INTRODUCTION This circuit automatically turns on a night lamp when bedroom light is switched

off. The lamp remains ‘on’ until the light sensor senses daylight in the morning. A yellow LED

is used as the night lamp. It gives bright and cool light in the room. When the sensor detects the

daylight in the morning, a melodious morning alarm sounds.

The circuit utilizes light-dependent resistors (LDRs) for sensing darkness and light in the

room. The circuit is designed around the popular timer IC NE555, which is configured as a

monostable. NE555 is activated by a low pulse applied to its trigger pin 2. Once triggered, output

pin 3 of NE555 goes high and remains in that position until timer is triggered again at its pin 2.

The musical tone of the alarm is generated by UM66 IC. The circuit can be easily assembled on

a general purpose PCB. Enclose it in a good-quality plastic case with provisions for LDR and

LED. Use a reflective holder for LED to get a spotlight effect for reading. Place LDRs away

from the LED, preferably on the backside of the case, to avoid unnecessary illumination. The

speaker should be small so as to make the gadget compact.

Circuit Diagram:

Components used: IC NE555N

RESISTORS 220Ω, 560Ω, 580Ω, 1k, 120k, 150k

LIGHT DEPENDENT RESISTOR

ZENER DIODE

TRANSISTOR BC548

CAPACITOR 0.01µF

NE555 TIMERIntroduction:

The 555 timer IC was first introduced around 1971 by the Signetics Corporation as the

SE555/NE555 and was called "The IC Time Machine" and was also the very first and only

commercial timer ic available. It provided circuit designers and hobby tinkerers with a relatively

cheap, stable and user-friendly integrated circuit for both monostable and astable applications.

The 555, come in two packages, either the round metal-can called the 'T' package or the more

familiar 8-pin DIP 'V' package. About 20-years ago the metal-can type was pretty much the

standard (SE/NE types). The 556 timer is a dual 555 version and comes in a 14-pin DIP package,

the 558 is a quad version with four 555's also in a 14 pin DIP case.I nside the 555 timer, are the

equivalent of over 20 transistors, 15 resistors, and 2 diodes, depending of the manufacturer. The

equivalent circuit, in block diagram, providing the functions of control, triggering, level sensing

or comparison, discharge, and power output. Some of the more attractive features of the 555

timer are: Supply voltage between 4.5 and 18 volt, supply current 3 to 6 mA, and a Rise/Fall

time of 100 nSec. It can also withstand quite a bit of abuse. The Threshold current determine the

maximum value of Ra + Rb. For 15 volt operation the maximum total resistance forR (Ra +Rb)

is 20 Mega-ohm. The supply current, when the output is 'high', is typically 1 milli-amp (mA) or

less.

General Description:

The LM555 is a highly stable device for generating accurate time delays or oscillation.

Additional terminals are provided for triggering or resetting if desired. In the time delay mode of

operation, the time is precisely controlled by one external resistor and capacitor. For astable

operation as an oscillator, the free running frequency and duty cycle are accurately controlled

with two external resistors and one capacitor. The circuit may be triggered and reset on falling

waveforms, and the output circuit can source or sink up to 200mA or drive TTL circuits.

Features:

• Direct replacement for SE555/NE555

• Timing from microseconds through hours

• Operates in both astable and monostable modes

• Adjustable duty cycle • Output can source or sink 200 mA

• Output and supply TTL compatible

• Temperature stability better than 0.005% per °C

• Normally on and normally off output

• Available in 8-pin MSOP package

Pin 1 (Ground):The ground (or common) pin is the most-negative supply potential of the device,

which is normally connected to circuit common (ground) when operated from positive supply

voltages.

Pin 2 (Trigger): This pin is the input to the lower comparator and is used to set the latch, which

in turn causes the output to go high. This is the beginning of the timing sequence in monostable

operation. Triggering is accomplished by taking the pin from above to below a voltage level of

1/3V+(or,in general, one-half the voltage appearing at pin 5).

