Analysis of the working principle of electronic mouse repeller

How it works

The electronic mouse repeller consists of four parts: a power supply circuit, a timing circuit, a cat-meowing circuit and a power amplifier circuit.

Turn on the power switch S1, and the 220V mains voltage is stepped down to AC6V through the fuse FU and transformer T. After rectification and filtering by the rectifier bridge FU and capacitor C1, VD1 stabilizes the voltage to +6V to supply power to the machine.

When the power is just turned on, since the voltage across capacitor C2t cannot change suddenly, the integrated block IC1 (2) and (6) pins are at low level, and the (3) pin outputs a high level, which is added to the IC2 (2) pin. The cat's meowing integrated block IC2 is triggered and starts, and the cat's meowing signal is output from the (3) pin. After being amplified by transistor VT1 and audio power amplifier integrated block IC3, a realistic cat's meowing sound is emitted by the speaker. As the voltage at the C2 terminal increases, the IC1 (2) and (6) pins change from high level to low level when it increases to about 4V (2/3VCC), the IC1 (3) pin changes from high level to low level, and the IC2 (2) pin is low level.

There is no output at the (3) pin. VT1 and IC3 stop working, and the meowing sound stops. At this time, the fully charged C2 is discharged through the internal circuit of RP2, VD2, and IC1 (7) pin. When the discharge causes the voltage of ICl (2) and (6) pins to drop to 2V (1/3 Vcc), the circuit flips, and IC1 (3) pin outputs a high level to start IC2 and IC3 output signals, and BL emits a meowing sound. This repetition has the effect of repelling mice. Adjusting RP2 can change the interval time of the meowing sound.

1. The cat's meowing sound became very unpleasant. Later, there was no cat's meowing sound at all when I turned on the computer.

Analysis of this phenomenon shows that the fault should be in the cat-calling integrated circuit or audio amplifier circuit, but after opening the shell, it was found that the FU fuse was blown. After checking the power supply circuit, it was found that the VD1 was short-circuited. After replacing it and the fuse, the power supply returned to the normal 6V, but the cat-calling sound was still unpleasant. Trying to replace the speaker, C9, IC3, C6, VT1, etc. did not work; trying with other music chips did not work either. So I thought that the power supply might be bad. Using a digital multimeter to measure the forward readings of the 4 diodes in the rectifier bridge were all around 0.65. The reverse reading was infinite, which was normal (otherwise, if one of them was short-circuited, it would cause the fuse to burn; if one of them was open-circuited, it would form a half-wave rectification. The rectified voltage and ripple were affected). Then check C1 and find that one of its pins was loose and poorly soldered. After re-soldering it, the fault was eliminated.

Analysis shows that the desoldering of C1 not only increases the ripple and causes poor sound quality, but also causes VD1 to be frequently subjected to current shock when the machine is turned on. If the quality of VD1 is poor, it may break down and short-circuit, thus causing this fault.

2. It can make a normal cat meow when the power is turned on, but it cannot meow again afterwards.

This phenomenon indicates that the power supply, music block and amplifier circuit are basically normal. The fault may be in the timing circuit. The timing circuit of this circuit is connected as an astable multivibrator. Under normal circumstances, the potential of its output terminal (3) changes with the charge and discharge of C2. However, after the power is turned on and a cat sound is emitted, the Ic1 (2) and (6) pins are always high level; the discharge terminal (7) pin is also always high level, with no discharge signs. It is suspected that IC1 is defective, but the fault still exists after replacement. Checking its peripheral components, it is found that the end of RP2 connected to VD2 is not connected to the other two ends, causing the C2 discharge circuit to be broken. The timing circuit no longer flips. IC2 can no longer be triggered to emit a meowing sound. After replacing RP2, the fault is eliminated.