Date: 17 - 21/03/2014 (Monday - Friday)
Title: Research and Findings
In this week, the research is based on systems that using basic photoresistor circuit for received circuit are widely used around the world for different kind of purpose. They are used light or dark sensor circuit. Because of my project is based on detect laser the circuit LED will off, I focused on researching about DARK sensor circuit.
Summarize of my research:
Dark- Activated Switch
In this project, i will go over how to build a
dark-activated switch.
This is a switch that will turn on a circuit or a
device when it gets dark.
So when it is bright and there is a lot of light, the
switch does not activated. However, once it gets dark, it activates and it
turns on whatever device the circuit is wired to turn on.
For example, if i connect an LED to the
dark-activated switch, once it gets dark, the switch will trigger and turn on
the LED. i can make our circuit power on any load. However, usually it will be
a light source, because that's the most useful thing to turn on when it gets
dark.
This type of circuit has extreme use because when it
gets dark, it will automatically turn on lights. This is extremely useful for
people who may live in a house that have vision problems and need the lights to
automatically turn on when it gets dark. It's also very useful for elderly
people who live in a home who may have difficulty walking to turn on lights
when it becomes dark.
In our circuit, we will turn on a LED as soon as the
level of light reaches a certain level of darkness. Thus, the place where i place this circuit will always be illuminated, either by the natural light of
the day or by the lamp of this circuit turning on when it becomes dark.
Components Needed
· Photoresistor
· 100KΩ
Resistor
· 330Ω
Resistor
· Variable
resistor 5Kohm
· 2N3904
NPN Transistor
· LED
or Buzzer
· 2
AA Batteries or DC Power Supply
One of the main components we will use for this
circuit is a photoresistor. Photoresistors are also called light-dependent
resistors (LDRs). This can be obtained at tayda electronics at the following
link: Tayda
Electronics- Photoresistor. If you don't have a photoresistor, just make
sure to use one that has a dark resistance of 2MΩ or greater and a light
resistance of about 20-30KΩ.
A photoresistor is a resistor whose resistance changes
according to the amount of light that it is exposed to. When exposed to total
darkness, the photoresistor's resistance is very high. This is called its rated
dark resistance. For example, in our case we are using a 2MΩ photoresistor.
This means that when exposed to total darkness, its resistance will be around
2MΩ. As the photoresistor is exposed to increasing amounts of light, its
resistance begins to drop significantly. There is another rating on the
datasheet labeled cell resistance @ illuminance. This is the
resistance that the photoresistor will drop to when exposed to bright light,
typically 10 lux. It is also called the light resistance (since it is the
resistance the photoresistor will have when exposed to bright light). For our
photoresistor, in particular the cell resistance @ illuminance is 20-30kΩ. This
means the resistance of the photoresistor will drop to 20-30KΩ when exposed to
10 lux level of light.
So a
photoresistor is basically a device that gives off very high resistance at dark
light levels and low resistance at high light levels. Being that it does this,
it can act as a sensor for light, or a photosensor.
With the photoresistor acting as the light sensor, the
other significant component is the 2N3904 transistor. The transistor in this
circuit will act for a dual purpose, functioning as a switch and an amplifier.
When sufficient current is driven into the base of the transistor, it acts a
switch, turning the transistor on. With the transistor on, now current can flow
across from the emitter of the transistor to its collector to power the load
connected to the collector side of the transistor. This is how the transistor
acts as a switch. It also acts as an amplifier because when a small current
passes through the base, turning the transistor on, a much larger current goes
from the emitter to the collector. In this way, it acts as an amplifier so that
sufficient current can be produced to power on the load connected to the output
of the transistor.
The power we will use for this circuit is 3 volts or
higher, which can be obtained from 2 AA batteries connected in series or from a
DC power supply.
Dark-activated Switch Schematic
The
schematic of the dark-activated switch circuit we will build is shown below:
Figure: Photoresitor dark circuit
So, for this circuit, 3 volts or higher is powering the circuit.
This 3 volts is in parallel to a 100KΩ resistor and the photoresistor. In the
middle of these 2 components is connected the base of the NPN 2N3904
transistor.
This is how the circuit works:
When exposed to bright light, the photoresistor's
resistance is very low. It drops to around 20-30KΩ. Current travels through the
100KΩ resistor and then has 2 paths- it can either go through the base of the
transistor or go through the photoresistor. The base of the resistor to the
collector has a resistance of around 400KΩ. Current always takes the path of
least resistance. When the photoresistor is exposed to bright light, its
resistance is about 20-30KΩ, which is significantly less than the 400KΩ of
resistance the base of the transistor has. Therefore, most of the current will
go through the photoresistor and very little will go to the base of the
transistor. So the base of the transistor is bypassed. Thus, the transistor
does not receive enough current to turn on and power on the LED. Thus, the LED
is off when there is a lot of light in the surroundings.
However, when it begins to get dark, the
photoresistor's resistance becomes very high. Its resistance goes up to over
2MΩ of resistance. This creates a very high-resistance path. Being that 2MΩ is
significantly greater than the 400KΩ of resistance that the base of the
transistor offers, most of the current will go through the base of the
transistor. This means that current does not go through the photoresistor when
it is dark, due to this high resistance. Instead current goes through the 100KΩ
resistor and through the base of the transistor. The transistor receives enough
current to power on and turn on the LED connected to the collector terminal.
So this is how a dark-activated light circuit can
work.
Again, as always, variations of this circuit can be
done. Instead of using an LED, we can use any other type of lighting fixture.
You may want to use a lamp. You may want to use multiple lights, so you can
place different lights in parallel in one another. All you may have to do is
adjust the power settings.
Just because lights seem like the most practical and
useful component to place in a dark-activated switch doesn't mean you have to
use any lighting component at all. Maybe you want a buzzer to go off when it
gets dark or a fan to automatically go off when it gets dark or water
sprinklers to go off when it gets dark. Use any component which you need to
achieve the purpose of the circuit. It could be water sprinklers that go off at
night time. It could be some type of sound.
Example i Build circuit on Breadboard
 |
| Figure: When laser OFF LED ON |
Figure: When laser ON LED OFF
After the research, I came out with the block diagram for my system to work.
Figure
: Block Diagram
Optional Output
- Spot
light, CCTV, LCD.
The basic concept of this project is the Laser will
sent light from the laser circuit, then mirror will to do the reflection to the
receiver. Receiver will receive the light and process it but when the receiver
do not receive the light from laser the alarm will trigger. This project is the
combination of hardware (Laser and Receiver circuit), software (Programming C
or Assembly language in Arduino circuit) to control the delay of system.
