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Showing posts with label Phototransistor Detector. Show all posts
Showing posts with label Phototransistor Detector. Show all posts
8:23 PM
Here's a simple problem: "How do you make an LED turn on when it gets dark?" 
You might call it the "nightlight problem," but the same sort of question comes up in a lot of familiar situations-- emergency lights, street lights, silly computer keyboard backlights, and the list goes on.
Solutions? Lots. The time-honored tradition is to use a circuit with a CdS photoresistor, sometimes called a photocell or LDR, for "light-dependent resistor." Photoresistors are reliable and cost about $1 each, but are going away because they contain cadmium, a toxic heavy metal whose use is increasingly regulated.
There are many other solutions as well. Look here for some op-amp based photodetector circuits with LED output, and check out some of the tricks used in well-designed solar garden lights, which include gems like using the solar cell itself as the sensor.
In this article we show how to build a very simple-- perhaps even the simplest-- darkness-activated LED circuit. To our LED and battery we add just three components, which cost less than thirty cents altogether (and much less if you buy in bulk). You can build it in less than five minutes or less (much less with practice)
What can you do with such an inexpensive light-controlled LED circuit? Almost anything really. But, one fun application is to make LED throwies that turn themselves off in the daytime to save power. Throwies normally can last up to two weeks. Adding a light-level switch like this can significantly extend their lifetime.
Here are our components: On top: a CR2032 lithium coin cell (3 V). On the bottom (L-R): the LED, an LTR-4206E phototransistor, a 2N3904 transistor, and a 1 k resistor. This LED is red, blindingly bright at 60 candela, in a 10 mm package. It casts a visible beam, visible for about twenty feet in a well-lit room. We got the LEDs and batteries on eBay, and the other parts are from Digi-Key, but Mouser has them as well. As we mentioned, the last three cost about $0.30 all together, and much less in bulk.
The LTR-4206E is a phototransistor in a 3mm black package. The black package blocks visible light, so it is only sensitive to infrared light-- it sees sunlight and incandescent lights, but not fluorescent or (most) discharge lamps-- it really will come on at night.
Our starting point is the simplest LED circuit: that of the LED throwie, which has an LED driven directly from a 3V lithium coin cell. From this, we add on the phototransistor, which senses the presence of light, and we use its output to control the transistor, which turns the LED on.
The circuit diagram looks like this; please ignore the messy handwriting. ;)
When light falls on the phototransistor, it begins to conduct up to about 1.5 mA, which pulls down the voltage at the lower side of the resistor by 1.5 V, turning off the transistor, which turns off the LED. When it's dark, the transistor is able to conduct about 15 mA through the LED. So, the circuit uses only about 1/10 as much current while the LED is off. One thing to note about this circuit: We're using a red LED. That's because the voltage drop across the transistor allows less than the full 3 V across the LED. The full three volts is really only marginal for driving blue LEDs anyway, so two-point-something really doesn't cut it. (Might be able to work around that with a cheap FET-- haven't tried yet.)
And now, let's build it. You can certainly put this together on a breadboard, but there's something more satisfying about the compact and deployable build that we walk through here.
First get the transistor and the resistor. The pins of the 2N3904 are called (left-to-right) Emitter, Base, Collector, when viewing it from the front such that you can read the writing. We're going to solder the resistor between the leads of the Base and Collector of the transistor. Unusual part: hold the resistor with its leads at 90 degrees to those of the transistor while you solder.
Read more
Stay safe when you do this.
After soldering, clip off the excess resistor lead that is attached to the transistor base (middle pin), as well as the excess length of the collector pin.
Next, we add the phototransistor. Note that it has a flatted side, much like an LED does. This pin on that side is the collector of the phototransistor. Solder the collector (flatted side) to the middle pin (the base) of the transistor, again at 90 degrees. The other pin of the phototransistor, the emitter, is left unconnected for the moment. (Here is an alternate view of what that should look like when you're done.)
Finally, we need to add the LED. To do so, we need to know which side is the "positive," or anode side of the device. Regrettably markings of LEDs are not consistent, so the best way to be sure is to test it with the lithium coin cell-- put the LED across the terminals of the cell and, when it lights up, note which side is touching the (+) terminal. (Usually, it's the one with the longer lead.) Solder the "positive" lead of the LED to the emitter pin of the transistor-- it's the one on the left, which doesn't have anything soldered to it. Trim away the excess lead of the LED that goes past the solder joint. Solder the other pin of the LED (the "negative" pin, or cathode) to the emitter of the phototransistor, the pin on the non-flatted side, which does not have anything connected to it yet.
