A pocket transmitter introduced to you here is very suitable for beginners. The circuit is simple and easy to manufacture, the cost is low, the output power does not exceed 5-8mW, the transmission range can be up to about 300 meters in the housing area, and an ordinary FM radio is used. Reception shows that its sensitivity and clarity are both excellent. The most challenging part of the circuit design is that it only needs a 3V power supply and a half-wave antenna to have such a transmission capability. In addition, because the circuit requires very few parts, it can be placed in a matchbox (larger than the domestic matchbox), which can be said to be an eavesdropper, but it is not limited to this aspect. For use, it can be placed in a baby room, gate or corridor passage to monitor the actual situation, and can also be used as a night security device.
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 extremely small. Test: After working for 8 hours, there is no need to recalibrate the receiver. The only effect on the output frequency is the condition of the battery. As the battery ages, the frequency changes slightly.
With this production, learning about FM transmission, you can understand its superiority, especially it produces extremely high-quality signals without noise, even if it is transmitted with low power, it is easy to obtain a good range.
Circuit working principle
It can be seen from the circuit in Figure (1) that there are two stages, one stage audio amplifier and one stage RF oscillator.
There is actually an FET hidden in the electret microphone. If you like it, it can be regarded as a first level. The FET amplifies the capacitance change of the front diaphragm of the microphone. This is why the electret microphone is very sensitive.
The audio amplifier stage is served by its emitter transistor Q1, with a gain of about 20 to 50, and sends the amplified signal to the base of the oscillation stage. 
The oscillation stage Q2 works at a frequency of about 88MHz, which is adjusted by the oscillation coil (a total of 5 turns) and a 47pF capacitor. The frequency is also determined by the transistor, 18pF return capacitor, and a few bias components, such as 470Ω emitter resistance And 22K base resistance.
When the power is turned on, the 1nF base capacitor is gradually charged through the 22K resistor, and 18pF is charged through the 470Ω resistance of the oscillating coil, but faster, the 47pF capacitor is also charged (although only a small voltage at both ends), the coil generates a magnetic field .
When the base voltage gradually rises, the transistor turns on and effectively connects the internal resistance on both sides of 18pF. When the 1nF capacitor is charged to the operating voltage of this pole, several messy cycles will occur, so we assume that the discussion is close to the operating voltage. 
The base voltage continues to rise. The 18nF capacitor attempts to prevent the movement of the emitter pressure. When the energy in the capacitor is exhausted and the movement of the emitter is no longer prevented, the base-emitter voltage decreases, the transistor is turned off, and the current flowing through the coil also stops. , The magnetic field collapses.
The magnetic field decays, producing a voltage in the opposite direction, and the collector voltage rises from the original 2.9V to exceed. 3V, and the 47pF capacitor is charged in the opposite direction. This voltage also affects the charging of the 18pF capacitor, and the voltage drop on the 470Ω emitter resistor causes the transistor to enter a deeper cut-off.
When the 18pF capacitor is charged, the injection voltage drops and falls to a certain crystal tube to start conduction, and the current flows into the coil to fight against the decaying magnetic field.
The voltage on the coil is reversed to form 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, shorting the 18pF capacitor, and the cycle begins to repeat.
Therefore, Q2 forms an oscillation here, generating an 88MHz AC signal. The amplified audio signal is broken into by 0.1uF capacitor! The base of Q2 changes the oscillation frequency to generate the required FM signal.
Production process
Before assembling, it is best to put the pre-prepared printed board and two batteries in the empty pear box to see how much space is available.
Although the space is limited, you still need to leave a small position for a single row of matches. These matches can be glued on the cardboard with glue to cover the circuit, making people think that it is just a box of matches and will not perceive it A bug.
Now put all the parts on the work table, separate the value of each part, and then sort them in order, this is very organized, to avoid welding the wrong parts. For tin wire, it is better to use extra-fine 0.6lmm resin (rosin) tin wire, because of its thin body, it is welded quickly and easy to tin.
