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.
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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