Rendering demonstration of a water level alarm designed with a self-excited multivibrator_Basic hardware circuit diagram explanation

Circuit diagram:

Circuit function introduction:

The water level alarm in this example can be used to sound an alarm when the water tank is full. When the water in the water tank reaches point B of the probe, the alarm will sound a reminder (the sound frequency is low at this time), and the reminder can close the water filling valve. If you ignore it at this time and continue to inject water, when the water reaches point C of the probe, the alarm will make a sharper sound (the sound frequency increases); in this way, when the water reaches the highest point D, the alarm will sound Sharper alarm sound.

Circuit working principle:

1. When someone in the water tank overflows probe B, probes AB are connected, so that the power supply voltage is added to the base of transistor Q1 through resistors R3, R2, and R1. Since the base of transistor Q1 is connected with a capacitor, the transistor The base voltage slowly increases. When the voltage at the base of transistor Q1 rises to 0.7V, transistor Q1 begins to conduct and enters the amplification zone.

2. After transistor Q1 is turned on, current will flow from the emitter to the base of transistor Q2, so that transistor Q2 will also start to conduct. At this time, transistor Q2 also works in the amplification area.

3. After transistor Q2 is turned on, current flows through the speaker; at the same time, the power passes through transistor Q1, and resistor R4 begins to charge capacitor C1. Here is a momentary state:

When the transistor Q2 is turned on, a voltage will be added to the right end of the capacitor C1, causing the potential of the right end of the capacitor C1 to have a sudden change; then according to the characteristic that the voltage across the capacitor cannot change suddenly, the left end of the capacitor C1 will also maintain a potential. The sudden change causes the base voltage of transistor Q1 to rise in a pulse, causing transistor Q1 to enter a saturated conduction state, and will also cause transistor Q2 to enter a saturated conduction state. In this way, the current drawn by the speaker will become larger.

Note: Capacitors C1 and R4 play a positive feedback role here; pay attention to the difference between potential and voltage in the above description.

4. After transistor Q1 is saturated and turned on, capacitor C1 starts to charge, and the charging voltage is negative on the left and positive on the right. However, the potential at the right end of the capacitor remains constant (equal to the voltage drop of the speaker), and then the charging voltage of the capacitor will increase, which will cause the potential at the left end of the capacitor, that is, the base voltage of transistor Q1, to slowly decrease.

5. When the base voltage drops to the point where the transistor enters the amplification area from the saturation area (both transistors will enter the amplification area), the voltage drop of the transistor Q2 increases, the voltage on the speaker decreases, and the current also decreases; As the base voltage of Q1 continues to decrease, transistor Q1 slowly enters the cut-off state, causing Q2 to also enter the cut-off state. At this time, the speaker stops sounding, and the potential at the right end of capacitor C1 drops to 0. According to the characteristic that the voltage across the capacitor cannot suddenly change, This will cause the potential at the left end of the capacitor, that is, the base of transistor Q1, to appear at a negative potential, causing transistor Q1 to completely enter the cut-off state.

6. Then the capacitor C1 passes through R4, and the speaker begins to discharge. During the discharge process, the base potential of the transistor Q1 begins to slowly increase, that is, it returns to the initial state of the circuit. The entire circuit will continue to oscillate like this.

To understand the entire circuit, pay attention to the following points:

1. Clarify the characteristics that the voltage across the capacitor cannot change suddenly.

2. As the water level rises, the resistance of the base of the transistor Q1 connected in the circuit will be reduced, the entire oscillation period will be reduced, and the frequency will become larger.

The parameters in the circuit are controlled as follows:

Resistor R4 and capacitor C1 control the positive pulse part of the oscillation cycle, that is, the sound generation part of the speaker. The larger the resistor R4 is, the longer the speaker will sound.

Resistors R3, R2, and R1 control the current flowing to the base of transistor Q1. The greater the resistance value, the smaller the base current, the slower the transistor base voltage rises, and the longer the speaker shut-off time. It controls the negative pulse part of the oscillation cycle.

-The Electronic Components Purchasing Network (www, oneyac, com) is a local component catalog distributor. It adopts the "small batch, spot, sample" sales model and is committed to meeting customers' procurement needs for multiple models, high quality, and fast delivery. Self-built and efficient intelligent warehousing, with over 50,000 self-operated inventories, provides one-stop genuine spot procurement, personalized solutions, option substitution and other diversified services. (This article is compiled from the Internet for the purpose of disseminating useful information and knowledge. If there is any infringement, please contact the administrator to delete it)

Circuit diagram:

Circuit function introduction:

This circuit changes the oscillation frequency of the self-excited oscillator through the charge and discharge of the electrolytic capacitor C1 to realize the speaker's tone-changing doorbell circuit.

Circuit working principle:

The core part of this circuit is the complementary self-oscillator circuit.

1. After the circuit is powered on, when the touch button S1 is not pressed, the transistors Q1 and Q2 are in a cut-off state, the entire self-excited oscillator does not work, and the speaker is silent.

2. When the touch button S1 is pressed, the power supply starts to charge the capacitor C1 through the resistor R1, causing the voltage of the base of the transistor Q1 to slowly rise. When the base voltage rises to 0.7V, the transistors Q1 and Q2 begin to conduct.

3. At the same time, the positive feedback network composed of resistor R3 and capacitor C3 causes the self-excited oscillator to start vibrating and the speaker produces sound.

Note: For a detailed explanation of the self-excited process of the self-excited oscillator, please see " Water Level Alarm Designed by a Self-Excited Multivibrator "!

4. In this circuit, the negative pulse part of the oscillator waveform is changed through changes in the charge and discharge current of electrolytic capacitor C1. As the button is pressed, the voltage across capacitor C1 continues to increase, and the time of the negative pulse part will also become shorter. The tone becomes higher; when the voltage on capacitor C1 stabilizes, the oscillation frequency also stabilizes. When the touch button S1 is released, the charge stored in the electrolytic capacitor C1 is discharged to the emitter junction of Q1 through the resistor R2, which causes the self-excited oscillator to continue to oscillate and the speaker to continue to sound. However, as the voltage on the electrolytic capacitor C1 continues to decrease, the negative pulse part of the oscillation waveform will slowly become longer, the oscillation frequency begins to become lower, and the tone of the speaker will also change accordingly. When the charge on the electrolytic capacitor C1 is almost discharged, the oscillator stops, the speaker stops sounding, and the entire circuit returns to the state where the button is not pressed.

-The Electronic Components Purchasing Network (www, oneyac, com) is a local component catalog distributor. It adopts the "small batch, spot, sample" sales model and is committed to meeting customers' procurement needs for multiple models, high quality, and fast delivery. Self-built and efficient intelligent warehousing, with over 50,000 self-operated inventories, provides one-stop genuine spot procurement, personalized solutions, option substitution and other diversified services. (This article is compiled from the Internet for the purpose of disseminating useful information and knowledge. If there is any infringement, please contact the administrator to delete it)