It is enough to use 15 to 2Ow small electric soldering iron. Before use, wipe the tip of the soldering iron clean with a sponge. The only thing that needs to be made is the coil. Copper wire or tinned copper wire.
Wrap around 5 coils on a 3mm diameter coil holder, such as a medium screwdriver, and then separate the coils by about 5.5mm.
When the frequency is finally adjusted, the output frequency is changed by compressing or stretching the coil back and forth. If your coil is made of enameled wire, you must peel off the paint from both ends of the wire and then apply a little tin.
The base plate can now be welded in accordance with the placement of the parts indicated in Figure (3), starting with the resistor, followed by the capacitor, transistor, coil, and words. The resistor stands upright on the base plate, but keeps the height to a minimum. The pins of the crystal tube should be inserted into the bottom plate as far as possible, so that the height of the tube does not protrude.
The two batteries are welded together with a switch, and then the two poles of the electric ground are connected to the bottom plate with a wire. Finally, connect a 10cm long copper wire to the "A" point of the bottom plate as an antenna. Even if the entire manufacturing process is completed,
why?
Are you wondering why the circuit is not working? How many times have you found that the circuit is not working properly after installation?
Please do not blame yourself, or swear at the magazine that taught you, often because of the so-called "error".
All parts manufactured by the manufacturer have their values, but this value only falls within the "difference", not the "normal" value printed on it. This difference is called error, if the error is said to be 5%. This means that the actual value of the part will be anywhere between 5% below its marked value and 5% above.
Errors are often applied to resistors, capacitors, transistors, and other components such as microphones, coils, and integrated circuits.
However, there is another factor, called the limit. Each element in the circuit has an allowable value range for the occasion. As long as the value is still within the range, or within these limits, the circuit will Appropriate work, when selecting each component, generally in the middle of this range.
Most circuits are not strictly limited. For example, if you choose another higher or lower value from the specified components, they generally work well. If not, the circuit is not strictly limited or the selected value is very inappropriate.
When you publish a line through a magazine, people from all walks of life will try to produce parts from various sources. Sometimes they use the specified value, sometimes they choose the next value. Also, some parts have a 1-5% error, while others are as high as 60% of the marked value. When these parameter differences and limits are mixed in any way, it is very common for you to encounter that the circuit does not work.
Take the microphone as an example, under 3V power supply, some words only need to use 100K load resistance (R1) to have very good sensitivity, others may need 4.7K to obtain only acceptable sensitivity, from the appearance you cannot Speaking of the difference between the two, they seem to be the same, but they are far from each other in electrical characteristics.
It can also be applied to transistors. The specifications may indicate that the characteristics of the two tubes are nearly the same. However, when they are connected to: a circuit, one job is satisfactory, and the other fails.
Please don't worry about failure due to seeing the above paragraph, as long as you carefully consider the circuit requirements for components and do it step by step, it can be completely successful.
Circuit tuning
After all parts have been soldered, it is best to visually inspect all solder joints for false soldering, or too much solder used to cause short-circuiting to the neighboring circuit. Only after thorough investigation can calibration and test performance be performed. The test steps are Add a short antenna (5 to 10 cm long) at point A on the bottom plate to tune-the FM radio is on the entire band to find the signal.
It is best to keep the transmitter and radio at a certain distance to prevent any harmonics or side waves from being detected.
If the radio fails to detect the carrier, it means that the frequency may be too low, stretch the oscillation coil slightly, and try again. If you use tin-clad copper wire to wind the coil, please note that the picture and the circle should not touch each other. If you use patented copper wire, you need to know the connectivity of the ring. You can measure it with the low barrier of the multimeter, or measure the circuit current, which should be about 4-6mA.
Once the carrier is detected, place the bug near the clock to check the sensitivity of the circuit. The radio should emit a clear and powerful "tick" sound, and the circuit should be more sensitive than your ears.
The load resistance (R1) of the microphone determines the sensitivity, which can be reduced to 10K or added to 47K, depending on the required sensitivity.
Make sure that the transmission frequency is far away from any local FM radio station, because the signal sent by the radio station is strong. When you test the distance, it will cover the bug.