By this point, there are only two pins sticking down below the components: One that goes to the resistor and collector (rightmost pin) of the transistor, and one that goes to the emitter of the phototransistor and to the cathode of the LED.
To test the circuit, squeeze the coin cell between these two terminals, positive side goes to the lead touching the resistor. You can't see the LED on here because these photos were taken with incandescent lighting-- it wouldn't turn on.
Bending the leads to contact the lithium cell a little more reliably, you can try it out a little more easily. In the photo on the right, I cupped my hand over the circuit-- so the LED turned on.
To make this into an actual "throwie," you still need to add some tape and a magnet, but that's quite easily done. This one makes a pretty good nightlight attached to the top of a doorframe-- when the room lights are off, it shines a bright, bright spot on the ceiling.
Where to go from here? While this little circuit can do something on its own, it would probably also be happy as part of a larger circuit. At a minimum, note that if you work with batteries that have lower internal resistance than the lithium coin cells, you should place an appropriate resistor in series with the battery before trying to operate this circuit-- or else you may put too much current through the LED. Certainly, this is one of the easiest and least expensive ways to control an LED with a photosensor.
You might call it the "nightlight problem," but the same sort of question comes up in a lot of familiar situations-- emergency lights, street lights, silly computer keyboard backlights, and the list goes on.Solutions? Lots. The time-honored tradition is to use a circuit with a CdS photoresistor, sometimes called a photocell or LDR, for "light-dependent resistor." Photoresistors are reliable and cost about $1 each, but are going away because they contain cadmium, a toxic heavy metal whose use is increasingly regulated.
There are many other solutions as well. Look here for some op-amp based photodetector circuits with LED output, and check out some of the tricks used in well-designed solar garden lights, which include gems like using the solar cell itself as the sensor.
In this article we show how to build a very simple-- perhaps even the simplest-- darkness-activated LED circuit. To our LED and battery we add just three components, which cost less than thirty cents altogether (and much less if you buy in bulk). You can build it in less than five minutes or less (much less with practice)
What can you do with such an inexpensive light-controlled LED circuit? Almost anything really. But, one fun application is to make LED throwies that turn themselves off in the daytime to save power. Throwies normally can last up to two weeks. Adding a light-level switch like this can significantly extend their lifetime.
Here are our components: On top: a CR2032 lithium coin cell (3 V). On the bottom (L-R): the LED, an LTR-4206E phototransistor, a 2N3904 transistor, and a 1 k resistor. This LED is red, blindingly bright at 60 candela, in a 10 mm package. It casts a visible beam, visible for about twenty feet in a well-lit room. We got the LEDs and batteries on eBay, and the other parts are from Digi-Key, but Mouser has them as well. As we mentioned, the last three cost about $0.30 all together, and much less in bulk.
The LTR-4206E is a phototransistor in a 3mm black package. The black package blocks visible light, so it is only sensitive to infrared light-- it sees sunlight and incandescent lights, but not fluorescent or (most) discharge lamps-- it really will come on at night.
Our starting point is the simplest LED circuit: that of the LED throwie, which has an LED driven directly from a 3V lithium coin cell. From this, we add on the phototransistor, which senses the presence of light, and we use its output to control the transistor, which turns the LED on.
The circuit diagram looks like this; please ignore the messy handwriting. ;)
When light falls on the phototransistor, it begins to conduct up to about 1.5 mA, which pulls down the voltage at the lower side of the resistor by 1.5 V, turning off the transistor, which turns off the LED. When it's dark, the transistor is able to conduct about 15 mA through the LED. So, the circuit uses only about 1/10 as much current while the LED is off. One thing to note about this circuit: We're using a red LED. That's because the voltage drop across the transistor allows less than the full 3 V across the LED. The full three volts is really only marginal for driving blue LEDs anyway, so two-point-something really doesn't cut it. (Might be able to work around that with a cheap FET-- haven't tried yet.)
And now, let's build it. You can certainly put this together on a breadboard, but there's something more satisfying about the compact and deployable build that we walk through here.
First get the transistor and the resistor. The pins of the 2N3904 are called (left-to-right) Emitter, Base, Collector, when viewing it from the front such that you can read the writing. We're going to solder the resistor between the leads of the Base and Collector of the transistor. Unusual part: hold the resistor with its leads at 90 degrees to those of the transistor while you solder.
Read more
Stay safe when you do this.
