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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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110 views

Can an optocoupler be used as a switch [closed]

I am trying to replace a analog switch (the switch is a momentary button in a low current DC situation) with optocoupler. Is it possible to use an optocoupler as a switch (i.e. just on/off)? ...
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2answers
91 views

Very low current (<10 microamps) darkness detector circuit that turns off in light

I've made a device that lights an LED at a very low current (<10 microamps) as a very dim indicator for use in the dark. At its simplest, the circuit is a 3V coin battery + resistor (300K-1M ...
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1answer
20 views

Phototransistor and Photodiodes capacitances when some in an array sense light

I'm using phototransistors to sense low speed data (about 7 bits a second). With one phototransistor and a 300K resistor, I found my sweet spot and can transmit correct data with lazer. I find if the ...
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1answer
118 views

Can you connect multiple photodiodes in parallel and treat it as one?

Im working on an IR sensor based off of the Texas Instr. particle sensor (TIDA-00378). They use just one photodiode in their design. I would like to use multiple photodiodes, lined up with multiple ...
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81 views

Optocoupler 24V input misbehaving

I have an optocoupler circuit that is used to accept 24V input (100 ms pulses) and interfaced to a microcontroller. The schematic is as follows: Opto: BL817C or TLP621. TLP621 datasheet As per my ...
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2answers
97 views

Optocoupler to drive LEDs (maybe with transistor)

I wonder if there is some simple solution to drive LEDs powered from single rectified voltage. Assume AC is 220 V, each LEDs is 3.0 - 3.2 V 150 mA. I reckon series connection made it simpler but not ...
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1answer
93 views

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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2answers
66 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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1answer
33 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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1answer
51 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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0answers
44 views

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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3answers
107 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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1answer
41 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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1answer
102 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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2answers
93 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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0answers
26 views

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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1answer
67 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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1answer
47 views

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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3answers
105 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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2answers
98 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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1answer
53 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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1answer
69 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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1answer
116 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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2answers
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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2answers
208 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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1answer
58 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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333 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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3answers
590 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
112 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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1answer
2k views

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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1answer
135 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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1answer
105 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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1answer
510 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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2answers
143 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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2answers
175 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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4answers
248 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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1answer
84 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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1answer
131 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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2answers
602 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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1answer
215 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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1answer
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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1answer
245 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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4answers
453 views

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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0answers
150 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
179 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 ...
2
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1answer
523 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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1answer
321 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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6answers
4k views

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
471 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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2answers
210 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 ...