## FM transmitter and Dark sensor

A dark sensor can be converted to a simple FM transmitter using the following components. This FM transmitter is very sensitive and it has transmitting range of 30 meters.

## Telephone misuse indicator

When the phone line is in idle condition (on hook), the full voltage of 48V DC from the exchange appears across the line. When the phone is engaged by lifting the hand set, the voltage across the line falls down to around 12V. Thus, under idle condition (48V), VR1 is adjusted to give a positive biasing voltage to the base of Q3. As Q3 conducts Q1 gets into reverse bias and stops conduction. If the phone is engaged, Q3 stops conducting due to voltage drop across the lines, and subsequently, Q1 and Q2 starts oscillating and beeping is heard. Thus, this circuit can be used as an indicator for telephone tapping and misuse by an unauthorized person.

## AM receiver from dark sensor

In this case, LDR has been replaced with capacitor C1-0.01uF and resistor R3-1K has been replaced with L1 & VC1. In place of a LED, a Piezo Diaphragm has been kept.

We just need to add inductor, capacitor and a piezo diaphragm to convert a dark detector to an AM radio.

## 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].

## Sound generator and Musical toy organ

One of the main objectives of LDR Engineering projects is to make students capable of developing their own logic for making devices. Here is an example that tells how LDR Engineering helps in developing ideas.

## How to use variable resistors

There are 3 pins/terminals on a preset. The maximum resistance that a preset can provide is written on it. If 100K is written on preset, it means that we can vary its resistance from 0 Ohm to 100K. A movable metal is rotated in clockwise or anticlockwise direction that changes the resistance of preset.

## How to use a relay

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches.

The relay’s switch connections are usually labeled COM(POLE), NC and NO:

COM/POLE= Common, NC and NO always connect to this, it is the moving part of the switch.

NC = Normally Closed, COM/POLE is connected to this when the relay coil is not magnetized.

NO = Normally Open, COM/POLE is connected to this when the relay coil is MAGNETIZED and vice versa.

A relay shown in the picture is an electromagnetic or mechanical relay.

## OR-gate realization

To make an object counter we can use a simple dark detector circuit and a normal calculator. Pole and NO (Normally Open) terminals of a relay have to be connected to the two pins which are used to activate ‘=’ key of calculator.
As we block the light falling on LDR, the calculator starts counting.
For the calculator to work as a counter, we first need to press ‘1’ and ‘+’ keys.

## Object counter using LDR and calculator

To make an object counter we can use a simple dark detector circuit and a normal calculator. Pole and NO (Normally Open) terminals of a relay have to be connected to the two pins which are used to activate ‘=’ key of calculator.
As we block the light falling on LDR, the calculator starts counting.
For the calculator to work as a counter, we first need to press ‘1’ and ‘+’ keys.

## Water Level Indicator

We can use the logic of making an automatic light detector to make a simple water level indicator.We just need to remove LDR from the circuit and put the wires into water, then it simply works as water level indicator. There is a specific resistance between the wires that are dipped into water and conduction of electricity through water gives biasing voltage to base of transistor.

The sensitivity can be adjusted using VR1. Put 470K variable resistance in VR1.

## Remote Tester

Remote tester circuit also resembles the logic of a “dark sensor (using BC557)”. LDR has been replaced with IR sensor TSOP 1738. The circuit works as a remote tester. Pressing a remote control switch near the IR sensor, switches on the LED D2.

## Toggle switch

This 555 timer circuit below toggles the LED when a button is pressed. Pins 2 and 6, the threshold and trigger inputs, are held at 1/2 the supply voltage by the two 10K resistors.

## 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)