Pocket Wireless Microphone

Here I introduce a pocket transmitter, which is very suitable for beginners. The circuit is simple and easy to make, the cost is low, the output power does not exceed 5-8mW, and the transmission range can reach about 300 meters in the housing area. It can be received by an ordinary FM radio, showing that its sensitivity and clarity are 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 ability. In addition, since the circuit requires very few parts, it can be placed in a matchbox (larger than the ordinary matchbox in China) as a listening device, which can be said to be unaware of the gods and ghosts. However, it is not limited to this use. It can be placed in the baby room, gate or corridor to monitor the actual situation. In addition, it can also be used as a night security device.

The current consumption of the circuit is less than 5mA, and two dry batteries can provide continuous operation for 80 to 100 hours.

The circuit is very stable under normal operation, and the frequency drift is very small. Test: After working for 8 hours, the receiver still does not need to be calibrated. The only thing that affects the output frequency is the condition of the battery. When the battery ages, the frequency changes slightly. Through this production, you can learn about FM transmission and understand its advantages, especially that it produces a very high-quality signal without noise, and it is easy to get a good range even with low power transmission.

How the Circuit Works

As can be seen from the circuit in Figure (1), it is divided into two stages, an audio amplifier and an RF oscillator.

There is actually a FET hidden inside the electret microphone. If you like, you can think of it as a stage. The FET amplifies the change in capacitance of the diaphragm in front of the microphone. This is why the electret microphone is very sensitive.

The audio amplifier stage is performed 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 operates at a frequency of about 88MHz, which is adjusted by the oscillation coil (5 turns in total) and the 47pF capacitor. The frequency is also determined by the transistor, 18pF feedback capacitor and a few bias components, such as the 470Ω emitter resistor and the 22K base resistor.

When the power is turned on, the 1nF base capacitor is gradually charged through the 22K resistor, while the 18pF is charged through the 470Ω resistor of the oscillation coil, but faster. The 47pF capacitor is also charged (although only a small voltage is obtained at its two ends), and the coil generates a magnetic field.

As the base voltage gradually rises, the transistor turns on and effectively connects the internal resistance to both sides of the 18pF. When the 1nF capacitor is charged to the operating voltage of the pole, several chaotic cycles will occur, so we assume that the discussion is close to the operating voltage. The base voltage continues to rise, and the 18nF capacitor tries to prevent the movement of the emitter voltage. When the energy in the capacitor is exhausted and no longer prevents the emitter from moving, the base-emitter voltage decreases, the transistor is turned off, the current flowing into the coil also stops, and the magnetic field decays.

The magnetic field collapses, generating a voltage in the opposite direction. The collector voltage in turn rises from the original 2.9V to more than 3V, and charges the 47pF capacitor in the opposite direction. This voltage also affects the charging of the 18pF capacitor and the voltage drop on the 470Ω emitter resistor, causing the transistor to enter a deeper cutoff.

When the 18pF capacitor is charged, the emitter voltage drops to a certain level, at which point the transistor begins to conduct, and current flows into the coil, counteracting 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, resulting in the transistor entering a deeper conduction, short-circuiting 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 injected into the base of Q2 through the 0.1uF capacitor, changing the oscillation frequency and generating the required FM signal.

Production process

Before assembly, it is best to place the prepared printed board and two batteries into an empty pear box to see how much space is available.

Although the space is limited, a small space must still be left for a single row of matches. These matches can be glued to the cardboard with glue to cover the circuit so that people think it is just a box of matches and will not notice that it is a stethoscope.

Now put all the parts on the work table, sort out the values ​​of each part, and then sort them in order. This is very organized and avoids soldering the wrong parts. It is best to use ultra-fine 0.6lmm resin (rosin) tin wire. Because it is thin, it is fast to solder and easy to tin. A 15 to 20w small electric soldering iron is enough. Wipe the soldering iron tip clean with a sponge before use. The only thing you need to make yourself is the coil, which needs a section of 22 BS (Ф0.5mm) or 24 BS (Фm.71mm) enameled copper wire or tinned copper wire.

Wind 5 turns on a 3mm diameter coil frame, or a medium-sized screwdriver, and then separate the turns by about 5.5mm.

