INDEX
Page 51 
Photo Devices 
The devices covered in this discussion are as follows: They are shown in approximate increasing sensitivity. The Photo Cell is the least sensitive while the Photo Darlington is the most sensitive:
The Photo Cell - (Photo resistor - LDR)
The LDR (Light Dependent Resistor - Photo Cell)
The Photovoltaic Cell - solar cell - produces an output voltage
The Photo Diode
The Photo Transistor
The Light Activated SCR
The Photo FET 
The Photo Darlington Transistor


All the Photo Devices in the Circuit Symbols Library will be described together in this article because they all do the same thing - detect light. The diagram below shows the symbols of the devices we will cover:

All Photo devices operate very similar to a variable resistor. When they are receiving no illumination, they have a HIGH resistance. When the illumination is bright, they exhibit a LOW resistance. 
The diagram below shows how the circuit sees the Photo device. The output is taken between the device and the load resistor. The load resistor can also be called a "stop" resistor or "limit" resistor as it limits the current taken by the device when it is detecting high illumination. 
Note: the output is shown to "fall" as the brightness increases. This means the output voltage drops as the illumination increases and must be detected by the next stage in the circuit.  See Page 2 for for details.  

Note: The multimeter is connected correctly because the negative 
lead of the multimeter is connected to the positive of the battery inside the meter. 

TWO CONDITIONS
Photo devices basically operate under two different conditions. Condition-one is a change from dark to light (or light to dark.  Condition-two is a change from medium illumination to brighter illumination (or brighter to darker).  
For the first condition, when the Photo device is connected to the multimeter, the needle will barely move when it is in darkness and produce almost full-scale deflection in bright light. In the second condition, the needle will sit somewhere on the scale and move only a very small amount. 
It will move very little for an LDR. It will produce a larger movement for the Photo transistor and even more movement for the Photo Darlington transistor. 
Depending on the amount of movement, you have the choice of using an expensive high-sensitivity device or a low-sensitivity device with additional gain being provided by an amplifying stage.  
Sometimes the high-sensitivity device is cheaper than a low-sensitivity device, so this will also influence your decision. The end-result will depend on the suitability; the availability of the component and the cost.
All the devices above produce no output voltage. They merely change in resistance when light is detected. The only device that produces a voltage when light is detected is the Photovoltaic Cell. It is a solar cell and produces both a voltage and current when exposed to light. As the light intensity increases, the voltage increases but the most important feature is the current. When both a voltage and current are produced, the cell is said to be capable of delivering ENERGY. 
Individual solar cells can have a voltage output from 0.45v or greater (actually almost any voltage). In actual fact, a number of pieces of semi-conductor material are connected together in series/parallel within the cell, to produce the output voltage and current. 
Cells can also produce current outputs from a few milliamps to hundreds of milliamps. This is due to the connection of many pieces of semi-conductor within the device.
The characteristics of each solar cell is provided at the time of purchase. 
A solar cell does not produce very much output (current output) under room illumination and they are fairly large in size. This may influence your decision for its use as a detector.

WHICH TYPE DO I USE?
Due to the general lack of Photo devices in hobby outlets, the circuit you choose will depend on the types available. For this reason we cannot provide particular type numbers or specifications. All the circuits in our discussion will be of a general nature. 
Sometimes it is cheaper to get a "hobby pack" of assorted types or go to a surplus outlet, as some original types are very expensive. 
These notes are designed to help you recognise "photo" transistors and diodes and show  typical circuits for their use. 
One point to note is the number of leads on a photo device.  Both photo-transistors and photo-diodes can have two leads as the base of a photo transistor is not connected to an outside circuit and thus it is not brought out of the package. This makes it extremely difficult to tell the difference between a photo diode and a photo transistor. In fact it is impossible without testing its performance.
A photo transistor will be between 10 -  100 times more sensitive than a photo diode. 

THE PHOTO TRANSISTOR
The Photo transistor is an ordinary transistor in a clear plastic case. If the case is dark, the transistor will respond to infra-red light only - otherwise the device can/will respond to both ordinary light and infrared light. 
All transistors will respond to light and that is why they have to be in a light-tight case if you don't want them to respond to light. The silicon material making up the junction of the transistor changes resistance when it sees the energy from a light-source and this change is detected between the collector and emitter leads. 
Sometimes a second transistor is also mounted inside the case and it amplifies the change in resistance to produce a more-sensitive device. The combination is called a PHOTO-DARLINGTON TRANSISTOR.  
Two less-sensitive devices are also available. They are a PHOTO-DIODE and PHOTO-RESISTOR (also called a LIGHT-DEPENDENT RESISTOR - LDR). 
Some devices detect visible light while others detect infra-red. 
Getting a photo-detecting circuit to work requires a little bit of experimentation. 
You need to determine if the device is sensitive enough for the application and then adjust the circuit for the lighting conditions.

The circuit above can be adjusted so that almost any light-level can be detected. When light is detected by the MEL-12 photo-transistor, its resistance DECREASES. This means the voltage-level sitting on the left-side of the 10u electrolytic is brought down towards the 0v rail. This causes the right-side of the electro to fall. The result is the voltage-level on the base of the first transistor is REDUCED. This turns the transistor OFF. The second transistor is turned ON and the voltage at point "x" changes from a HIGH to a LOW. This voltage swing is passed to the CD 4011 NAND gate to operate a further circuit. 
The 50k mini-trim pot changes the gain of the photo-transistor. It makes the transistor  more-sensitive as the resistance of the pot is decreased. The result is the voltage-fall on the left-side of the 10u electrolytic is greater for any given change in light and this means a very small change in light-level will cause the circuit to operate. 
The circuit will only detect AN INCREASE IN LIGHT. When the light-source is removed or reduced, the circuit does not respond. 
The circuit below is designed to detect when the light-source turns OFF:

In the following circuit, the A-to-D stage and 4011 NAND gate have been replaced by a Schmitt Trigger.
The MEL-12 Darlington Photo-transistor is connected directly to the input of the Schmitt gate and the circuit can be adjusted via the 50k mini-trim pot.

When the MEL-12 receives light, the  voltage on the input of the Schmitt gate drops from a HIGH value to a LOW value. For the circuit to work, the LOW value must be 33% of rail voltage or less. 
When a light-source is removed from a MEL-12, the voltage on the input of the Schmitt gate rises to a HIGH value. For the gate to change state, the voltage must rise to 66% of rail voltage or higher. 
The animation below shows how the mini-trim pot changes the gain of the photo-transistor.

If the change in light intensity is not sufficient to operate the Schmitt gate, you can add the A-to-D stage described above and the 10u electrolytic.

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