Compress the coil to reduce the frequency; increase the frequency to increase the frequency. This eliminates the need for trimming capacitors and saves the cost of the machine. However, you can use trimming capacitors if you like.
By the way, it is best to use a 39pF ceramic capacitor for C4, and add another 10pF or 22pF fine-tuning capacitor together, so that the circuit can be adjusted more carefully. The coil adjustment can easily deviate from the FM band.
In theory, the sensor should also be adjusted to maintain the L / C ratio of the tuning circuit, but the range we need is very small, so there is no limit.
An FM receiver with an adjustment indicator can be used to determine the output power of the unit. It is necessary to make a comparison. The indicator indicates four unit degrees, indicating a very good output. When testing the unit, use 10cm The long antenna is placed horizontally, 10 meters away from the tuner. Based on four unit degrees, that is, a half-wave antenna is known. (170cm long), this machine can launch as far as about 300 meters.
What if I do n’t work?
The carrier wave from the bug cannot be received on the FM receiver. First, it should be assumed that the frequency is lower than the normal 88-108MHz FM band. This is the most likely cause.
Measure the current of the circuit. If there is 4-6mA, it means that the circuit is working. Slightly stretch the coil and scan the entire wave band. When touching any component on the bottom plate, only use a non-metal screwdriver and leave the battery , Because the capacitive effect caused by the skin on your hand will cause the circuit to be significantly out of adjustment and may stop the 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 distributed capacitance of the circuit as shown in (3). Once the circuit works, the arrangement can be changed, but in the initial test step, each component must be placed as shown in the figure.
The oscillator works at about 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 above-mentioned test instrument, you need to use a multimeter for DC voltage measurement to see if the oscillation tube Q2 has the correct value.
Measure the base voltage and emitter voltage. An ordinary multimeter will indicate that these two points are about 2V due to its effect on the circuit. Only high-impedance meters, such as FET voltmeters, indicate that the emitter has 2V and the base. There is 2.5V. (Recommended to use digital table)
If voltage is present at both test points, it is assumed that the transistor is working normally, but it is possible to emit the wrong frequency.
The 18pF feedback capacitor is used in conjunction with 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 copper foil short-circuit breakage on the bottom plate, poor solder joints, or the use of unnumbered parts, etc. This is often a possibility, especially if the numbers or values ​​printed on those parts are ambiguous. If in doubt, it should be replaced immediately.
If only the carrier wave is received but there is no pure tone flood number, the fault is at the audio level or microphone. The so-called carrier without pure tone is to tune the radio to one place, the silence is received, there is no rustling, but the pure tone signal from the transmitter cannot be heard.
These two parts can be checked with an oscilloscope to test whether a tone signal is sent to the oscillation stage.
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 that the microphone sensitivity or full operation.
There is a 1.4V voltage on the collector of the audio amplifier, which means that the transistor is on. If it is lower than 0.8V, the transistor is saturated, or it may be damaged in some way, or it may mean that the transistor has a very high gain, which is not suitable.
If the voltage exceeds 2.5V, this level is not enough to conduct the inspection transistor and bias resistor, and replace it if necessary. The oscilloscope also shows the sensitivity of the microphone. Increasing or decreasing the load resistance can change the gain of the FET. For parts with extremely high sensitivity, the load resistance should not be lower than 10K. Sometimes it may need to be as high as 47K or more.
If you want to increase the sensitivity of any type of microphone, you can increase the resistance of the load resistor. The final value depends on the quality of the microphone.
The above are all inspections that can be done with simple test equipment. If you still cannot find the fault, you need to try again.
The bottom plate and the battery are placed together in the match box. If the bottom plate can be set aside, it will occupy the smallest space. Covering the circuit with a row of matches, the matches can be glued to a piece of thin cardboard, and the antenna is led out from one end of the match box.
Open a small hole at the other end to let the sound go into the microphone, but it is not necessary to do so, because even if the box is closed, the sound seems to penetrate. As long as a short antenna, about 10cm, can have a transmission range of 30 meters, enough room communication, even larger houses can cope.
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