📚 CIE A-Level Physics: Types of Output Devices in Electronic Sensing Systems | CIE A-Level 物理：电子传感系统中的输出设备类型
In any electronic sensing system, information flows from a sensor to a processor and then to an output device. The output device is the part that interacts with the user or with the environment, converting a small electrical signal into a visible, audible, or mechanical response.
在任何电子传感系统中，信息从传感器流向处理器，再流向输出设备。输出设备是与使用者或环境互动的部分，将微弱的电信号转化为视觉、听觉或机械响应。
Typical sensors include thermistors, light-dependent resistors (LDRs) and strain gauges. Their resistance changes with temperature, light or strain, so a potential divider can convert this into a voltage. The processor, often an operational amplifier or a switching transistor, then decides whether a threshold has been passed and switches the output device accordingly.
典型传感器包括热敏电阻、光敏电阻（LDR）和应变片。它们的电阻随温度、光照或应变而变化，因此分压器可将这种变化转化为电压。处理器通常是运算放大器或开关晶体管，用来判断是否越过阈值，并据此切换输出设备。
1. What Are Output Devices? | 什么是输出设备？
An output device is a component that converts an electrical signal into a non-electrical form. For example, an LED converts current into light, a loudspeaker converts a varying current into sound, and a relay uses a small current to control a large current in a separate circuit.
输出设备是将电信号转化为非电形式的元件。例如，LED 将电流转化为光，扬声器将变化的电流转化为声音，继电器用一个小电流来控制另一个电路中的大电流。
Output devices are the final stage of a sensing system. They allow the system to do something useful, such as switching on a warning light, ringing a bell, or opening a valve.
输出设备是传感系统的最终环节。它们使系统能够完成有用的动作，例如开启警示灯、响铃或打开阀门。
2. Light-Emitting Diodes (LEDs) | 发光二极管
An LED is a p-n junction diode made from a semiconductor such as gallium arsenide phosphide. When forward biased, electrons recombine with holes at the junction and release energy as photons of visible light or infrared radiation.
LED 是由砷化镓磷等半导体材料制成的 p-n 结二极管。当正向偏置时，电子在结处与空穴复合，并以可见光或红外辐射的形式释放能量。
E = hf = hc/λ
The photon energy E is related to the frequency f, and hence to the wavelength λ, by the Planck constant h and the speed of light c. A semiconductor with a wider energy gap emits shorter wavelengths, so the colour of the LED depends on the material used.
光子能量 E 通过普朗克常量 h 和光速 c 与频率 f 相关，从而也与波长 λ 相关。能隙较宽的半导体发射较短波长，因此 LED 的颜色取决于所用材料。
- LEDs require a forward potential difference of about 1.5 V to 3.5 V, depending on colour. LED 所需要的正向电压约为 1.5 V 到 3.5 V，具体取决于颜色。
- Typical operating currents are 10 mA to 20 mA. 典型工作电流为 10 mA 到 20 mA。
- Above the threshold voltage the current rises very rapidly, so a series resistor is essential to limit the current. 超过阈值电压后，电流上升非常迅速，因此必须串联电阻来限流。
3. Calculating the Series Resistor for an LED | LED 串联电阻的计算
When an LED is connected to a supply, the potential difference across the LED is approximately equal to its forward voltage VLED. The resistor drops the remaining supply voltage Vs and sets the desired current I.
当 LED 连接到电源时，其两端电压约等于正向电压 VLED。串联电阻承担剩余的电源电压 Vs 并设定所需电流 I。
R = (Vs − VLED) / I
Example: A 5.0 V supply is connected to an LED with VLED urrent of 15 mA is required. The series resistor is R = (5.0 − 2.0) / (15 × 10⁻³) = 200 Ω.
例：5.0 V 电源与 VLED = 2.0 V 的 LED 连接，要求电流为 15 mA。则串联电阻 R = (5.0 − 2.0) / (15 × 10⁻³) = 200 Ω。
The exact value is not critical. A standard 220 Ω resistor would normally be chosen, because the LED current must simply be kept safely below its maximum rating.
该数值并不要求非常精确。通常可以选择标准 220 Ω 电阻，因为只需将 LED 电流安全地保持在其最大额定值以下即可。
4. Liquid Crystal Displays (LCDs) | 液晶显示器
An LCD does not emit light. It contains a thin layer of liquid crystals between two crossed polarising filters. In its normal state, light passing through the first polariser has its plane of polarisation rotated by 90° by the liquid crystal, so it can pass through the second filter.
LCD 本身并不发光。它由两层正交偏振片之间的薄液晶层构成。在自然状态下，穿过第一个偏振片的光被液晶将偏振面旋转 90°，因此能够穿过第二个偏振片。
Applying a small alternating voltage to a segment changes the molecular arrangement of the liquid crystal. The polarisation is then no longer rotated, so that segment appears dark. In this way, selected segments can form letters or numbers.
对某个分段施加一个小交流电压会改变液晶的分子排列。此时偏振面不再被旋转，该分段就会变暗。通过这种方式，可以选择不同分段来显示字母或数字。
- LCDs require alternating current, usually at a low frequency, to prevent permanent chemical changes in the liquid crystal. LCD 需要低频交流电，以防止液晶发生不可逆的化学变化。
- They consume very little power, making them ideal for battery-operated devices. 它们功耗很低，非常适合电池供电的设备。
- Because an LCD is a passive device, it must be illuminated by ambient light or by a backlight. 由于 LCD 是被动器件，必须借助环境光或背光源才能观看。
5. Relays | 继电器
A relay is an electromagnetic switch. It consists of a coil of wire wound around an iron core, an armature, and a set of contacts. When a current passes through the coil, the iron core becomes magnetised and attracts the armature, which closes or opens the contacts in a separate circuit.
继电器是一种电磁开关。它由绕在铁芯上的线圈、衔铁和一组触点组成。当线圈中有电流通过时，铁芯被磁化并吸引衔铁，从而接通或断开另一个电路中的触点。
Relays are used in sensing circuits when the processor is too weak to switch a high-current device such as a mains lamp or motor. The relay provides electrical isolation: the low-voltage control circuit is completely separate from the high-voltage output circuit.
当处理器太弱，无法开关大电流设备（如市电灯泡或电动机）时，传感电路中使用继电器。继电器提供电气隔离：低压控制电路与高压输出电路完全分离。
To protect the transistor that drives the coil, a diode is usually connected across the coil in the reverse direction. When the coil current is switched off, the collapsing magnetic field induces a back e.m.f. that could damage the transistor; the diode provides a safe path for this current.
为了保护驱动线圈的晶体管，通常在线圈两端反向并联一个二极管。当线圈电流被切断时，磁场的消失会感应出反电动势，可能损坏晶体管；该二极管为这股电流提供安全通路。
6. Loudspeakers and Buzzers | 扬声器与蜂鸣
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