Pin 3 (Output): The output of the 555 comes from a high-current totem-pole stage made up of

transistors Q20 - Q24. Transistors Q21 and Q22 provide drive for source-type loads, and their

Darlington connection provides a high-state output voltage about 1.7 volts less than the V+

supply level used. The state of the output pin will always reflect the inverse of the logic state of

the latch, and this fact may be seen by examining Since the latch itself is not directly accessible,

this relationship may be best explained in terms of latch-input trigger conditions. To trigger the

output.

Pin 4 (Reset): This pin is also used to reset the latch and return the output to a low state. The

reset voltage threshold level is 0.7 volt, and a sink current of 0.1mA from this pin is required to

reset the device. These levels are relatively independent of operating V+ level; thus the reset

input is TTL compatible for any supply voltage. The reset input is an overriding function; that is,

it will force the output to a low state regardless of the state of either of the other inputs. It may

thus be used to terminate an output pulse prematurely, to gate oscillations from "on" to "off", etc.

Delay time from reset to output is typically on the order of 0.5 µS, and the minimum reset pulse

width is 0.5 µS.

Pin 4 (Reset): This pin is also used to reset the latch and return the output to a low state. The

reset voltage threshold level is 0.7 volt, and a sink current of 0.1mA from this pin is required to

reset the device. These levels are relatively independent of operating V+ level; thus the reset

input is TTL compatible for any supply voltage. The reset input is an overriding function; that is,

it will force the output to a low state regardless of the state of either of the other inputs. It may

thus be used to terminate an output pulse prematurely, to gate oscillations from "on" to "off", etc.

Delay time from reset to output is typically on the order of 0.5 µS, and the minimum reset pulse

width is 0.5 µS.

Pin 5 (Control Voltage):This pin allows direct access to the 2/3 V+ voltage-divider point, the

reference level for the upper comparator. It also allows indirect access to the lower comparator,

as there is a 2:1 divider (R8 - R9) from this point to the lower-comparator reference input, Q13.

Use of this terminal is the option of the user, but it does allow extreme flexibility by permitting

modification of the timing period, resetting of the comparator, etc. When the 555 timer is used in

a voltage-controlled mode, its voltage-controlled operation ranges from about 1 volt less than V+

down to within 2 volts of ground (although this is not guaranteed). Voltages can be safely

applied outside these limits, but they should be confined within the limits of V+ and ground for

reliability. By applying a voltage to this pin, it is possible to vary the timing of the device

independently of the RC network. The control voltage may be varied from 45 to 90% of the Vcc

in the monostable mode, making it possible to control the width of the output pulse

independently of RC.

Pin 6 (Threshold):Pin 6 is one input to the upper comparator (the other being pin 5) and is used

to reset the latch, which causes the output to go low.

Resetting via this terminal is accomplished by taking the terminal from below to above a voltage

level of 2/3 V+ (the normal voltage on pin 5). The action of the threshold pin is level sensitive,

allowing slow rate-of-change waveforms. The voltage range that can safely be applied to the

threshold pin is between V+ and ground. A dc current, termed thethreshold current, must also

flow into this terminal from the external circuit. This current is typically 0.1µA, and will define

the upper limit of total resistance allowable from pin 6 to V+. For either timing configuration

operating at V+ = 5 volts, this resistance is 16 Mega- ohm. For 15 volt operation, the maximum

value of resistance is 20 MegaOhms.

Pin 7 (Discharge):This pin is connected to the open collector of a npn transistor (Q14), the

emitter of which goes to ground, so that when the transistor is turned "on", pin 7 is effectively

shorted to ground. Usually the timing capacitor is connected between pin 7 and ground and is

discharged when the transistor turns "on". The conduction state of this transistor is identical in

timing to that of the output stage. It is "on" (low resistance to ground) when the output is low and

"off" (high resistance to ground) when the output is high. In both the monostable and astable

time modes, this transistor switch is used to clamp the appropriate nodes of the timing network to

ground. Saturation voltage is typically below 100mV (milli-Volt).