After soldering, clip off the excess resistor lead that is attached to the transistor base (middle pin), as well as the excess length of the collector pin.
Next, we add the phototransistor. Note that it has a flatted side, much like an LED does. This pin on that side is the collector of the phototransistor. Solder the collector (flatted side) to the middle pin (the base) of the transistor, again at 90 degrees. The other pin of the phototransistor, the emitter, is left unconnected for the moment. (Here is an alternate view of what that should look like when you're done.)Finally, we need to add the LED. To do so, we need to know which side is the "positive," or anode side of the device. Regrettably markings of LEDs are not consistent, so the best way to be sure is to test it with the lithium coin cell-- put the LED across the terminals of the cell and, when it lights up, note which side is touching the (+) terminal. (Usually, it's the one with the longer lead.) Solder the "positive" lead of the LED to the emitter pin of the transistor-- it's the one on the left, which doesn't have anything soldered to it. Trim away the excess lead of the LED that goes past the solder joint. Solder the other pin of the LED (the "negative" pin, or cathode) to the emitter of the phototransistor, the pin on the non-flatted side, which does not have anything connected to it yet.
By this point, there are only two pins sticking down below the components: One that goes to the resistor and collector (rightmost pin) of the transistor, and one that goes to the emitter of the phototransistor and to the cathode of the LED.
To test the circuit, squeeze the coin cell between these two terminals, positive side goes to the lead touching the resistor. You can't see the LED on here because these photos were taken with incandescent lighting-- it wouldn't turn on.
Bending the leads to contact the lithium cell a little more reliably, you can try it out a little more easily. In the photo on the right, I cupped my hand over the circuit-- so the LED turned on.
To make this into an actual "throwie," you still need to add some tape and a magnet, but that's quite easily done. This one makes a pretty good nightlight attached to the top of a doorframe-- when the room lights are off, it shines a bright, bright spot on the ceiling.Where to go from here? While this little circuit can do something on its own, it would probably also be happy as part of a larger circuit. At a minimum, note that if you work with batteries that have lower internal resistance than the lithium coin cells, you should place an appropriate resistor in series with the battery before trying to operate this circuit-- or else you may put too much current through the LED. Certainly, this is one of the easiest and least expensive ways to control an LED with a photosensor.
8:56 PM
- The LEDs Are ON When The Phototransistors Are Dark -(The Outputs Are LOW When The Inputs Are HIGH)
The circuit on this page is for a visible and infrared light detector circuitboard that has 8 detectors. LM339 voltage comparators are the active element. These detectors can be used as part of other light detector circuits shown on other pages at this site such as these Light Activated Detector Circuits at this site.
A) Basic Inverting Detector Circuit
The following diagram shows the basic circuit on the Inverting circuitboard.
Selecting A Value For The Input Resistor (R1)
The value of resistor R1 depends on the type of sensor and the desired sensitivity. See below for more details.
For phototransistors a value of 470K ohms will work for most room light situations. If the light is dim, selecting a higher value resistor such as 1 Megohm will give better sensitivity. This High Impedance Test Voltmeter circuit can also be used for testing phototransistors installations.
For CdS photocells it is usually best to install the cell and then measure its resistance under the normal lighting conditions. A resistor with a value that is 3 to 5 times the measured resistance of the cell is then selected for R1.
Selecting A Value For The Output Resistors (R4)
The value of resistor R4 is chosen to give a desired current flow though the LEDs See below for more details.
A 1K ohm resistor will allow about 10 milliamps to flow through a typical LED if the supply voltage is 12 volts. The value of the resistors at the outputs of the comparators can changed depending on the desired current through the LEDs.
B) 8 - Photo-Detector PCB Circuit
The following diagram shows the circuit that is on the printed circuit board. There are 8 independent photo-detectors with open collector outputs that can sink up to 15 milliamps each.
Circuit Notes
i) All of the comparators on the PCB are wired so that when the photosensors are dark, the output of the comparators will be LOW and the LEDs will be ON.
ii) The detection voltage level for the circuit as shown is set at 1/2 of the supply voltage. If a lower or higher detection level voltage is needed, the values of resistors R9 / R10 and R19 / R20 can be adjusted.
iii)This circuit does not need a regulated power supply and can operate on supply voltages of up to 32 volts.
iv)The 1K output resistors can be replaced by jumper wires if they are not needed such as for inputs to control or signals circuits that have their own current limiting resistors.
v) WARNING - If the polarity of the power supply for this circuit is reversed or the circuit is connected to an AC or DC source this circuit will be damaged. The maximum supply voltage for this circuit is 15 Volts.
