The high signal-to-noise ratio micro wireless walkie-talkie is shown in the figure below
The machine uses earphones for receiving, and the earplug cable also serves as a transceiver antenna. When the earplug is not in use, the power supply is cut off, so there is no need to add a power switch. It is compact and easy to install and debug. The communication distance is about 100m. It can be used as a wireless microphone with FM radio. The working principle is divided into two parts: transmitting and receiving a) Transmitting part: Capacitor three-point type and high-frequency amplifier (composed of T4, L7, C15) composed of T3, C13, C11, L6, students of this part of the circuit are already familiar with it and will not repeat them. b) The synchronization receiving part is to realize the four functions of mixing, local oscillator, synchronous detection and low-amplification. L1 and C1 form the input loop and are tuned to the received signal frequency (90MHz for this unit), while the resonance frequency of the local oscillator loop L2 and C5 is one-half of the received frequency. The difference frequency signal output after mixing the second harmonic of the received signal and the local oscillator signal is just within the audio frequency range. The strength of the audio signal is approximately equal to the ratio of the audio load impedance to the R2 impedance. Whenever a signal is received, its audio output amplitude can reach tens of millivolts, which is independent of the strength of the received signal. And because the output conductance of T1 is controlled by its collector current, T1 not only has the function of controlling the local oscillator frequency, but also the magnitude of the second harmonic component in the collector current of T1 greatly affects the operating characteristics of the receiving part . Therefore, in order to increase the amplitude of the second harmonic, the value of the feedback capacitor C4 is not less than 2 to 3 times when the fundamental wave oscillates. In addition to being controlled by the operating characteristics of T1, the LO frequency is very important because it is controlled by the input signal frequency within a certain range. Because when the frequency of the second harmonic of the local oscillation signal is close to the frequency of the received signal, the low frequency component of the T1 collector current will be mixed after the two are mixed, so that the output conductance of the transistor will change accordingly, so the local oscillation frequency will also change , So as to be synchronized by the frequency of the external signal, this is the synchronous receiving process of this machine. As the local oscillator and synchronous detection stage in the figure, T1 is connected as a common base circuit as a mixing frequency, and T1 is connected as a common emitter circuit. T2 is an emitter follower, which is matched with headphones with lower impedance. Component selection This machine has no special requirements for components. T1, T3, T4 must use the model with cut-off frequency above 200MHz, such as 9018, 9011, and the β value should be greater than 80. The capacitors use high-frequency ceramic capacitors. D1 ～ D3 used for voltage stabilization are replaced with silicon transistors, and silicon diodes are also acceptable. Small solid core potentiometers for R1, R4, R6, R8. L1, L2, L6, L7 must be self-made. They are all wound on the ballpoint pen core with an enameled wire with a diameter of 0.51mm, and the distance between each turn is about 1mm, and then reborn. L1 is wound around 10 turns and tapped at 5 turns; L2 is wound around 15 turns; L6 is wound around 7 turns and tapped at 2 turns; L7 is wound around 8 turns and tapped at 2 turns. When using commercially available Φ3.5mm earplugs, you need to use tweezers to bend one of the moving break contacts into a moving close contact. The power supply of this machine uses two AAA batteries. The whole machine is installed in a small box made of 110mm × 30mm × 15mm tinplate, easy to carry. Production and debugging As long as the installation is error-free debugging is relatively simple. First set the transceiver selection switch to the transmitting position, adjust R6 to make the working current of T3 between 0.6 and 1mA, and adjust R8 to make the working current of T4 between 1 and 2mA. When adjusting, the ammeter should be connected to the regulated power supply, but the collectors of T3 and T4 cannot be disconnected. At this time, the FM radio should be able to receive the signal sent by the machine. Adjust C11 and C15 to avoid the band of the broadcast station. Fine-tune R18 and C15 to make the transmission distance farthest and the sound clearest, even if the transmission part Adjusted. Then adjust the receiving part of another intercom. Adjust C1 and C5 to enable the signal from the previous interphone. Adjust the tightness of R1 and fine-tuning L1 so that the sound signal heard is the largest without distortion, even if the receiving part is adjusted. Then adjust the receiving part of the previous intercom and the transmitting part of the latter one in the same way as before. |
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Practical voice-controlled quasi-duplex wireless intercom ![Practical voice-controlled quasi-duplex wireless intercom]()
Editor's note: This is a quasi-duplex walkie-talkie designed by a middle school student, that is, it is automatically launched when speaking, not speaking ...
Published on 2006-04-16 19:50 • 278 times read
Three IC wireless intercom 
Posted at 2006-04-16 19:21 • 265 times read
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