Pin 8 (V +):The V+ pin (also referred to as Vcc) is the positive supply voltage terminal of the

555 timer IC. Supply-voltage operating range for the 555 is +4.5 volts (minimum) to +16 volts

(maximum), and it is specified for operation between +5 volts and +15 volts. The device will

operate essentially the same over this range of voltages without change in timing period.

Actually, the most significant operational difference is the output drive capability, which

increases for both current and voltage range as the supply voltage is increased. Sensitivity of

time interval to supply voltage change is low, typically 0.1% per volt. There are special and

military devices available that operate at voltages as high as 18 volts.

555 Monostable-multivibrator-operation

The operation of the circuit is explained below:

Initially, when the output at pin 3 is low i.e. the circuit is in a stable state, the transistor is on and

capacitor- C is shorted to ground. When a negative pulse is applied to pin 2, the trigger input

falls below +1/3 VCC, the output of comparator goes high which resets the flip-flop and

consequently the transistor turns off and the output at pin 3 goes high. This is the transition of the

output from stable to quasi-stable state, as shown in figure. As the discharge transistor is cutoff,

the capacitor C begins charging toward +VCC through resistance RA with a time constant equal

to RAC. When the increasing capacitor voltage becomes slightly greater than +2/3 VCC, the

output of comparator 1 goes high, which sets the flip-flop. The transistor goes to saturation,

thereby discharging the capacitor C and the output of the timer goes low, as illustrated in

figure.Thus the output returns back to stable state from quasi-stable state. The output of the

Monostable Multivibrator remains low until a trigger pulse is again applied. Then the cycle

repeats. Trigger input, output voltage and capacitor voltage waveforms are shown in figure.

3.7. Monostable Multivibrator Design Using 555 timer IC

The capacitor C has to charge through resistance RA. The larger the time constant RAC, the

longer it takes for the capacitor voltage to reach +2/3VCC. In other words, the RC time constant

controls the width of the output pulse. The time during which the timer output remains high is

given as

tp=1.0986RAC where RA is in ohms and C is in farads. The above relation is derived as below.

Voltage across the capacitor at any instant during charging period is given as

vc = VCC (1- e-t/RAC)

Substituting vc = 2/3 VCC in above equation we get the time taken by the capacitor to charge from 0 to +2/3VCC.

So +2/3VCC. = VCC. (1 – e-t/RAC) or t – RAC loge 3 = 1.0986 RAC

So pulse width, tP = 1.0986 RAC s 1.1 RAC

The pulse width of the circuit may range from micro-seconds to many seconds. This circuit is

widely used in industry for many different timing applications.

In this mode of operation, the timer functions as a one-shot. The external capacitor is

initially held discharged by a transistor inside the timer. Upon application of a negative trigger

pulse of less than 1/3 VCC to pin 2, the flip-flop is set which both releases the short circuit

across the capacitor and drives the output high. The voltage across the capacitor then increases

exponentially for a period of t = 1.1 RA C, at the end of which time the voltage equals 2/3 VCC.

The comparator then resets the flip-flop which in turn discharges the capacitor and drives the

output to its low state. Since the charge and the threshold level of the comparator are both

directly proportional to supply voltage, the timing interval is independent of supply.

LIGHT DEPENDENT RESISTER (LDR)

A light dependent resistor or photo resistor is a resistor whose resistance decreases with

increasing incident light intensity. It can also be referenced as a photoconductor. A photo resistor

is made of a high resistance semiconductor. If light falling on the device is of high enough

frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump

into the conduction band. The resulting free electrons conduct electricity, thereby lowering

resistance. Photo resistors come in many different types. Inexpensive cadmium sulfide cells can

be found in many consumer items such as camera light meters, street lights, clock radios, alarms,

and outdoor clocks.

LDRs or 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 1000 000 ohms, but when

they are illuminated with light resistance drops dramatically.

