Light control alarm circuit diagram, light control alarm circuit principle

The real world is the world of electronic technology - analog circuits + digital circuits. What travels in analog circuits are continuous signals, that is to say, various functions based on time as parameters, while digital circuits are much simpler, with only two states: high level or low level (strictly speaking, there are three types, but There is one between high and low levels, the status is unknown). Learning electronic technology is mainly through textbooks + computer simulation software such as mulTIsim + hands-on practice. After you have a certain theoretical foundation, read and analyze schematic diagrams.

The picture above is a simple light-controlled alarm circuit. Let's analyze it and appreciate the wonderful electronic technology. First, we see the right bright resistor R, transistors VT1 and VT2, and the buzzer HA in the circuit. Then we can probably guess that its working principle is due to the change in the resistance of R. The change in the voltage across the resistance of R causes the state of the transistor to change. , thus affecting the working status of the buzzer. Build the following circuit diagram in the mulTIsim simulation software. This is a resistor voltage divider circuit. The greater the resistance of the photoresistor, the higher the voltage at both ends.

The key component in the light-controlled alarm circuit is the triode. Generally speaking, whether it is an NPN or PNP transistor, it has three pins: B base, C collector and E emitter. Just like when we talk about diodes, we think of one-way conductivity, and when we talk about triodes, we think of "current amplification." Everyone compares triodes to faucets in real life (B is the valve, C is the water tank, and E is the water pipe). It's easy to understand. Taking the NPN triode as an example, when the voltage between BE reaches the trigger voltage (0.7V), it is equivalent to the faucet valve opening. At this time, the water in the water tank can flow to the water pipe. The specific flow rate is related to the degree of valve opening, that is, the triode acts on Enlarge the area; when the valve is opened to the maximum extent, the flow of water in the water tank to the water pipe is fixed, that is, the triode acts in the conduction area; of course, there is no water flow when the valve is closed, that is, the triode acts in the cut-off area.

Returning to the original light-controlled alarm circuit, when the resistance of the photoresistor increases to a certain value, the triode VT1 conducts (note that the conduction here is different from the "conduction" term in the operating characteristics of the triode, it is just to illustrate this When the triode is opened like a faucet valve, water flows through it, and the same is true for VT2 below). At this time, the collector current of VT1 flows to the emitter, which means that there is current flowing through the base of triode VT2 at this time, and a voltage is generated at the same time. Drop; if the transistor VT2 is turned on, the buzzer will sound. The schematic diagram built through mulTIsim software is as follows. When we adjust the resistance of R3 (photoresistor) to 40K, the base voltage of transistor Q2 is 0.75V, and the buzzer makes a beeping sound.