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Questions tagged [phototransistor]

Phototransistors are in effect a bipolar transistor with a transparent enclosure so light can reach the junction of the base and collector. Compared to a photodiode, they offer increased gain at the cost of a slower response time.

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How to find polarity of photo interrupter

I have scavenged some photo interrupters from multiple printers but I am trying to figure out the polarity and the voltage. I've tried searching up photo interrupter polarity but couldn't find ...
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60 views

Why did Adafruit pick a 10K resistor for this phototransistor breakout?

Adafruit has an ALS-PT19 Analog Light Sensor Breakout. It looks like it has a 10K resistor on it: The ALS-PT19 datasheet says that resistor is to convert the output current of the transistor to ...
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27 views

How do I determine the capacitor to use when converting current to voltage?

I'm trying to figure out how to use an ALS-PT19-315C/L177/TR8 phototransistor (or something like it). The datasheet has this sample circuit for converting current to voltage so that I can measure it ...
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45 views

How do current and voltage relate to load?

I think I've lost the plot somewhat on how current and voltage relate to the load in a circuit and I'm hoping for some really basic level clarification. In fact, I'm not really sure how to ask the ...
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How to connect photo-transistors to trigger Arduino event

I'm busy doing my finals project, and I need to have 5 photo-transistors come together on the same pin to trigger an event whenever one of these transistors drops it's voltage because an object is ...
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102 views

Can a series resistor be used to limit the power consumption of a phototransistor?

I’m hoping to use several QRD1113 IR proximity sensors in a device, and they work well but have higher power consumption than I would like. I’ve tested many other ones, but I haven’t found another ...
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35 views

Multiple phototransistors one OR output

I am trying to wrap my mind around a design for a long light sensor bar. This is actually a laser sensor that needs to sense a moving laser beam continuously over a certain distance. I would need this ...
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53 views

Photodiode temperature compensation

The goal is to design a circuit that can accurately measure light intensity with little temperature dependency. Common application uses photodiodes to do so. Photodiodes can be measured either in ...
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49 views

Do NPN and PNP phototransistor switch complementarily?

I am trying to understand whether there is a difference in the switching characteristics of NPN and PNP phototransistors. My understanding is: NPN: current flows when light is incident, no current ...
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Lowest resistor value for enclosed light sensor unit with nearby lighting without losing sensitivity

This is a drawing I made of a light sensor unit I'm creating. Inside of it, I have a circuit board containing 3 7-segment displays and I have two photo-transistors (yellow half-circles in the picture) ...
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55 views

Sensor range and speed based on resistor

I have 4 of these sensor circuits and can't quite figure out the optimal resistor value to use. Here's what I assume (which somewhere I may be wrong): The datasheet for the IC states leakage current ...
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75 views

How to shield ambient light phototransistor from LED

My design requires a surface mount ambient light sensing phototransistor and a surface mount LED. The phototransistor's purpose is to sense the room's light level, and I don't want the LED light to be ...
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Can sending correct data via UART qualify as valid manchester encoding?

I'm currently in the process of having a laser beam detected by the phototransistor. While the parts in themselves work, I want to detect when the beam has been detected. I understand that a 38Khz ...
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96 views

Transistors with different loads in parallel

My circuit roughly looks like this: I want to send a signal to uC and turn on a LED when there is light shining on phototransistor Q3. Currently I'm using stm32f4 (discovery board) as a signal ...
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81 views

Add tilt switch to this photodiode circuit

I'd like to take this phototransistor switch circuit: And add a tilt-ball or similar switch in series. That switch needs to be heavily debounced, so that it stays "switched" more or less for several ...
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46 views

calculating maximum sense speed of amplified phototransistor circuit

I made the following photo-transistor circuit to sense light with the exact parts shown. Resistors are 5% tolerance 1/4 watt. The circuit does work, but what I would like to know is how do I calculate ...
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58 views

Determining collector-emitter voltage versus collector current for a phototransistor

I'm using a Vishay TEPT4400 phototransistor (datasheet) and want to design a circuit that gives an output (Vce) of 1V at 200 lux and 5V at 0 lux. I'm using a simple common-emitter configuration , but ...
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66 views

LED as transmitter and photodetector in the same circuit?