The animation opposite shows that when the torch is turned on, the resistance of the LDR

falls, allowing current to pass through it. When the light level is low the resistance of the LDR is

high. This prevents current from flowing to the base of the transistors. Consequently the LED

does not light. However, when light shines onto the LDR its resistance falls and current flows

into the base of the first transistor and then the second transistor. The LED lights. The preset

resistor can be turned up or down to increase or decrease resistance, in this way it can make the

circuit more or less sensitive.

A photoresistor or light dependent resistor or cadmium sulfide (CdS) cell is a resistor whose

resistance decreases with increasing incident light intensity. It can also be referred to as a

photoconductor.A photoresistor is made of a high resistance semiconductor. If light falling on

the device is of high enough frequency, photons absorbed by the semiconductor give bound

electrons enough energy to jump into the conduction band. The resulting free electron (and its

hole partner) conduct electricity, thereby lowering resistance.

A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own

charge carriers and is not an efficient semiconductor, e.g. silicon. In intrinsic devices the only

available electrons are in the valence band, and hence the photon must have enough energy to

excite the electron across the entire bandgap. Extrinsic devices have impurities, also called

dopants, added whose ground state energy is closer to the conduction band; since the electrons

do not have as far to jump, lower energy photons (i.e., longer wavelengths and lower

frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms

replaced by phosphorus atoms (impurities), there will be extra electrons available for conduction.

This is an example of an extrinsic semiconductor.

Applications

Photoresistors come in many different types. Inexpensive cadmium sulfide cells can be found in

many consumer items such as camera light meters, street lights, clock radios, alarms, and

outdoor clocks. They are also used in some dynamic compressors together with a small

incandescent lamp or light emitting diode to control gain reduction. Lead sulfide (PbS) and

indium antimonide (InSb) LDRs (light dependent resistor) are used for the mid infrared spectral

region. Ge:Cu photoconductors are among the best far-infrared detectors available, and are used

for infrared astronomy and infrared spectroscopy.

TRANSISTOR

FEATURES

Low current (max. 100 mA)

Low voltage (max. 65 V).

APPLICATIONS

General purpose switching and amplification.

DESCRIPTION

NPN transistor in a TO-92; SOT54 plastic package.

PNP complements: BC548.

DIODES

There are a number of different electronic devices which tend to be called diodes. Although

they're made differently they all have three things in common.

• They have two leads like a resistor.

• The current they pass depends upon the voltage between the leads.

• They do not obey Ohm's law!

The function of a diode is to allow current in one direction and to block current in the opposite

direction. The terminals of a diode are called the anode and cathode.

Figure 1

The schematic symbol for a diode. When current flows in the direction of the triangle (anode to

cathode) then it is in forward bias Characteristics and Equations

Forward Voltage Drop

Electricity uses up a little energy pushing its way through the diode, rather like a person pushing

through a door with a spring. This means that there is a small voltage across a conducting diode.

It is called the 'forward voltage drop' and is about 0.7V for all normal diodes which are made

from silicon. The forward voltage drop of a diode is almost constant whatever the current passing

through the diode.

Reverse Voltage

When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a

very tiny current of a few µA or less. This can be ignored in most circuits because it will be very

much smaller than the current flowing in the forward direction. However, all diodes have a

maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and

pass a large current in the reverse direction; this is called 'breakdown'.

Zener Diodes

Zener diodes are used to maintain a fixed voltage. They are designed to 'breakdown' in a reliable

and non-destructive way so that they can be used in reverse to maintain a fixed voltage across

their terminals.

Light Emitting Diodes

Very similar to the basic diodes already talked about but with the added bonus that they light up

when in forward bias (and the voltage is high enough).

Figure 3 Schematic symbol of an LED.

Packages

Most diode packages have at least one indicator as to which leg is the anode and which is the

cathode. The cathode is usually marked by a band at one end and has the shorter of the two legs.

LED's will also have a flattened edge on the cathode side so you can still tell the difference easily

once you've chopped the legs off to solder them in!

