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.
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.