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LIGHT DETECTORS

Page 1: Basic Electronics
              The capacitor - how it works ..
               The Diode - how the diode works
              Circuit Symbols - EVERY Circuit Symbol
              Soldering - videos -
              Light Detectors - this page
Page 2: The Transistor
              - PNP or NPN  Transistor TEST
Page 2a: The 555 IC 
                The 555 - 1
                The 555 - 2
                The 555 - 3
                The 555 TEST
Page 3: The Power Supply   download as .pdf (900kB)
            3a:   - Constant Current
            3b:   - Voltage Regulator
 Page 4: Digital  Electronics
            4a:   - Gates  Touch Switch Gating
Page 5: Oscillators
Page 6: Test - Basic Electronics (50 Questions)
Page 7: The Multimeter
Page 8: Constructing a Project
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LIGHT DETECTORS - Input Devices - Sensors
 There are many different types of light sensors - from the very sensitive - to the low-sensitivity types.
And every type has a range of sensitivity, depending on its construction.
So, you have to know which type to use and how to test them and how to adjust their sensitivity.
All light sensors are simply a variable resistor and form a VOLTAGE DIVIDER with the supply resistor and/or the sensitivity mini trim pot.
There are two things you need to know.
1. The level of brightness you want to detect.
2. And the brightness range. such as a very small change or a large change.
This could be from daylight to darkness or room-light to lack of room light.
And then you need to know the characteristics of the input.
This is how much the input needs to change to detect the two input values.
Does it detect the slightest change from a certain value to another value or does the value have to change a lot?
This gap is called the “hysteresis.”
A wide gap is best so you don’t have the circuit turning “on and off” at the critical lighting levels.
A microcontroller and a 555 IC and a Schmitt trigger IC has a wide gap.
A transistor has a narrow gap. And some digital “gates” or “blocks” have a narrow gap.

Interfacing is very easy.
Use any voltage from 3v to 12v.
Use a “stopper resistor” (current limiting resistor) of 1k to 2k2 and a 50k to 100k mini trim pot.
Place these in series to the supply and connect the two leads of your light detecting device between the 0v rail and the mini trim pot.
This is called a HIGH IMPEDANCE circuit and you can only measure its output voltage with a digital multimeter. An ordinary analogue multimeter will load the circuit and produce a lower reading.
Measure across the device and take voltage measurements.
Change the mini trim pot from low resistance to high resistance and take more readings.
For a microcontroller, the voltage must go below 1v to register a LOW
For a 555 and Schmitt trigger, the voltage must go below 33% and higher than 66%. To register a LOW and a HIGH.

Now you need to experiment and see what the mini trim pot does to the voltage. Do this before connecting the sensor to any circuitry.
If you don’t do this before connecting to a circuit, you will just get confused.
You wont know which way to turn the mini trim pot.
If the pot needs to be turned to very low resistance, something is wrong.
In dark conditions all the light sensing devices have a very high resistance.
The LDR (Light Dependent Resistor) can be fitted either way but a Photo Diode and Photo Transistor must be fitted around the correct way.
An LDR is also called a PE Cell. Photo Electric Cell.
Everything you are doing is based on the VOLTAGE DIVIDER CIRCUIT.
The voltage across each component in a series circuit depends on the resistance of each component. A high resistance has a higher voltage across.
When the sensor sees darkness, it has a high voltage across it.
This is because it has a high resistance.
But this high resistance is different for each type of sensor.
You have to see what happens via your experimenting. I cannot provide any figures.
Each sensor will change differently because they have different sensitivities. And the output voltage will change when the mini trim pot is adjusted. In most cases the mini trim pot will be in the mid-position.
This is called DETERMINING THE SENSITIVITY.

Diagram for light detector
When light is detected by any of the light detectors, the resistance decreases.
All light detectors are placed in a circuit with a CURRENT LIMITING RESISTOR in series with the detector.
This turns the circuit into a VOLTAGE DIVIDER ARRANGEMENT.
When the resistance of the detector decreases, the voltage across decreases.
When the resistance of the current limiting resistor is a high value, a very small current will flow in the combination.
Here’s the fact you have to understand.
When light is detected by the sensor, its resistance decreases and the voltage across it decreases because the voltage is directly connected to the resistance.
Each one-kilo-ohm of resistance develops say one volt across it

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