Please go here to see the full model!
The circuit on this page is for a visible and infrared light detector circuitboard that has 8 detectors. LM339 voltage comparators are the active element. These detectors can be used as part of other light detector circuits shown on other pages at this site such as these Light Activated Detector Circuits at this site.
A) Basic Inverting Detector Circuit
The following diagram shows the basic circuit on the Inverting circuitboard.
Selecting A Value For The Input Resistor (R1)
The value of resistor R1 depends on the type of sensor and the desired sensitivity. See below for more details.
For phototransistors a value of 470K ohms will work for most room light situations. If the light is dim, selecting a higher value resistor such as 1 Megohm will give better sensitivity. This High Impedance Test Voltmeter circuit can also be used for testing phototransistors installations.
For CdS photocells it is usually best to install the cell and then measure its resistance under the normal lighting conditions. A resistor with a value that is 3 to 5 times the measured resistance of the cell is then selected for R1.
Selecting A Value For The Output Resistors (R4)
The value of resistor R4 is chosen to give a desired current flow though the LEDs See below for more details.
A 1K ohm resistor will allow about 10 milliamps to flow through a typical LED if the supply voltage is 12 volts. The value of the resistors at the outputs of the comparators can changed depending on the desired current through the LEDs.
B) 8 - Photo-Detector PCB Circuit
The following diagram shows the circuit that is on the printed circuit board. There are 8 independent photo-detectors with open collector outputs that can sink up to 15 milliamps each.
Circuit Notes
i) All of the comparators on the PCB are wired so that when the photosensors are dark, the output of the comparators will be LOW and the LEDs will be ON.
ii) The detection voltage level for the circuit as shown is set at 1/2 of the supply voltage. If a lower or higher detection level voltage is needed, the values of resistors R9 / R10 and R19 / R20 can be adjusted.
iii)This circuit does not need a regulated power supply and can operate on supply voltages of up to 32 volts.
iv)The 1K output resistors can be replaced by jumper wires if they are not needed such as for inputs to control or signals circuits that have their own current limiting resistors.
v) WARNING - If the polarity of the power supply for this circuit is reversed or the circuit is connected to an AC or DC source this circuit will be damaged. The maximum supply voltage for this circuit is 15 Volts.
Please go here to see the full model!
8:02 PM
For this Section, Please read Infrared Light Photo-Detector Circuit First!
A Practical Quad Photo-Detector Circuit
The next circuit is for a practical 4 photo-detector circuit using an LM339 Quad comparator IC. Although phototransistors are shown, photocells could also be used with the corresponding change in values for resistors R1 through R4.
This circuit can also be used for infrared detector circuits as shown HERE!
The values for resistors R7 through R10 can also be changed depending on the required LED current.
A Practical Quad Photo-Detector Circuit
The next circuit is for a practical 4 photo-detector circuit using an LM339 Quad comparator IC. Although phototransistors are shown, photocells could also be used with the corresponding change in values for resistors R1 through R4.
This circuit can also be used for infrared detector circuits as shown HERE!
The values for resistors R7 through R10 can also be changed depending on the required LED current.
7:49 PM
For this Section, Please read Infrared Light Photo-Detector Circuit First!
Basic Phototransistor Detector
In this circuit the light falling on the phototransistor will be from an Infrared Light Emitting Diode (IrLED) but otherwise it is the same as the phototransistor circuit shown above.
When the light falling on the phototransistor (Q1) is blocked, its conductance will decrease and the voltage across Q1 will rise. When the voltage rises above 1/2 of the supply voltage the output of the comparator will turn ON and the LED will be lit.
Basic Phototransistor Detector
In this circuit the light falling on the phototransistor will be from an Infrared Light Emitting Diode (IrLED) but otherwise it is the same as the phototransistor circuit shown above.
When the light falling on the phototransistor (Q1) is blocked, its conductance will decrease and the voltage across Q1 will rise. When the voltage rises above 1/2 of the supply voltage the output of the comparator will turn ON and the LED will be lit.
7:11 PM
Basic Visible and Infrared Light Detectors
This page features basic, visible light photo-detector circuits that can be used to detect trains or other light blocking objects.
The sensors used for these circuits are silicon phototransistors or Cadmium Sulfide (CdS) photocells. Both of these sensors allow less current to flow when they are dark. (Phototransistors change their 'conductance' while photocells change their resistance depending on the intensity of the light falling on them.)
The phototransistor or photocell would normally be placed between the rails in the circuits on this page.
