Thursday, October 1, 2009

Description of the circuit components

General description

This circuit is build by using resistor, capacitor, relay, high speed diode, 555 IC timer, and light dependent resistor (LDR).
The main component of this circuit is the 555 IC timer. This timer will control the entirely circuit which it will making the circuit be a switch.

Component description

555 timer IC

Diagram A

Diagram B

Generally the 555 timer IC is an integrate circuit (chip) as show as above diagram A and also the pin number in the diagram B. It use in implementing a variety of timer and multivibrator application. Depending on the manufacture, the standard 555 package include over 20 transistor, 2 diode and 15 resistor on a silicone chip installed in an 8 pin mini dual-in-line package (DIP-8). The internal schematic is show as below.

Explanation of the pin of the IC






Ground, low level (0V)



A short pulse high-to-low on the trigger starts the timer



During a timing interval, the output stays at +VCC



A timing interval can be interrupted by applying a reset pulse to low (0V)



Control voltage allows access to the internal voltage divider (2/3 VCC)



The threshold at which the interval ends (it ends if U.thr 2/3 VCC)



Connected to a capacitor whose discharge time will influence the timing interval



The positive supply voltage which must be between 3 and 15 V

Normally the 555 timer IC has three operating mode that is :

(A) Monostable mode

In this mode, the 555 function as a “one-shoot”. Application include timers missing pulse detection, bouncefree switches, frequency divider, capacitance measurement, pulse-width modulation (PWM) etc.

(B) Astable mode

Free running mode : the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks tone generation, security alarms, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation, etc.

(C)Bistasble modes or Schmitt trigger
The 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bouncefree latched switches, etc.

High-speed diode

For this circuit, we are using the 1N4148 high-speed switching diodes fabricated in planar technology, and encapsulated in hermetically sealed glass SOD27 (DO-35) packages. This diode contain maximum speed 4ns, maximum continuous reverse voltage 100V, maximum repetitive peak reverse voltage 100V, and maximum repetitive peak forward current 450mA which get from the data sheet.

Light dependent resistor (LDR)

Light Dependent Resistors are very useful especially in light/dark sensor circuits. Normally the resistance of an LDR is very high, sometimes as high as 1000000 ohms, but when they are illuminated with light resistance drops dramatically.


A relay is an electrical operated switch. Electrical current through the coil of the delay creates a magnetic field that attracts a leaver and changes the switch contacts. The coil current can be on or off so relays have two switch position and they have to switch position and they are double-throw ( changeover ) switches.The general concept of this circuit diagram is shown as below.

Actual relay design’s diagram is also shown as below.

For this mini project, we are using 5-pins relay. A 5-PIN relay has a single control circuit, but two separate current paths for the switch: one when the relay is de-energized (OFF – no current through the coil) and the other the energized (ON – current is flowing continuity. When the 5-PIN relay is de-energized (OFF), pins 4 and 5 have continuity. When the relay is energized (ON), pins 3 and 5 have continuity.

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