Zener diode

A Zener diode is a type of diode that permits current not only in the forward direction like a

normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage

known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener,

who discovered this electrical property.

Current-voltage characteristic of a Zener diode with a breakdown voltage of 17 volt. Notice the

change of voltage scale between the forward biased (positive) direction and the reverse biased

(negative) direction.

A conventional solid-state diode will not allow significant current if it is reverse-biased below its

reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a

conventional diode is subject to high current due to avalanche breakdown. Unless this current is

limited by circuitry, the diode will be permanently damaged. In case of large forward bias

(current in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-

in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor

material and the doping concentrations.

A Zener diode exhibits almost the same properties, except the device is specially designed so as

to have a greatly reduced breakdown voltage, the so-called Zener voltage. By contrast with the

conventional device, a reverse-biased Zener diode will exhibit a controlled breakdown and allow

the current to keep the voltage across the Zener diode at the Zener voltage. For example, a diode

with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2 V if reverse bias

voltage applied across it is more than its Zener voltage. The Zener diode is therefore ideal for

applications such as the generation of a reference voltage (e.g. for an amplifier stage), or as a

voltage stabilizer for low-current applications.

The Zener diode's operation depends on the heavy doping of its p-n junction allowing electrons

to tunnel from the valence band of the p-type material to the conduction band of the n-type

material. In the atomic scale, this tunneling corresponds to the transport of valence band

electrons into the empty conduction band states; as a result of the reduced barrier between these

bands and high electric fields that are induced due to the relatively high levels of dopings on both

sides. The breakdown voltage can be controlled quite accurately in the doping process. While

tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%.

Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to 200

volts.

Another mechanism that produces a similar effect is the avalanche effect as in the avalanche

diode. The two types of diode are in fact constructed the same way and both effects are present

in diodes of this type. In silicon diodes up to about 5.6 volts, the Zener effect is the predominant

effect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche

effect becomes predominant and exhibits a positive temperature coefficient[1]. In a 5.6 V diode,

the two effects occur together and their temperature coefficients neatly cancel each other out,

thus the 5.6 V diode is the component of choice in temperature-critical applications. Modern

manufacturing techniques have produced devices with voltages lower than 5.6 V with negligible

temperature coefficients, but as higher voltage devices are encountered, the temperature

coefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode.

All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella term

of "Zener diode".

Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage

across small circuits. When connected in parallel with a variable voltage source so that it is

reverse biased, a Zener diode conducts when the voltage reaches the diode's reverse breakdown

voltage. From that point on, the relatively low impedance of the diode keeps the voltage across

the diode at that value.

LIGHT EMITTIG DIODE (LED)

INTRODUCTION

A light-emitting-diode lamp is a solid-state lamp that uses light-emitting diodes (LEDs) as the

source of light. Since the light output of individual light-emitting diodes is small compared to

incandescent and compact fluorescent lamps, multiple diodes are used together. LED lamps can

be made interchangeable with other types. Most LED lamps must also include internal circuits to

operate from standard AC voltage. LED lamps offer long life and high efficiency, but initial

costs are higher than those of fluorescent lamps.

Technology overview

General purpose lighting requires white light. LEDs by nature emit light in a very small band of

wavelengths, producing strongly colored light. The color is characteristic of the energy bandgap

of the semiconductor material used to make the LED. To create white light from LEDs requires

either mixing light from red, green, and blue LEDs, or using a phosphor to convert some of the

light to other colors.

The first method (RGB-LEDs) uses multiple LED chips each emitting a different wavelength in

close proximity to create the broad white light spectrum. The advantage of this method is the fact

that one can adjust the intensities of each LED to "tune" the character of the light emitted. The

major disadvantage is the high manufacturing cost, which is important in commercial success.

