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46 votes
Accepted

Optical audio transmitter can't handle speech

It seems to me that you are probably using a music source that is stereo and, instead of it delivering a mono signal as you expected (L plus R), the connection of the transformer to L and R channels ...
Andy aka's user avatar
  • 460k
36 votes

My teacher said a BJT phototransistor doesn't have a base

I think you and your teacher are both right - you are just not using the same definitions and thus your wording disagrees. From your description: Shouldn't be transistor without base be just a ...
Joren Vaes's user avatar
  • 12.6k
27 votes

Non-invasive bubble sensor

I have made something like this. The trick was to use total internal reflection at the liquid to air interface. This project worked very well. Aim the light source at ~45 degree angle (angle can be ...
evildemonic's user avatar
  • 9,229
23 votes

Will a 950 nm IR emitter work with a 870 nm phototransistor?

Side by side: - The receiver will be slightly off centre compared with the transmitter (purple lines I added are sitting at 950 nm) but the receiver will still be above 80% of its optimum sensitivity....
Andy aka's user avatar
  • 460k
19 votes

Will a 950 nm IR emitter work with a 870 nm phototransistor?

Yes, your system will work. Consider the system spectrum (red line below), which is each sensitivity at each wavelength times the emission at that wavelength. You can see that the transmitter is ...
jonathanjo's user avatar
  • 14.1k
12 votes

Triggering generic transistors with UV or green light?

Easiest would be to buy a TO-5 or TO-18 (or even TO-3) transistor and grind or saw the top off the can. Eg. 2N2222, a pretty common type even today. I would expect similar spectral response to an ...
Spehro Pefhany's user avatar
11 votes
Accepted

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

The resistor adds additional hysteresis beyond what the chip provides. With the nature of what the internet is these days quite possibly you may have found a circuit idea that was designed before the ...
Michael Karas's user avatar
11 votes

Triggering generic transistors with UV or green light?

The old OC71 germanium transistor used to be packaged in a painted glass tube. All you needed to get a photo-transistor was to scrape the black paint off. De-lidding a T018 or T05 is probably the ...
Neil_UK's user avatar
  • 167k
10 votes

ICEO, ICBO physical interpretation in BJT

ICBO = Reverse Leakage Current between Collector and Base while Emitter is Open. (IE=0) ICEO = Reverse Leakage Current between Collector and Emitter while Base is Open. (IB=0) ...
Mohammad Ansari's user avatar
10 votes

Will a 950 nm IR emitter work with a 870 nm phototransistor?

Yes, it will work. Look at the sensitivity/emission vs wavelength curves. It is a spread and there is a point of overlap where both devices have reasonably high values.
DKNguyen's user avatar
  • 56.8k
9 votes

My teacher said a BJT phototransistor doesn't have a base

The base connection is often not used and would thus serve mostly to pick up noise. You can find optocouplers (4-pin) which have no base connection and others (eg. 4N35) which bring the base ...
Spehro Pefhany's user avatar
8 votes
Accepted

Understanding voltage and current of a phototransistor w/ collector resistor

Read the datasheet. It should tell you what the expected phototransistor current will be. That is apparently not enough to cause 5 V across a 10 kΩ resistor. Do the math. At full ...
Olin Lathrop's user avatar
8 votes
Accepted

Why are most IR phototransistors slow?

Photodiode + transimpedance amplifier can give you much faster response. Quite a bit of good information has been written. Anything by Phil Hobbs will be useful. By keeping a constant bias voltage ...
Spehro Pefhany's user avatar
7 votes
Accepted

What's the difference in a digital optocoupler and a non digital

Phototransistor-output optocouplers are cheaper and suit for slow digital applications and sometimes for analog applications such as switching power supply feedback. When you calculate the actual ...
Spehro Pefhany's user avatar
7 votes

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

Ica (what they are the "collector light current") is the current through the collector and out the emitter vs. Vce (collector to emitter voltage) at different intensity of light impinging on ...
jwh20's user avatar
  • 7,937
7 votes

How to extend useful life of IR LED and photodiode pair

First you should isolate the cause of the problem. If it is due to dirt, contamination then the answer will be somewhat different than if it is due to the obvious potential problem of LED aging or ...
Spehro Pefhany's user avatar
7 votes
Accepted

4N35 difference between two versions

As commented, the different lines "001Q" and "027Q" are just date and assembly codes. Image source: OnSemi 4N35 Optocoupler Datasheet The two optocouplers should behave the same. ...
SamGibson's user avatar
  • 17.9k
6 votes
Accepted

How do I bias a phototransistor (TCRT5000) to scale the effective range?