The Photo-detectors on this page use LM339 (Quad) or LM393 (Dual) voltage comparator, integrated circuits to detect the change in voltage across the sensor.
All of the circuits on this page are configured to have the LED's turn on when the sensor element is dark (covered by a train.) The LED's can also be made to turn off when a train is detected. This will be explained in the NOTES sections of this page.
The supply voltage for the circuits is specified as regulated 12 volts DC but this can be changed if needed. In some cases the values of some resistors may have to be adjusted to compensate.
Visible Light Photo-Detector Circuits
A) Basic Phototransistor Detector
In this circuit, when the light falling on the phototransistor (Q1) is blocked, its conductance will decrease and the voltage across Q1 will rise. When the voltage rises above 1/2 of the supply voltage the output of the comparator will turn ON and the LED will be lit.
The only critical part of this circuit is the value of resistor R1 which in most cases can be 470K ohms but may have to be increase if the room is dark or decreased if the room is well lit.
Increasing the value of R1 will cause the sensitivity of the sensor to decrease. This may be necessary when the light falling on the cell is not very strong or shadows can affect the phototransistor.
There are a number of phototransistors sizes and case styles. The smaller cases will be easier to hide but connecting wires may be more difficult.
B) Basic CdS Photocell Detector
In this circuit, when the light falling on the photocell (PC 1) is blocked, its resistance will increase and the voltage across PC 1 will rise. When the voltage rises above 1/2 of the supply voltage the output of the comparator will turn ON and the LED will be lit.
Due to wide variations in CdS photocells it is usually best to install the cell and then measure its resistance under normal lighting conditions. A resistor with a value that is approximately 3 to 5 times the measured resistance of the cell is then selected for R1. For example; If the cell resistance is measured at 400 ohms then a 1200 to 2200 ohms resistor would be used.
Increasing the value of R1 will cause the sensitivity of the sensor to decrease. This may be necessary when the light falling on the cell is not very strong or shadows can affect the photocell.
This circuit can be adapted for use in dark areas by placing a small light above the photocell.
This page features basic, visible light photo-detector circuits that can be used to detect trains or other light blocking objects.
The sensors used for these circuits are silicon phototransistors or Cadmium Sulfide (CdS) photocells. Both of these sensors allow less current to flow when they are dark. (Phototransistors change their 'conductance' while photocells change their resistance depending on the intensity of the light falling on them.)
The phototransistor or photocell would normally be placed between the rails in the circuits on this page.
The Photo-detectors on this page use LM339 (Quad) or LM393 (Dual) voltage comparator, integrated circuits to detect the change in voltage across the sensor.
All of the circuits on this page are configured to have the LED's turn on when the sensor element is dark (covered by a train.) The LED's can also be made to turn off when a train is detected. This will be explained in the NOTES sections of this page.
The supply voltage for the circuits is specified as regulated 12 volts DC but this can be changed if needed. In some cases the values of some resistors may have to be adjusted to compensate.
Visible Light Photo-Detector Circuits
A) Basic Phototransistor Detector
In this circuit, when the light falling on the phototransistor (Q1) is blocked, its conductance will decrease and the voltage across Q1 will rise. When the voltage rises above 1/2 of the supply voltage the output of the comparator will turn ON and the LED will be lit.
The only critical part of this circuit is the value of resistor R1 which in most cases can be 470K ohms but may have to be increase if the room is dark or decreased if the room is well lit.
Increasing the value of R1 will cause the sensitivity of the sensor to decrease. This may be necessary when the light falling on the cell is not very strong or shadows can affect the phototransistor.
There are a number of phototransistors sizes and case styles. The smaller cases will be easier to hide but connecting wires may be more difficult.
B) Basic CdS Photocell Detector
In this circuit, when the light falling on the photocell (PC 1) is blocked, its resistance will increase and the voltage across PC 1 will rise. When the voltage rises above 1/2 of the supply voltage the output of the comparator will turn ON and the LED will be lit.
Due to wide variations in CdS photocells it is usually best to install the cell and then measure its resistance under normal lighting conditions. A resistor with a value that is approximately 3 to 5 times the measured resistance of the cell is then selected for R1. For example; If the cell resistance is measured at 400 ohms then a 1200 to 2200 ohms resistor would be used.Increasing the value of R1 will cause the sensitivity of the sensor to decrease. This may be necessary when the light falling on the cell is not very strong or shadows can affect the photocell.
This circuit can be adapted for use in dark areas by placing a small light above the photocell.
