November 29, 2020

## Sound generator using 555

Circuit given below is configured on astable mode of operation. In place of a fixed resistance, LDR has been kept for getting variable resistance for the 555 timer to work in astable mode.

## Mode of NE555- Astable

This circuit has been configured in Astable mode. The output pulses are determined by the values of resistors R1, R2 and the timing capacitor C1. The formula for the frequency of pulses is: f= 1.44/[(R1+2R2)*C1].

The high and low time of each pulse can also be calculated.

## Mode of NE555- Monostable

Monostable mode of operation

Monostable mode of operation of 555 IC can be explained using a simple touch sensitive circuit. Here, timing constant (T) of monostable timer= 1.1*R1*C1. The following circuit has been configured in monostable mode. A monostable circuit produces a single pulse(for a certain duration determined by the Timing Constant(T)=1.1*R1*C1) when triggered at pin 2.

## Voltage Divider Rule

A Voltage divider consists of two resistances R1 and R2 connected in series across a supply Voltage Vs. The supply Voltage is divided up between the two resistances to give an output Voltage Vo which is the Voltage across R2. This depends on the value of R2 relative to R1:

If R2 is much smaller than R1, Vo is small (low, almost 0V)
(because most of the Voltage is across R1)
If R2 is about the same as R1, Vo is about half Vs
(because the Voltage is shared about equally between R1 and R2)
If R2 is much larger than R1, Vo is large (high, almost Vs)
(because most of the Voltage is across R2)

## Building a circuit on a breadboard

A breadboard is a device for testing temporary electronics projects. Components used to test a circuit can be reused for other projects. None of the components are damaged.

Almost all people start to learn electronics using breadboard because it is the simplest way of making circuit. The photograph shows a typical small breadboard which is suitable for beginners building simple circuits with one or two ICs (chips).

## Dark/Light sensor using transistor

Automatic Light detector using variable resistor

A light detector senses light. As the light level increases and LDR meets the lowest threshold resistance, the circuit automatically turns on the LED D1. We can adjust the sensitivity using the preset VR1-10K.

LESS RESISTANCE(VR1)-> LESS DARKNESS TO SWITCH OFF THE LED

HIGH RESISTANCE(VR1)-> MORE DARKNESS TO SWITCH OFF THE LED