Digital circuit production 05-555 dual tone alarm circuit

1. Circuit simulation and principle explanation

      This circuit mainly consists of two NE555 chips U1 and U2 to form a multivibrator circuit. It can be seen from the figure that pin 5 of U2 is connected to pin 3 of the output terminal of U1. The control voltage of U2 is controlled by U1. When pin 3 of U1 outputs high level, the control terminal voltage of U2 is higher than 2/3* The charging and discharging time of VCC and C4 capacitor becomes longer, the output cycle becomes larger, and the frequency decreases; when pin 3 of U1 outputs low level, the U2 control terminal voltage is lower than 2/3*VCC, and the charging and discharging time of C4 capacitor becomes shorter. , the output period becomes smaller, and the frequency increases. Within one cycle of the output of pin 3 of U1, U2 generates two different frequencies. When an external buzzer or speaker is connected, it will produce sounds of alternating frequencies.

2. Precautions

(1) When simulating, please switch to the simulation mode, and the device needs to be called in the simulation library;
(2) If you need to use an oscilloscope to test the waveform, just disconnect the buzzer for testing;
(3) Pay attention to the chip during the welding process When installing, do not insert it backwards;
(4) The buzzer can be replaced by a speaker. The speaker has no positive and negative poles, just install it.