外文翻译——基于单片机的交通灯控制

外文翻译——基于单片机的交通灯控制内容摘要：

eaders, level control, character recognition etc. Nowadays packing containing an LED and a photodiode, called „optocoupler‟ is used for switching on power or control circuits. Because the light source (LED) and photodiode are physically kept separated (with 2mm) in the package, isolation upto 2500V can be had. 555 timer The buffer circuit39。 s input has a very high impedance (about 1M ) so it requires only a few 181。 A, but the output can sink or source up to 200mA. This enables a high impedance signal source (such as an LDR) to switch a low impedance output transducer (such as a lamp). It is an inverting buffer or NOT gate because the output logic state (low/high) is the inverse of the input state: Input low ( 1/3 Vs) makes output high, +Vs Input high ( 2/3 Vs) makes output low, 0V When the input voltage is between 1/3 and 2/3 Vs the output remains in its present state. This intermediate input region is a deadspace where there is no response, a property called hysteresis, it is like backlash in a mechanical linkage. This type of circuit is called a Schmitt trigger. If high sensitivity is required the hysteresis is a problem, but in many circuits it is a helpful property. It gives the input a high immunity to noise because once the circuit output has switched high or low the input must change back by at least 1/3 Vs to make the output switch back. LIGHT EMITTING DIODE Light emitting diode (LED) is basically a PN junction semiconductor diode particularly designed to emit visible light. There are infrared emitting LEDs which emit invisible light. The LEDs are now available in many colours red, green and yellow. A normal LED emits at and consumes MA of current. The LEDs are made in the form of flat tiny PN junction enclosed in a semispherical dome made up of clear coloured epoxy resin. The dome of a LED acts as a lens and diffuser of light. The diameter of the base is less than a quarter of an inch. The actual diameter varies somewhat with different makes. The mon circuit symbols for the LED are shown in Fig. It is similar to the conventional rectifier diode symbol with two arrows pointing out. There are two leads one for anode and the other for cathode. LEDs often have leads of dissimilar length and the shorter one is the cathode. All manufacturers do not strictly adhere this to. Sometimes the cathode side has a flat base. If there is doubt, the polarity of the diode should be identified. A simple bench method is to use the ohmmeter incorporating 3volt cells for ohmmeter function. When connected with the ohmmeter: one way there will be no deflection and when connected the other way round there will be a large deflection of a pointer. When this occurs the anode lead is connected to the negative of test lead and cathode to the positive test lead of the ohmmeter. If low range (Rxl) of the ohmmeter is used the LED would light up in most cases because the low range of ohmmeter can pass sufficient current to light up the LED. Another safe method is to connect the test circuit shown in Fig. 2. Use any two dry cells in series with a current limiting resistor of 68 to 100 ohms. The resistor limits the forward diode current of the LED under test to a safe value. When the LED under test is connected to the test terminals in any way: if it does not light up, reverse the test leads. The LED will now light up. The anode of the LED is that which is connected to the “A” terminal (positive pole of t。