The second method, phosphor converted LEDs (pcLEDs) uses a single short wavelength LED

(usually blue or ultraviolet) in combination with a phosphor, which absorbs a portion of the blue

light and emits a broader spectrum of white light. (The mechanism is similar to the way a

fluorescent lamp produces white light from a UV-illuminated phosphor.) The major advantage

here is the low cost, while the disadvantage is the inability to fine tune the character of the light

without completely changing the phosphor layer. So while this will not yield high CRI (color

rendering index) values without sacrificing some other performance property, the low cost and

adequate performance makes it the most suitable technology for general lighting today.

To be useful as a light source for a room, a number of LEDs must be placed close together in a

lamp to add their illuminating effects. This is because an individual LED produces only a small

amount of light, thereby limiting its effectiveness as a replacement light source. If white LEDs

are used, their arrangement is not critical for color balance. When using the color-mixing

method, it is more difficult to generate equivalent brightness when compared to using white

LEDs in a similar lamp size. Furthermore, degradation of different LEDs at various times in a

color-mixed lamp can lead to an uneven color output. LED lamps usually consist of clusters of

LEDs in a housing with both driver electronics, a heat sink and optics.

Application

LED lamps are used for both general lighting and special purpose lighting. Where colored light

is required, LEDs come in multiple colors, which are produced without the need for filters. This

improves the energy efficiency over a white light source that generates all colors of light then

discards some of the visible energy in a filter.

White-light light-emitting diode lamps have the characteristics of long life expectancy and

relatively low energy consumption. The LED sources are compact, which gives flexibility in

designing lighting fixtures and good control over the distribution of light with small reflectors or

lenses. Due to the small size of LEDs, control of the spatial distribution of illumination is

extremely flexible,[1] and the light output and spatial distribution of a LED array can be

controlled without efficiency loss.

LED lamps have no glass tubes to break, and their internal parts are rigidly supported, making

them resistant to vibration and impact. With proper driver electronics design, an LED lamp can

be made dimmable over a wide range; there is no minimum current needed to sustain lamp

operation. LEDs using the color-mixing principle can produce a wide range of colors by

changing the proportions of light generated in each primary color. This allows full color mixing

in lamps with LEDs of different colors.[2] LED lamps contain no mercury.

However, some current models are not compatible with standard dimmers. It is not currently

economical to produce high levels of lighting. As a result, current LED screw-in light bulbs offer

either low levels of light at a moderate cost, or moderate levels of light at a high cost. In contrast

to other lighting technologies, LED light tends to be directional. This is a disadvantage for most

general lighting applications, but can be an advantage for spot or flood lighting.

The world's first mass-installation of LED lighting is in the Manapakkam, Chennai office of the

Indian IT company iGate.[3] It spent Rs. 37 lakh (U$80,000) to light up 57,000 sq feet of office

space. The company expects the LED lighting to completely pay for itself within 5 years.

In 2008, SSL (Solid-State Lighting) technology advanced to the point that Sentry Equipment

Corporation in Oconomowoc, Wisconsin, USA, was able to light its new factory almost entirely

with LEDs, both interior and exterior. Although the initial cost was three times more than a

traditional mixture of incandescent and fluorescent bulbs, the extra cost will be repaid within two

years from electricity savings, and the bulbs should not need replacement for 20 years.

CAPACITORS

INTRODUCTION

Capacitors store electric charge and have many uses. In series with a resistor they take time to

charge up and can be used in timing circuits. Since they take time to charge and discharge they

can also be used to smooth (slow the rate of change) signals. once they are full they block DC

currents so are used to filter signals and remove DC offsets. Capacitors are rated in Farads (F)

and this is a measure of how much charge they can store. The higher the rating the bigger the

charge resevoir. Usually larger capacitors (> 1&muF) are polarised and smaller ones (< 1&muF)

are un polarised - read on to find out what this means.

Unpolarised

Unpolarised capacitors don't mind which direction they are charged up from, the potential

difference across them can be in either direction.

Figure 1 - The schematic symbol of a capacitor, measured in Farads (F).

Polarised

Polarised capacitors have a positive and a negative connection, if connnected the wrong way

wround they will leak and often go pop! While not a huge disaster, it does make a mess you will

have to clear up and the fluids inside them can be quite nasty so be careful when using them.

