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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What resistance should be used with a phototransistor to have fast rise/fall time?

I am trying to accurately capture the video latency and response time curve of an LCD display from a black image to a white image for the purpose of reviewing the accuracy of a couple of video latency ...
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How to choose a phototransistor or photodiode?

I've found a photoresistor (yes, resistor, probably 30 years old) in my scrap box, attached it to an ATtiny as ambient light sensor, with a 100kΩ resistor as voltage divider towards +5V. On power on, ...
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Ambient light sensor to a microcontroller

I am looking to make a small sensor board for something at home. I am familiar with embedded systems but my electronics knowledge may not be the best. For my situation, I would like to connect a ...
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11 votes
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Triggering generic transistors with UV or green light?

Flashes of UV light can create electron-hole pairs in transistors. Or at least reports of the "UV flash bug" claim this happens in microprocessors. People observed raspberry pies reset when ...
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What does the VCC pin do in this Optocoupler?

I have this optocoupler, and I am trying to figure out how it works. What is the VCC pin supposed to do? How does it supply current to the gate pin of the transistor? https://www.digikey.com/en/...
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Does a phototransistor have a minimum focal window? - Velocity Measurement of High Speed Object

My goal is to measure the velocity of a moving object that has several different changes in velocity throughout the actuation. I am planning on using a phototransistor to detect black and white lines ...
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Optocoupler circuit to shift from 3.3V to 15V

I am trying to use the circuit below to shift a squared wave form (2kHz) with a peak of 3.3VDC to one with a peak at 15VDC. I am trying to do that with the circuit below, I am using the optocoupler ...
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Problem in sensor circuit using a Phototransistor for the Schmitt Trigger circuit

This is my Schmitt Trigger design, which trips at \$2.3V\$ and \$1.7V\$- Here, $$+V_{cc} = 9V$$ $$R1 = 51k \Omega$$ $$R2 = 22k \Omega$$ $$R3 = 330k \Omega$$ \$V_{in}\$ is a square-wave with 0-7V peak ...
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Help identifying 3-pin open-collector photo-transistor-like component

Can someone help identify the 3 pin photo device (not the entire module) on this "Icstation 5V Light Sensor Module"? (may be same as this one It seems to act as a photo-transistor, but yet ...
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Amplifying a small signal from a phototransistor

I am building an amplifier for a phototransistor. It works well under normal lighting conditions. In my application, the light is very dim and barely produce any readable output. I tried to increase ...
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Convert flame sensor ADC value to wavelength

I am working on a fire detector project using a flame sensor. This sensor can detect IR light wavelength from 760-1100 nm using a YG1006 phototransistor. I have read that if the wavelength is bigger ...
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BC639 16 exploding on custom design

On a board I designed, a transistor, BC639 16, is exploding. A pinball machine has one board that has some design flaws. Its schematics are these: This PCB has 8 optos (QVE11233.0086) that are ...
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1 answer
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Calculation for Resistor in Phototransistor Emitter

I decided to ask here for help, I am still new at electronics, I have been trying to figure out the definitions and graphs of this phototransistor, the PT1302 (datasheet here), my doubt is how can I ...
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Understanding the phototransistor

I am trying to learn how a phototransistor works, and I have made a schematic to show how I understand the photo-diode and NPN transistor to be linked in a phototransistor. I think that when light ...
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How to get the desired signal from a photodiode and phototransistor multisim?

The above is a visualization of an IR transmitter and receiver circuit I'm building. Sending a 2v and 1khz sine wave through a photodiode to a phototransistor. The figure above shows the outputs I ...
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Understanding how a NPN transistor works

I have been trying to understand how an NPN transistor works, specifically in the case of a photo-transistor. As far as I can tell, it should be something like this: I don’t understand how current ...
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When base is open, is the base-emitter junction reverse biased? How does leakage current from collector to emitter flow?