When it comes to adjusting the frequency, you need to change the output frequency by compressing or stretching the coil forward and backward. If your coil is made of enameled wire, you need to peel off the paint on both ends of the wire and then apply a little tin.

Now you can solder the baseplate according to the parts placement indicated in Figure (3), starting with the resistor, followed by the capacitor, transistor, coil and telephone. The resistor should be placed upright on the baseplate, but the height should be kept to a minimum. The pins of the transistor should be inserted into the baseplate as far as possible so that the height of the tube does not protrude.

Two batteries are welded together using a switch, and then the two poles of the power supply and ground are connected to the bottom plate with wires. Finally, a 10cm long copper wire is connected to the "A" point of the bottom plate as an antenna, and the entire manufacturing process is complete. Why? Are you wondering why the circuit does not work? How many times have you found that the circuit does not work properly after installation?

Please don't blame yourself or curse the magazine that taught you. Many times, it is caused by so-called "errors".

All parts produced by the manufacturer have their values, but this value is only within the "difference", not the "normal" value printed on it. This difference is called the error. If the error is 5%, it means that the actual value of the part will be anywhere between 5% below and 5% above its marked value.

Errors are often found in resistors, capacitors, transistors, and other components such as microphones, coils, and integrated circuits.

However, there is another factor, called limits. Each component in the circuit has a range of allowable values ​​for the occasion. As long as the value remains within this range, or within these limits, the circuit will work properly. When choosing each component, it is generally in the middle of this range.

Most circuits are not strictly limited and will generally work fine if you select another higher or lower value from a given component. If it does not work, then either the circuit is not strictly limited or the value you selected is inappropriate.

When you publish a circuit through a magazine, people from all walks of life will try it out, getting the parts they need from all sorts of sources. Sometimes they use the values ​​specified, sometimes they go with the next best value. Also, some parts have 1-5% error, while others are as high as 60% of the stated value. When these parameter differences and limits are mixed in any way, it is very common to encounter circuits that do not work.

Take a microphone for example. With a 3V power supply, some microphones only need a 100K load resistor (R1) to have excellent sensitivity, while others may need 4.7K to achieve acceptable sensitivity. You cannot tell the difference between the two from the appearance. They look the same, but the electrical characteristics are very different.

The same thing can be applied to transistors; the specification sheet may say that two transistors have nearly identical characteristics, but when they are connected to a circuit, one works well and the other doesn't.

Please do not worry about failure after reading the above paragraph. As long as you carefully consider the circuit's component requirements and do it step by step, you can succeed.

Circuit Adjustment

After all parts are soldered, it is best to check all soldering points with the naked eye to see if there is false soldering or too much solder causing a short circuit with adjacent parts. Only after a thorough check can calibration and performance testing be performed. The test procedure is to add a short antenna (5 to 10 cm long) to point A on the bottom plate and tune an FM radio across the entire band to look for the signal.

It is best to keep the transmitter at a certain distance from the radio to prevent any harmonics or side waves from being picked up. If the radio fails to detect the carrier, it means that the frequency may be too low. Lengthen the oscillation coil slightly and try again. If tinned copper wire is used to wind the coil, be careful that the coils should not touch each other. If lacquered copper wire is used, it is necessary to know the continuity of the coil. You can measure it with the low resistance of a multimeter, or measure the circuit current, which should be about 4-6mA.

Once the carrier is detected, place the receiver near a clock and check the sensitivity of the circuit. The radio should emit a clear and strong "ticking" sound. The circuit should be more sensitive than your ears.

The microphone load resistor (R1) determines the sensitivity and can be reduced to 10K or increased to 47K, depending on the required sensitivity.

Be sure to transmit on a frequency completely away from any local FM radio stations, as the signals from these stations are so strong that they can obscure your listener when you are testing the distance.

By compressing the coil, the frequency will be lowered; by stretching it, the frequency will be increased. This will avoid the need for fine-tuning capacitors and save the cost of the machine. However, you can use fine-tuning 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 in common, so that the circuit can be adjusted more carefully. It is easy to deviate from the FM band when adjusting with a coil.