Variable

You can also get variable capacitors which do exactly what they say on the tin! However unlike

resistors it is not always possible to reduce them to zero capacitance so they will usually specify

a minimum as well as a maximum capacitance value.

Figure 3- Schematic symbol of a variable capacitor

10.5. Characteristics and Equations

The charge that a capacitor can store is given by the equation:

Q=CV Equation 1

Where Q is the charge in coloumbs, C is the capacitance in Farads and V is the voltage in Volts.

The current in a capacitor (I) is equal to the capacitance multiplied by the voltage change with

time (dV/dt) :

I=CdV/dt

Equation 2

As can be seen above, if there is no change in the voltage then there's no current flowing,

because of this, in a DC circuit once a capacitor has reached its aiming voltage it will stop

conducting current until it has been discharged or a larger voltage is applied across it.

Because of the above characteristic a capacitor's impedance is related to the frequency of the

signal flowing though it:

ZC=-1/jωC

Equation 3

Where ω is the angular velociy and j is the complex conjugate showing phase shift, this shows

that the current in a capacitor is 90 degrees ahead of the voltage.

Figure 4 - Capacitors in Parallel

When capacitors are connected in parallel (figure 4) their combined capacitance is equal to the

individual capacitance added together. For example if capacitors C1 and C2 are connected in

series their combined resistance, C, is given by:

C=C1+C2

Equation 4

This can be extended for more capacitors:

C=C1+C2+C3+C4+...

DESCRIPTION

This circuit automatically turns on a night lamp when bedroom light is switched off. The lamp

remains ‘on’ until the light sensor senses daylight in the morning. A super-bright white LED is

used as the night lamp. It gives bright and cool light in the room. When the sensor detects the

daylight in the morning, a melodious morning alarm sounds. The circuit is powered from a 6V

DC supply. The circuit utilises light-dependant resistors (LDRs) for sensing darkness and light in

the room. The resistance of LDR is very high in darkness, which reduces to minimum when LDR

is fully illuminated. LDR1 detects darkness, while LDR2 detects light in the morning. The circuit

is designed around the opular timer IC NE555 (IC2), which is configured as a monostable. IC2 is

activated by a low pulse applied to its trigger pin 2. Once triggered, output pin 3 of IC2 goes

high and remains in that position until IC2 is triggered again at its pin 2. When LDR1 is

illuminated with ambient light in the room, its resistance remains low, which keeps trigger pin 2

of IC2 at a positive potential. As a result, output pin 3 of IC2 goes low and the white LED

remains off. As the illumination of LDR1’s sensitive window reduces, the resistance of the

device increases. In total darkness, the specified LDR has a resistance in excess of 280 kiloohms.

When the resistance of LDR1 increases, a short pulse is applied to trigger pin 2 of IC2 via

resistor R2 (150 kiloohms). This activates the monostable and its output goes high, causing the

white LED to glow. Low- value capacitor C2 maintains the monostable for continuous operation,

eliminating the timer effect. By increasing the value of C2, the ‘on’ time of the white LED can

be adjusted to a predetermined time. LDR2 and associated components generate the morning

alarm at dawn. LDR2 detects the ambient light in the room at sunrise and its resistance gradually

falls and transistor T1 starts conducting. When T1 conducts, melody-generator IC UM66 (IC3)

gets supply voltage from the emitter of T1 and it starts producing the melody. The musical tone

generated by IC3 is amplified by single-transistor amplifier T2. Resistor R7 limits the current to

IC3 and zener diode ZD limits the voltage to a safer level of 3.3 volts. The circuit can be

easily assembled on a general-purpose PCB. Enclose it in a good-quality plastic case with

provisions for LDR and LED. Use a reflective holder for white LED to get a spotlight effect for

reading. Place LDRs away from the white LED, preferably on the backside of the case, to avoid

unnecessary illumination. The speaker should be small so as to make the gadget compact.