I'm just going to take some of your numbers and guess from there. You are running your IR emitter LED at about \$35\;\textrm{mA}\$ and you are getting returned collector currents from around \$47\;\mu\...
jonk's user avatar
  • 78.1k
6 votes

What component does this symbol represent?

The round symbol that contains several smaller circles is likely to be a DIN connector. There are several versions: - And, it looks like yours is the SV50. Is the circuit from a midi controller? Here'...
Andy aka's user avatar
  • 460k
6 votes
Accepted

Optocoupler design - why does this circuit work?

A few notes: CTR is only rarely a guaranteed value that you can use for accurate linear isolated measurements. In the case of the TLP621-4, the datasheet only guarantees that it's somewhere between ...
Hearth's user avatar
  • 33k
6 votes

How to extend useful life of IR LED and photodiode pair

In practice, these devices get damaged very soon. (Some people that use this circuit report this problem). If by "get damaged" you mean "they age" rather than "they get ...
Kuba hasn't forgotten Monica's user avatar
5 votes
Accepted

Confusing dark sensor circuit diagram

This is a crappy circuit. Run away. It seems the intent is that when there is lots of light, LDR1 goes down in resistance, which turns on Q1, which turns off Q2. When it's dark, Q1 doesn't turn on ...
Olin Lathrop's user avatar
5 votes

FPT100B alternative

BPW76A and BPW76B are in-production devices that should be acceptable in this application. The package has roughly same dimensions. The 4.7kohm resistor may need some adjustment, but otherwise it ...
Kuba hasn't forgotten Monica's user avatar
5 votes

Non-invasive bubble sensor

You can try using a photo-transistor in binary mode: it might work, but it will be difficult to find the right component that triggers just right for your application. Another solution is to use some ...
Sandro's user avatar
  • 7,337
5 votes
Accepted

Simple phototransistor circuit to turn on P-channel MOSFET

The circuit looks correct in principle. But you missed some details: simulate this circuit – Schematic created using CircuitLab Your thinking is: When there's no light the Vgs of the MOSFET is -...
Rohat Kılıç's user avatar
4 votes

When wiring IR Phototransistor, why is a pulldown resistor needed?

The pull down to ground is used to create a voltage divider, with the top half being the relative resistance of the Photo transistor. Without the pull down, you only have a series resistor, which ...
Passerby's user avatar
  • 73.1k
4 votes

Transistors with different loads in parallel

Ground the emitter of Q2, and take the signal from its collector. Leave R2 there, but increase its value to 3 - 5K. This will invert the signal, but that is easily fixed in software. You should ...
Peter Bennett's user avatar
4 votes

How to detect IR pulse using phototransistor and voltage comparator (subtracting constant light)

The secret of successful IR transmissions in ambient light conditions, with it's combined many mixed frequencies, is modulation. The transmit source produces an on/off carrier frequency and then "...
Michael Karas's user avatar
4 votes

Phototransistor amplifier

Actually, that is not a TIA. It's a fairly simple AC amplifier with a gain of about 45 (100k / 2.2k). I presume the emitter of Q1 is tied to ground. Then the voltage at the collector will be 10k ...
WhatRoughBeast's user avatar
4 votes

Reflective optical sensor

"Daylight" is full of various wavelengths of light, including infrared. In daylight, your sensor will "see" infrared from the mishmash that is sunlight, and react the same as if it ...
JRE's user avatar
  • 71.8k

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