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 ...
- 473k
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 ...
- 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 ...
- 9,504
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....
- 473k
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 ...
- 16.2k
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 ...
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 ...
- 57.9k
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 ...
- 174k
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)
...
- 101
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.
- 57.3k
10
votes
Accepted
Can I make my circuit ‘sleep’?
Can a phototransistor be used to open circuit a power rail so circuit is only active during daylight hours?
On its own? No. The gradual turn-on as the light level increased and decreased would cause ...
- 180k
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 ...
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 ...
- 315k
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 ...
8
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. ...
- 18.3k
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 ...
- 7,977
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 ...
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'...
- 473k
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 ...
- 38.5k
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 ...
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 ...
- 315k
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 ...
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 ...
- 8,118
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 -...
- 38.5k
5
votes
Accepted
DIY pulse oximeter circuit - phototransistor shows no reading
The SFH 203 IR diode you list is a photodiode, not an LED. It's meant to be used as a sensor, similar to the SFH-313-FA phototransistor you are using. I'm not sure what should happen if you try to use ...
- 27.9k
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 ...
- 60.7k
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 "...
- 57.9k
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 ...
- 61k
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 ...
- 73.6k
4
votes
Accepted
What is the "collector light current" of a phototransistor and how does it relate to the collector current?
"I'm interested in estimating Ic based on the Irradiance I calculated (~0.5mW/cm^2)"
Ic mainly depends on:
How precise is the estimated value (mW/cm^2) or what is expected range
The value ...
- 9,638
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