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The two most common applications of the switching characteristics of transistors in digital circuits are control and drive functions. Control involves using a microcontroller to indirectly manage other parts of the circuit. For example, as shown in Figure 3-7, we can regulate the backlight of a display by controlling the base of a transistor through the microcontroller’s IO pin. This method is widely used and familiar.
Another form of control is managing the transition between different voltage levels. Suppose your microcontroller operates at 5V, but you need to interface with a 12V system. Directly connecting the 12V to the microcontroller’s IO port would risk damaging it. To prevent this, a transistor is used. The transistor has a higher working voltage than the microcontroller's IO, allowing the 5V signal to control the 12V circuit, as illustrated in Figure 3-8.
In Figure 3-8, when the IO port outputs a high level (5V), the transistor turns on, pulling the OUT pin to 0V. When the IO port goes low, the transistor turns off, and the pull-up resistor R2 pulls the OUT pin to a high level. This setup enables low-voltage control of a high-voltage system.
Drive applications refer to the ability of the transistor to provide sufficient current. In Figure 3-9, two LED control circuits are compared. The top circuit uses the microcontroller’s IO directly to turn the LED on or off. However, the bottom circuit shows that even if the IO is high, the LED may not light up due to limited current output from the microcontroller.
Microcontrollers are primarily control devices, and their IO ports have limited current capabilities. Typically, an IO port can only source or sink tens to hundreds of microamps—less than 1 mA. This is insufficient for driving LEDs effectively. A transistor, however, can handle much higher currents, such as 500mA or more, making it ideal for driving LEDs. As shown in Figure 3-10, when the IO port is high, the transistor turns on, amplifying the current to illuminate the LED.
Even when using a low-level signal to turn on an LED, the MCU’s IO port still cannot supply enough current. According to the STC89C52 datasheet, the total current per IO port should not exceed 6mA, and the overall MCU current must stay under 50mA. If multiple LEDs are connected, the current demand increases significantly, risking damage to the microcontroller.
In Figure 3-11, the circuit diagram shows eight LEDs connected via a bus structure. All the LEDs share the same bus lines, which are labeled clearly. Understanding these connections is crucial for interpreting the circuit correctly.
In Figure 3-12, each LED branch has a 330Ω current-limiting resistor. With a 5V supply and an LED voltage drop of about 2V, each LED draws approximately 8mA. If all eight LEDs are lit simultaneously, the total current reaches 64mA—far beyond what the microcontroller can safely handle.
Some might suggest increasing the resistor value to reduce current, such as using 1kΩ instead of 330Ω. While this reduces the current to around 3mA per LED, it also dims the LED, which may be problematic if the display requires brightness. Additionally, the microcontroller’s IO ports are not designed for heavy current loads, and overloading them could cause instability or permanent damage.
To address this, driver ICs like the 74HC245 can be used. These chips act as current buffers, isolating the microcontroller from the load. As shown in Figure 3-13, the 74HC245 is a bidirectional buffer that allows the microcontroller to control multiple LEDs without exceeding its current limits. It can handle up to 70mA, making it suitable for driving several LEDs simultaneously.
The 74HC245 also includes a decoupling capacitor (0.1μF) and pull-up resistors (R10–R17). The direction pin (DIR) determines whether the data flows from A to B or B to A. In our case, since the microcontroller controls the A side, the DIR pin is set to high. The OE (output enable) pin must be low for the buffer to function; otherwise, the chip remains inactive.
As shown in Figure 3-14, the microcontroller’s P0 port connects directly to the A side of the 74HC245. This configuration ensures that the microcontroller can control the LEDs efficiently, without overloading its IO ports. The 74HC245 acts as a bridge, providing the necessary current while protecting the microcontroller from excessive load.
In summary, transistors and driver ICs play essential roles in digital circuits. Transistors allow low-voltage signals to control high-voltage systems, while driver ICs help manage current requirements, ensuring safe and stable operation of peripheral components.