine. However, if you like, you can also use the trimmer capacitor.
By the way, C4 is best to use a 39pF ceramic capacitor and put another 10pF or 22pF trimming capacitor together, so that the circuit can be adjusted more carefully. It is easy to deviate from the FM band with coil adjustment.
In theory, the inductor should also be adjusted to maintain the L/C ratio of the tuned circuit, but the range we need is small and there is no limit.
Using an FM receiver with an adjustment indicator, you can determine how much output power the unit has. It needs to be compared. The indicator indicates four units of power, indicating a very good output. Use 10cm when testing the unit. The long antenna is placed horizontally, up to 10 meters from the tuner. Based on the four unit degrees, it is known to use a half-wave antenna. (170cm long), the unit can launch as far as about 300 meters.
The carrier from the eavesdropper cannot be received on the FM receiver. It should first be assumed that the frequency is lower than the normal 88-108 MHz FM band, which is the most likely cause.
What if I don't work?
Measuring the current of the circuit, if there is 2.5-3.5mA, indicating that the circuit is working, slightly elongating the coil, and scanning the entire band, when contacting any component on the bottom plate, only use a non-metallic screwdriver and leave The battery, because of the capacitive effect caused by your skin, can cause the circuit to be significantly out of tune and may stop output completely. Also, it is important to maintain a 3V power supply and keep the battery close to the backplane.
The entire wiring must maintain the same circuit distribution capacitance as in (2). Once the circuit is working, its arrangement can be changed, but in the initial test step, each component must be placed as shown in the figure.
The oscillator operates at approximately 88MHz, unless you have a 100MHz oscilloscope, it is difficult to see its waveform, or the antenna is directly connected to the 75Ω input of the frequency meter.
If there is no such test instrument, use a multimeter for DC voltage measurement and see if the oscillation tube Q2 has the correct value.
Measuring the base voltage and the emitter voltage, an ordinary multimeter will indicate that both points are about 2V because of its action on the circuit. Only a high-impedance meter, such as a FET voltmeter, indicates that the emitter has 2V and the base. There is 2.5V. (Recommended to use the digital meter)
If there is voltage present at both test points, it is assumed that the transistor is working normally, but it is possible to transmit the wrong frequency.
The 18pF return capacitor is matched to the BC547 transistor. If you plan to use another number, you can reduce the capacitor value to 10pF or 5.6pF. Change this capacitor first, then the transistor.
Other simple things, such as short-circuiting of copper foil on the bottom plate, poor solder joints, or the use of unnumbered parts, etc., are often a possibility, especially if the numbers or values ​​printed on those parts are ambiguous, If you have doubts about it, you should replace it.
If only the carrier is received but there is no pure tone nickname, the fault is at the audio level or on the microphone. There is no pure tone in the carrier. The radio is tuned to one place. The silence is received, there is no rustling, but the pure tone signal from the transmitter is not heard.
These two parts can be checked with an oscilloscope to test whether there is a knee signal sent to the oscillation level.
Without an oscilloscope, there are certain difficulties in testing. Even if there is a voltage between 0.7V and 1.5V on the microphone, this does not mean the sensitivity of the microphone or it works completely.
The audio amplifier has a voltage of 1.4V on the collector, indicating that the transistor is conducting. If it is lower than 0.8V, the transistor is saturated, or it may be damaged in some way, it may also indicate that the transistor has a very high gain, which is not suitable.
If the voltage exceeds 2.5V, this stage is not sufficient to conduct a check transistor and a bias resistor, and replace it if necessary. The oscilloscope also displays the sensitivity of the microphone, increases or decreases the load resistance, and can change the gain of the FET. For extremely sensitive parts, the load resistance should not be lower than 10K, and sometimes it may need to be as high as 47K or more.
For any type of microphone, if you want to increase its sensitivity, you can increase the resistance of the load resistor. As for the final value, it depends on the quality of the microphone.
All of the above are checks that can be done with a simple test instrument. If you still can't find the fault, you will need to come back again.
Put the camouflage box bottom plate and the battery into the match box. If the bottom plate can be placed sideways, the space occupied is the smallest. Cover the circuit with a row of matches, which can be glued to a thin cardboard that is pulled out from one end of the matchbox.
Open a small hole at the other end to let the sound into the microphone, but it is not necessary to do so, because even if you close the box, the sound seems to penetrate. As long as a short antenna, about 10cm, can have a launch range of 30 meters, enough room communication, even larger houses can cope. The unlimited electronic website has been sampled and the frequency is very stable. This is related to the cutoff frequency of the oscillating launch tube. The higher the fT, the better. It is better to use a tube such as SC3356, but there is another problem in making a bug. For a longer launch time, you must have a battery with enough energy. The battery is large and the bug is too small. This bug is in the frequency range, and the length of the antenna affects the launch distance.
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