I've been studying photo transistors, but I'm rusty on leakage current in bjt. I don't get why CE leakage current is \$ 1+\beta \$ times CB leakage current. It seems like the bjt amplifies CB leakage ...
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Why is there an additional resistor in this phototransistor circuit?

I'm currently working on a phototransistor that is part of a reflective optical sensor. Now I saw a circuit, which looks like this and I'm wondering why there is the (additional) resistor \$R_3\$ ? ...
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Transimpedance amplifier design advice

I'm working on a transimpedance amplifier for a phototransistor in order to create a sensor that counts objects falling down a pipe. This is the sensor: https://www.digikey.it/short/0vq01q7h Datasheet:...
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4 votes
2 answers
536 views

What is the "collector light current" of a phototransistor and how does it relate to the collector current?

I found the "collector light current" term in this datasheet (Vishay TEFT4300) but I can't find a definition for it on the internet nor figure out how it relates to the collector current. I ...
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Optocoupler design - why does this circuit work?

I am busy with some calculations regarding the components around the optocoupler and I am now finding myself in a bit of a mental struggle. I have done quite a lot of reading (at least here, here, ...
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Phototransistors in parallel

We are trying to hook up phototransistors in parallel so we can sum the output currents and put it over a resistor to get an output voltage. When we do this, the first phototransistor (closest to the ...
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1 answer
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Need help identifying an SMD component - top marking PJt

I'm wondering if anyone can help identify this component, with top marking PJt. It's a 6-pin package and it's connected to a phototransistor. The collector is connected to pin 6 and the emitter is ...
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Making a light detector using a phototransistor and an open drain comparator

I want to make a light detector using a phototransistor. To do that I connected a phototransistor and apotentiometer to the open drain comparator as in the diagram below. When the light is dim, ...
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Calculating Values of ADC for Phototransistor on Digital Pin

I would like to read a phototransistor from a digital GPIO pin on Raspberry Pi or Arduino microcontroller. Phototransistors are analog devices and to read the value properly I know that I will need an ...
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Determine if phototransistor is in cutoff

I'm attempting to determine if the following phototransistor HOA1870-031 is in cutoff. The HOA1870-031 we have in stock have the cathode of the IR emitter tied to the emitter of the IR detector (...
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Quadrature encoder (unknown) pinout

I have a quadrature photoresistor from a printer. I want to determine the pinout. I can't find a datasheet for it. The case only shows "80058 - 96315" and I don't know the pinout. The two ...
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4 answers
103 views

Dual photo transistor component reference (like used in old mechanical mouses)

Context : I'm trying to implement a way to detect the rotation speed of a motor (no problem) and also get it's direction. I know all the basics, don't worry :) (placement difference between 2 photo ...
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Create differential signal from single ended encoder

I have a problem with the output of an DC motor encoder feedback. I have an 5v single ended encoder (AB signal), but my control needs an differential signal. I am not sure how to do this as I have no ...
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Attempting to build photosensitive alarm-clock - circuit questions

I'm working on building a photosensitive alarm clock for a school project. My supplies require me to use a DC power source, and my speaker requires an AC current. In the circuit I posted below, I ...
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How does one find what base current to use in simulating a phototransistor?

How does one find what base current to use in simulating a phototransistor? I wish to use LTspice to simulating a phototransistor (LPT80), so I downloaded the model but I have not found information on ...
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Working principle confusion based on the photodiode, phototransistor and LED circuit

I need help understanding the working principle of this circuit that I simulated in Multisim. I'm doing this for my lab report. When the switch is closed, the photodiode, phototransistor and LED are ...
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EE-SX1140 datasheet confusion

In the datasheet it is said that the collector current shall not surpass 20mA. Then there is VCC = 5 V, RL = 100 Ω, IL = 5 mA which should be with a voltage drop from collector to emitter of 0.1V -&...
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Why does CNY70 output peak at a distance only?

