Triodes are widely used in digital circuits due to their switching characteristics, primarily for two key applications: control and drive. In control applications, as shown in Figure 3-7, the base of the triode can be controlled via a single-chip microcontroller (MCU) to indirectly manage the backlighting on the rear side. This is a common practice. Another form of control involves managing the transition between different voltage levels. For example, if your MCU operates at 5V but needs to interface with a 12V system, directly connecting the IO port could damage the MCU. To avoid this, a triode is used, which can handle higher voltages. The triode’s operating voltage is sufficient to manage the 12V circuit, allowing the 5V IO port to control it effectively, as seen in Figure 3-8.
Voltage Conversion
In Figure 3-8, when the IO port outputs a high level of 5V, the triode turns on, causing the output (OUT) to go low at 0V. When the IO port is low, the triode turns off, and the OUT goes high due to the pull-up resistor R2. This setup allows low-voltage signals to control high-voltage systems. The second main application of triodes is driving, which refers to their ability to provide current. Let's compare two circuits in Figure 3-9 to understand this better.
LED Small Light Control Method Comparison
The upper LED in Figure 3-9 behaves like the ones discussed in Lesson 2. When the IO port is high, the LED turns off; when low, it turns on. However, the lower circuit shows that simply applying a high level may not work as expected. MCUs are mainly control devices with limited current output. A typical IO port can only supply tens to hundreds of microamps—less than 1mA. This is insufficient to light an LED brightly. To solve this, we use a triode. Our board’s transistor can handle up to 500mA, making it ideal for such tasks, as shown in Figure 3-10.
In Figure 3-10, when the IO port is high, the transistor turns on, amplifying the current from the base to the collector. This allows mA-level current to flow through the LED, lighting it up effectively. Even though the MCU’s low level can directly illuminate an LED, the IO port’s current capacity is still limited. For instance, the STC89C52 has a maximum total current of 50mA, with each IO port not exceeding 6mA. Some enhanced models allow up to 25mA, but again, the total is constrained. Now, looking at the circuit in Figure 3-11, we see eight LEDs connected in parallel.
To understand the circuit, note that all the LED lines on the right are connected via a bus structure. This means that DB0 on the left corresponds to DB0 on the right, but not to other pins like DB1. This is important for correctly interpreting the connections. In Figure 3-12, we analyze the current flow. Subtracting the LED voltage drop and the transistor’s Vce, with a 330Ω current-limiting resistor, each branch draws about 8mA. If all LEDs are lit, the total current would be 64mA, which is too much for the MCU’s IO ports to handle safely.
Some might suggest increasing the resistor value to reduce current, like using 1kΩ instead of 330Ω. While this reduces the current to around 3mA per LED, it also dims the brightness. Since these LEDs are part of a digital display, brightness matters for visual clarity. Moreover, the MCU’s IO ports are not designed for high current output. Even if they could handle it temporarily, long-term use could lead to instability or damage. Therefore, direct connection is not recommended for multiple LEDs.
So, how do we control multiple LEDs with an MCU? Besides triodes, driver ICs like the 74HC245 are useful. These chips act as current buffers without adding logic functions. The 74HC245 can handle up to 70mA, making it suitable for buffering multiple LED circuits. As shown in Figure 3-13, it is placed between the MCU’s P0 port and the LEDs. The chip has a direction pin (DIR), which determines signal flow, and an output enable (OE) pin that must be low to activate the buffer.
In Figure 3-13, you’ll notice a 0.1µF decoupling capacitor, which helps stabilize power. The 74HC245 is a bidirectional buffer, so when DIR is high, signals from A propagate to B. The OE pin is active low, meaning it must be grounded to function. The pull-up resistors R10-R17 ensure proper signal integrity. Some students might wonder why we need both a triode and the 74HC245. The answer lies in understanding circuit design: any current path must be clear and capable of handling the required load. The 74HC245 removes bottlenecks by boosting current capability, ensuring reliable operation of the entire system.
Mini Industrial Pc,Industrial Mini Pc,Fanless Indutrial Pc,Industrial Computer,Embedded Industrial PC
Shenzhen Innovative Cloud Computer Co., Ltd. , https://www.xcypc.com