In theory, the inductor should also be adjusted to maintain the L/C ratio of the tuned circuit, but the range we need is very small, so there is no limit.

Using an FM receiver with an adjustment meter, you can determine the output power of the machine. What you need is to make a comparison. The meter indicates four unit degrees, which means a very good output. When testing the machine, use a 10cm long antenna to place it horizontally, 10 meters away from the tuner. Based on the four unit degrees, you know that using a half-wave antenna (170cm long), the machine can transmit as far as about 300 meters.

What if you don’t work?

If the carrier wave sent by the monitor cannot be received on the FM receiver, the first thing to assume is that the frequency is lower than the normal 88-108MHz FM band. This is the most likely reason.

Measure the current of the circuit. If there is 4-6mA, it means the circuit is working. Stretch the coil slightly and scan the entire band. When touching any component on the bottom plate, only use a non-metallic screwdriver and leave the battery, because the capacitance effect caused by the skin on your hand will cause the circuit to be significantly out of adjustment and may completely stop output. In addition, it is also important to maintain a 3V power supply and keep the battery close to the bottom plate.

The entire wiring must be as shown in Figure (2) to maintain the same circuit distributed capacitance. Once the circuit is working, its arrangement can be changed, but in the initial testing steps, each component must be placed as shown in the figure.

The oscillator runs at about 88MHz, which is difficult to see unless you have a 100MHz oscilloscope, or connect the antenna directly to the 75Ω input of a frequency meter.

If the above test instruments are not available, a multimeter is needed to measure the DC voltage to see if the oscillator Q2 has the correct voltage value.
Measure the base voltage and emitter voltage. An ordinary multimeter will indicate that both points are about 2V due to its effect on the circuit. Only a high-impedance meter, such as a FET voltmeter, will indicate that the emitter has 2V and the base has 2.5V. (A digital meter is recommended)

If there is voltage at both test points, it is assumed that the transistor is operating normally, but it may be emitting the wrong frequency.

The 18pF feedback capacitor is used with the BC547 transistor. If you plan to use another number, you can reduce the capacitance to 10pF or 5.6pF. Replace the capacitor first, then the transistor.

Other simple things such as short circuit and breakage of copper foil on the bottom board, poor soldering points, or the use of unnumbered parts, etc., are often a possibility, especially when the numbers or values ​​printed on those parts are unclear. If there is any doubt, they should be replaced immediately.

If only the carrier is received but no pure tone signal is received, the fault is in the audio level or microphone. The so-called carrier without pure tone is that when the radio is tuned to a certain point, all you receive is silence, no rustling sound, but you can't hear the pure tone signal from the transmitter.

These two parts can be checked with an oscilloscope to test whether there is a knee signal sent to the oscillation stage.

Without an oscilloscope, testing is somewhat difficult. Even if there is a voltage between 0.7V and 1.5V on the microphone, this does not mean that the microphone is sensitive or fully operational.

If there is a 1.4V voltage on the collector of the audio amplifier, it means that the transistor is turned on. If it is lower than 0.8V, the transistor is saturated or may be damaged in some way. It may also mean that the transistor has a very high gain and is not suitable.

If the voltage exceeds 2.5V, the level is not enough to conduct electricity. Check the transistor and bias resistor and replace them if necessary. The oscilloscope also shows the sensitivity of the microphone. Increasing or reducing the load resistance can change the gain of the FET. For extremely sensitive parts, the load resistance should not be less than 10K, and sometimes it may need to be as high as 47K or more.

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 checks that can be done with simple testing instruments. If the fault cannot be found, you will need to start over.

The bottom plate and the battery are placed together in the matchbox. If the bottom plate can be placed on its side, it will take up the least space. Use a row of matches to cover the circuit. You can stick the matches on a piece of thin cardboard and lead the antenna out from one end of the matchbox.

A small hole is made at the other end to allow the sound to enter the microphone, but this is not necessary because even if the box is closed, the sound seems to be able to penetrate. With just a short antenna, about 10 cm, you can have a transmission range of 30 meters, which is enough for communication in a room, or even a larger house.

Warning: This circuit is for reference only, please do not use it for illegal purposes.