As in this answer I plan to use a 5V neopixel RGB LED as both transmitter and receiver to transmit information between ATMega328 boards via fibre optic cable. (This is the Forrest Mimms strategy, for ...
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103 views

Replacing obsolete photodarlington with phototransistor

I've got an old circuit design I need to replicate, function wise, to fit into an existing larger design. Logic I can usually handle, anything with transistors starts to get me lost. My strength is ...
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57 views

covering sides of LEDS without adding heat

I'm doing a project in which a 5mm LED is near a 5mm phototransistor. There are times in which the sensor must pick up external light even with the LED on. I understand one option is to turn the LEDs ...
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165 views

Photo-diode for a high-frequency tachometer

I intend to build a tachometer, which is capable of detecting rotations up to 1kHz (60 000 RPM). From the Internet I know that this is possible to do with a magnet and a Hall detector. However, I ...
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51 views

Is the LED-like device in the front of an Arduino flame sensor a photo transistor?

This is a common Arduino flame sensor found in the Elegoo 37 sensors kit V2. Is the led in front of it a photo transitor?
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307 views

How to calibrate color sensor to sense RGB colors on the scale of 0-255 if the color source distance varies from 1-25 cm?

I am currently using a TCS3200 RGB color sensor but the sensed values for same color varies with the distance between sensor and color source. How should I calibrate my sensor so as to get values ...
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454 views

How to transmit data with a laser beam? [closed]

I am working on a project about data transmission with lasers. The project is very young so I can modify my current planned setup if necessary. This is the actual planned setup: Every raspberryPi ...
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1answer
92 views

Saturating BJT with phototransistor

simulate this circuit – Schematic created using CircuitLab I'm trying to get a phototransistor to saturate a BJT so that when a small amount of light is present, the BJT is on. At the moment, ...
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Optical audio transmitter can't handle speech

This is a question relating to my project on an optical audio transmitter. I wired up a very simple device in which I use an audio signal transformer (EI14) to modulate the intensity of a laser beam (...
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103 views

Diode in 6N138 optocoupler

I'm currently in the process of getting MIDI input to my Raspberry PI through UART, and in order to follow the standards, I should optically insulate the MIDI cable from the RPI by using an ...
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104 views

Unusual harmonic data reading from phototransistor in a simple arduino setup

I have a very simple circuit for detecting ambient light readings with an arduino, but I'm noticing a sinusoidal pattern to the readings I'm getting that are hard to pin down the source of. How do I ...
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435 views

Why are most IR phototransistors slow?

Most NPN silicon phototransistors that I see (Everlight, Kingbright, etc.) have fairly poor rise and fall times, on the order of 10us. The best one I've seen so far is the QT Brightek QSD8T120B with ...
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127 views

Decrease the time it takes to transition from saturation to cutoff of a 2 terminal phototransistor?

Consider a two terminal phototransistor (collector and emitter) that is saturated and the light source is then removed. What can one do to speed the transition from saturation to cutoff ? Edit: ...
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152 views

Phototransistor with digital potentiometer

As per the circuit diagram below, I'm trying to use a phototransistor with a digital potentiometer so that the sensitivity can be adjusted. PH_TR connects to an ...
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214 views

Optocoupled MOSFET drive

I have a high power 12V rail that I would like to keep separate from the logic supply, and so would like to optocouple the N-MOSFET drive for an external high power LED light. Does this seem like a ...
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81 views

Do phototriac optoisolators (e.g. MOC3021) require pulse trains?

