The IC 555 is used widely in different types of alarm circuits. Let us try to understand one of these circuits, which can be used for light detection.Circuit Design
A 50 kilohm resistance is connected between Pin 8 and Pin 7 and a 1 kilohm resistance is connected between Pin 7 and Pin 6. Pin 2 is shorted to Pin 6. The positive terminal of a 0.047 microfarad capacitor is connected to Pin 6 and the other end of the capacitor is grounded. The speaker is connected to the output Pin 3 through a 4.7 mircofarad capacitor. The other terminal of the speaker is grounded.
For detecting light, an LDR (Light Dependent Resistor) is used. The LDR is connected between Pin 5 and Pin 8 using DPDT (Double Pole, Double Throw) switch.Another DPDT switch is connected in between Pin 4 and Pin 8 with a 10 kilohm resistance in between.
Finally we connect Pin 8 to the 5 Volt power supply and Pin 1 is grounded.
Now, whenever the LDR is exposed to direct light or when the LDR is place in complete darkness, the speaker will produce an alarm. What is explained above, is a simple alarm circuit using the 555 timer. IC 555 can be used similarly, in major alarm systems. 
The 555 timer applications that have been explained in this article, are the miniature versions of their actual applications. The circuits are used in large numbers by major consumer electronics goods manufacturers. Using this IC simply eliminates the need of an external timer circuit.

  • What is the element that is used to detect light?
  • If you are not connecting Pin8 and Pin1 what happens?

Figure 1. Switch Debouncing Circuit Diagram Using the 555 Timer
This is a switch debouncing circuit that employs a 555 timer IC. One problem with any pushbutton switch is the oscillation that occurs when it makes electrical contact to close the circuit.  This can result in 'bounces' or undesired multiple pulses that can lead to errors in digital circuits.

The circuit above can be used to 'debounce' a pushbutton switch.  It gets its initial trigger from the pushbutton switch and outputs a single clean pulse that the receiving circuit can use error-free.  In this circuit, the 555 timer is essentially configured as a 555 monostable multivibrator.

•The 555 timer can be used in many ways
•Common uses are for precision timing, pulse generation, sequential timing, time delay generation
Answer these questions by placing your responses in the comments section.
  • What is the applications of timer?
  • How many pins of IC555?
Pin Configuration:-
     Pin1: ground
     Pin2: trigger input to lower comparator
     Pin3: output high ( 200mA max)
     Pin4: reset latch to low state

     Pin5:gives access to voltage divider 
     Pin6: input to upper comparator
     Pin7: an open collector to discharge
     Pin8: power source
This is the first timer blog we've done.

I have collected information from the Internet and some friends.

We hope that you benefit from timer blog and that you like. If you want to add any information or anything else you can send us an email.

Now watch the video on the opposite page.

When completed, answer the following 2 questions and place your responses in the comments section.

  1. What was the main point of the video?
  2. How can you develop this circuit?   

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    We are Khaled Alanzi and Abdulrhman and we study in TTC riyadh and we specialization in electronics



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    December 2012
    November 2012