I am using the CNY70 sensor similar as in Understanding the CNY70 Reflective Optical Sensor (FTR: V=3.3V (instead of 5V) R=1k (instead of 200Ω) and X=10k putting the output voltage in the range of 0–0....
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Photodiode and Phototransistor circuit

How can I design a circuit that characterizes photodiodes and phototransistors (as output/input)? That circuit should communicate electronically over an isolated link where one side runs a low voltage ...
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Low precision flash detector

I'm interested in flash detection, what would be equivalent to 1000 FPS. High speed cameras that do this are too expensive and too heavy and provide too much information. In the long run I'm going to ...
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2 votes
2 answers
147 views

Detecting small AC current signal generated by phototransistor

I'm trying to detect a small AC signal (~10kHz) generated by a phototransistor on a magnitude of 10nA~1uA using the following circuit by converting the signal to voltage signal first and then filter ...
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3 votes
2 answers
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Reflective optical sensor

I am working on a project that uses a TCRT5000 reflective optical sensor. Sensor datasheet is here I tested this sensor in a room, the voltage at the output pin is high it is above 2.5V. While ...
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What's the reason for keeping a base line in optocoupler? [duplicate]

Image Source: https://www.vishay.com/docs/83725/4n25.pdf The pin no 6 of the optocoupler IC is connected to the base of the NPN transistor. Should I keep it open? Or grounded? Does it anything to do ...
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How do these phototransistors circuits work?

In the figure 1, When there is insufficient light, the darlington transistor is biased on, keeping the relay energized. When there is sufficient light, phototransistor FT turns on; I imagine this ...
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Vs1838B break beam problem

So im using ESP32 to output 38khz 40% duty cycle wave to ir led and connecting a vs1838b on the other side. The vs1838b works as it outputs LOW when i send a signal from my remote. Anyways, after the ...
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How to simulate a phototransistor in a circuit, prefereably in ltSpice?

I copied a circuit from a working pcb, which is detecting small IR intensity changes. I like to simulate it in ltSpice but I dont know how?! notes on circuit: the transistor is bc847, op amp is lm324, ...
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7 votes
3 answers
3k views

Why would you use a feedback resistor on a logic buffer?

Please excuse my poorly drawn photo-interrupter circuit. It is used for external home sensing where a flag will interrupt the light source, turning off the phototransistor. I'm just curious as to ...
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Switching a 5V power relay with optocoupler

I have looked at the schematics of a few 5V power relays and noticed different designs. Some use just a transistor to draw the higher current to switch the coil and some use optocoupler (surge ...
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4 answers
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Will a 950 nm IR emitter work with a 870 nm phototransistor?

I'm planning to build a simple IR light barrier across a distance of 15 mm. I plan to use this receiver with this emitter. The reason is to have some SMD PNP capable components. The emitter and ...
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Why are capacitors used parallel to transistors

In the circuit attached. There are two capacitors. One is parallel to "PWR LED". Although I understand it delays the turning on and off of the LED, why would you need that? Also, the other capacitor ...
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2 votes
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Can reverse breakdown voltage be used to simulate lifetime aging of phototransistors?

As phototransistors age, their responsivity to light decreases (a fixed amount of light generate less photocurrent). To simulate long-term aging of phototransistors in a short period of time, one can ...
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How to build fast visible light detector

I need a circuit which can detect flickering of light too fast. I have Digital LDR Module which is too slow can I replace LDR with L14G2. Digital LDR Module - Digital LDR Circuit - Can we ...
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Phototransistor amplifier

In the following picture: I have screenshotted a picture of the front end of a phototransistor receiver circuit from Electronic Gadgets for the Evil Genius. It appears to be a phototransistor-based ...
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Phototransistor Datasheet Confusion

I am going through this datasheet of a photo-transistor from Fairchild Semiconductor. Problem-1: I cant figure out the gain of the photo-transistor (like in other normal BJTs). There is something ...
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