I have a project where I need to make a microcontroller trigger an old, non-controlled, camera flash (think of discharging a capacitor through a xenon lamp). First idea was use a transistor-based ...
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116 views

How to allow current over a certain voltage

I am new on electronics and i was practicing on transistors. I have a working circuit below. The LDR I use allows current and letting LED smoothly based up to amount of darkness. The Voltage after 2.3 ...
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548 views

In a photo diode in which region light should be focused to fall?

In this question correct answer given is option \$(D)\$ that means in a photo diode light should be focused to fall in whole P and N region including the deplection region but when I googled this ...
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190 views

Photodiodes over Phototransistors

Current solar panels being manufactured are not considered efficient. I have also read articles saying there is talk or even ongoing research about four P-N layers (i.e: PNPN or vice versa) for solar ...
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101 views

Safe to open BJT?

I am experimenting with optically controlling BJTs. In my current project, I take transistors in metal cans and use a diamond saw lubricated with isopropyl alcohol to slice the top off. This generates ...
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198 views

phototransistor amplifier quirk

As an amateur to breadboarding I'd been learning about transimpedance amps and photodiodes earlier this year for a project. Eventually I picked up a phototransistor (3DU5C) at a hobby shop, mistaking ...
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Match phototransistor to LED source

Phototransistors are a new tool for me, and I'm wondering how to select one that will match a specified source (LED worklight, color temp 5000K, wavelength unspecified). My prototype works but ...
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115 views

How to get the RGB values from a color sensor which changes color from yellow, green, grey, purple?

I am analysing a color sensor which will be immersed in water and the color of the sensor changes from yellow, green, purple when the chemical properties of water changes. Precautions to get better ...
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139 views

how to identify mystery phototransistor

I've recently torn down and old printer as a learning experience and to maybe build something kinda fun out of it. As many will know that have taken apart printers, most newer ones have ceased using ...
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3answers
167 views

Clean water turbidity sensor

I'm working on a project to determine the water clarity, using a MCU with a turbidity sensor. I have tested sensors such as: Cheap Dishwasher sensor, and while it works for changes in turbidity in ...
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1answer
505 views

PWM on phototransistor

I am pulsing the VDD with 50% duty cycle at a frequency of 188 Hz (PWM). I get some weird spikes on my output TP3. Am I missing something about phototransistors? Here is the picture of the IR circuit ...
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318 views

Weird behavior of phototransistor

Tonight I debugged a weird issue with a simple IR LED/phototransistor circuit that I was meaning to integrate in a RPM sensor. With the IR LED pointed towards the phototransistor, with a gap of 1-2cm ...
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My teacher said a BJT phototransistor doesn't have a base

But I find it to be absolutely incorrect. How can a BJT transistor even work without a base? Shouldn't a transistor without base be just a semiconductor (PP, NN)? Does there exist some special NASA-...
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2answers
398 views

Theory and question on using a photo interrupter as a switch [closed]

Edit, answer: I managed to figure it out with help from passerby's answer. I needed a pull down resistor for the PNP transistor. Here's the final schematic: When the beam of the photo interrupter is ...
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201 views

increasing laser beam without increasing power (and possibly breaking law)

I bought some laser diodes on ebay recently that were advertised as 5mw which look like the image shown: To comply with law, I hooked it up to a 9V battery with a 22K resistor. I made a simple ...
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276 views

How to approximate the behavior of a phototransistor in the circuit editor?

While phototransistors are often used in this common turbidity sensor application (at least in part) because of their gain, their response is not linear since the gain will depend on the intensity of ...
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138 views

i need to control rgb leds brightness using phototransistor plus transistor and at the same time send light value to analog pin on Arduino

Schematic is missing led resistor. i need to control rgb leds brightness using phototransistor plus transistor and at the same time send light value to analog pin on .I'm not sure where do I go from ...
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364 views

How to use a Phototransistor and photodiode?

I don't expect my parents to spend the money required to get a ballistics chronometer for me so I decided it would be easier to make one from parts laying around in my room. I have the general plans ...