[Photo] Wireless microphone (9018)

A wireless microphone, like a miniature radio station, emits radio waves that can cover hundreds of square meters of surrounding area. The wireless microphone introduced in this article is an FM transmitter. It is received by an FM radio and can clearly receive the sound signal emitted by the wireless microphone within a dozen meters.
The electrical principle is shown in Figure 2-10-1, and Figure 2-10-2 is the circuit board. T1, C2, C3, C4, L1, and C6 form an oscillator. The oscillation frequency is controlled by the sound signal sent from the microphone. The changed oscillation wave signal is sent to T2, which is amplified and transmitted from the antenna to the air.

Figure 2-10-1

Figure 2-10-2

Recognize

Measure

1. Measure the resistance (R), the method is shown in Figure 2-10-3, the range of the multimeter is set to Ω file × 1K, and the zero is adjusted. R1 is about 1K, R2 is about 15K, and R4 is about 10K. The range of the multimeter is set at × 10K, and the zero is adjusted. The measured R3 is about 200Ω and R5 is about 56Ω.

Figure 2-10-3 Figure 2-10-4

2. Measure electrolytic capacitor C1, see Figure 2-10-4. When the meter bar just touched the capacitor lead, the pointer oscillated greatly and then returned to its original position. Otherwise, it means that the leakage is too large to be used.
3. Measuring capacitance C1, C2, the method is shown in Figure 2-10-5. When the watch bar just touches the capacitor lead, the pointer returns to its original position after swinging. The other capacitors are small-capacity capacitors, and the deflection of the pointer is not obvious when measuring with × 1K.
4. Measure the transistor (T), the method is shown in Figure 2-10-6. The range of the multimeter is set to the NPN file, and the e, b, and c of the diode are inserted into the e, b, and c of the NPN file of the multimeter, and the (hFE) value is greater than 80.

Figure 2-10-5 Figure 2-10-6

Give it a try

1. Homemade coils L1, L2.
L1 uses enameled wire with a diameter of 0.41 mm and winds 7 turns flat on a ballpoint pen core with a diameter of 3 mm, in which some paint is scraped off at the fourth turn and a piece of wire is welded as a tap. In the same way, L2 is wound 6 times in a flat manner. After being reborn, the paint is scraped off the lead at both ends of the coil to be tinned.
2. Scrape the lead wires of each component and put them on tin for use. When assembling each component, the pin should be as far as possible, and the component should be close to the circuit board.
3. Assembly welding R1, R2, R3, R4, R5.
4. Assemble and weld C1, make sure that the leads have positive and negative poles, install C2, C3, C4, C5, C6, C7, C8 and C9.
5. Install transistors T1 and T2. The transistors e, b, and c lines cannot be mistaken. Install the battery clip, pay attention to the positive and negative poles.
6. Welding L1, L2, the final assembly is simple, the microphone lead has polarity, can not be installed wrong. See Figure 2-10-7.
7. A piece of flexible wire with a length of about 30 cm is soldered to L2 of the circuit board as an antenna. Check the circuit diagram carefully, and then connect the power supply after confirming that it is correct.

8. Turn on the FM radio, put the wireless microphone close to the radio, and listen in the range of 88-108MH2. If you cannot hear it, adjust the length of L1. When adjusting, avoid the receiving position of the local broadcasting station. If you still can't receive it, you should check whether the circuit starts to vibrate. Set the multimeter to V position 2.5 and connect it to the R3 terminal. If you find that the pointer is deflecting in the reverse direction, just change the meter bar. If the circuit starts to vibrate, the pointer will shake slightly. After receiving, gradually increase the distance and continue to adjust L2 until the sound is loud and clear.
The production requirements are high, so be careful and patient during the production. As long as the components are intact and the installation is correct, they are generally successful.

Figure 2-10-7

Think about it

In addition to being a microphone, what other applications can wireless microphones be used for?