Analysis of the circuit principle of audio remote control switch

Every time a sound of a specific frequency is emitted, the audio remote control switch will be controlled to change its state. The circuit has a strong anti-interference ability to sounds of other frequencies emitted in the environment.

The schematic diagram of the audio remote control switch circuit is shown in the attached figure. The circuit consists of an audio amplifier circuit, a phase-locked loop circuit and a switch output circuit. The audio amplifier circuit amplifies the sound signal received by the microphone. The external sound signal is connected to the input end of the phase-locked loop circuit after amplification. There is a voltage-controlled oscillator inside the phase-locked loop circuit, which usually works at the set working frequency. When the external sound signal is transmitted to the phase-locked loop circuit, whenever a sound signal with the same working frequency as the voltage-controlled oscillator is received, the phase-locked loop circuit is in a locked state, and the voltage of the output circuit changes from low to high, controlling the switch output circuit to change the switch state.

The circuit can be controlled by whistling. Adjust the receiving frequency of the circuit so that the circuit only responds to a specific whistle. Carefully adjust the circuit to achieve the highest sensitivity and the longest control distance. If the maker is more experienced, he can also use his mouth to whistle to adjust the circuit.

The circuit has relatively high requirements for the power supply voltage, and the power supply voltage should be kept as stable as possible; if a 9V power supply is used, the 4046 built-in voltage regulator can be used to output a 5V operating voltage.

The 4046 phase-locked loop circuit consists of three parts: a phase comparator, a low-pass filter, and a voltage-controlled oscillator. The function of the phase comparator is to compare the center frequency of the external sound signal with the internal pulse signal. When the frequencies are the same, a high potential is output from pin 1. Whether it is an external sound signal or an internal oscillation signal, the frequency will drift. In fact, the frequencies of the two cannot be the same, and the phase-locked loop circuit may be in an unlocked state. In order to solve this problem, the phase-locked loop circuit can feed back the voltage output by the phase comparator to the voltage-controlled oscillator through a low-pass filter, changing the frequency of the internal oscillation pulse to make it the same as the frequency of the external signal. This input signal frequency range allowed from the initial unlocked state to the final locked state is called the frequency capture range.

The disadvantage of some existing frequency-selective switches using phase-locked loop circuits is that they have almost no selectivity. They use voltage-controlled oscillation and can receive almost all interference signals close to the frequency of external signals. The characteristic of this circuit is that the control voltage output by the phase comparator is divided, and only a small part of the control voltage is allowed to control the voltage-controlled oscillator, so the capture range is relatively narrow. Of course, this range can be easily adjusted by changing the voltage ratio.

Specific measures:

(1) The control voltage of the voltage-controlled oscillator consists of two parts;

(2) The basic voltage determines that the operating frequency of the voltage-controlled oscillator is consistent with the frequency of the received signal. This voltage is provided by potentiometer R6;

(3) The lock voltage output by the 4046 phase-locked loop is distributed through the voltage divider composed of R8 and R7, and then added to the voltage-controlled oscillator together with the base voltage;

(4) By controlling the ratio of the two voltages, the frequency selection capture range of the frequency selection switch can be adjusted arbitrarily; the larger the proportion of the basic voltage, the better the selectivity and the narrower the capture range; the larger the proportion of the locking voltage output by the phase-locked loop, the poorer the selectivity and the wider the capture range.