Timeline for Light sensor using Photo Transistor
Current License: CC BY-SA 3.0
23 events
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| Jun 22, 2012 at 12:20 | history | edited | Oli Glaser | CC BY-SA 3.0 |
Added more details
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| Jun 22, 2012 at 12:13 | comment | added | Oli Glaser | @Steven - In general yes, but we'll have to agree to disagree on this one I think :-) For this particular circuit posted and with this particular OP (from previous questions) I thought keeping things as simple as possible was very important. Part of the learning can be finding out for himself why this circuit would be not ideal in most situations, and then trying it other ways. In any case he has plenty of info about both versions now :-) | |
| Jun 22, 2012 at 11:13 | comment | added | stevenvh | @Oli - I don't agree here. He's doing Bad Things™, and you should always point this out, also to a beginner. After all, it's just adding a resistor, that's less radical than the schematic you posted. | |
| Jun 22, 2012 at 10:43 | comment | added | Oli Glaser | @Steven - I added a bit more, hopefully this makes things a bit clearer. Initially I was trying to keep things as simple as possible as it's clear the OP is very new to the subject (e.g. we still aren't quite sure whether he is using a phototransistor or LDR). I thought changing the circuit might just confuse things too much. | |
| Jun 22, 2012 at 10:39 | history | edited | Oli Glaser | CC BY-SA 3.0 |
Added requested detail
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| Jun 22, 2012 at 8:29 | comment | added | stevenvh | @Oli - You should definitely explain that his common collector circuit is very bad, because it doesn't have any feedback. The BC337's \$H_{FE}\$ is between 100 and 600, and without feedback the collector current will also vary in that range. Other transistors don't even give an upper limit for \$H_{FE}\$. For the time being -1, until you add this. | |
| Jun 22, 2012 at 8:26 | comment | added | stevenvh | @Umer - if you disconnect the transistor, can you measure a varying voltage across the phototransistor, depending on light level? | |
| Jun 22, 2012 at 3:21 | comment | added | Oli Glaser | Not sure then, maybe your phototransistor is faulty. If you have another one try that. You could maybe try a bit higher (e.g. 20k) but I think your phototransistor should easily drop the base current if it's working. Make sure you read the datasheet and know which pin is which. Also check the NPN pins to make sure they are correct. It's late here - I will check back tomorrow to see how you got on. | |
| Jun 22, 2012 at 3:15 | comment | added | Umer Farooq | yes I moved up to 13K | |
| Jun 22, 2012 at 3:15 | comment | added | Oli Glaser | Did you change the resistor value first? | |
| Jun 22, 2012 at 3:14 | comment | added | Umer Farooq | I have tried to change the orientation but still it is not working. What do i do | |
| Jun 22, 2012 at 3:11 | comment | added | Oli Glaser | The 2N3904 has a lower gain than the BC337, so you don't really want to go higher than 20k as there won't be enough current to light the LED properly (there is likely something wrong with your phototransistor if it still doesn't work with above 10k - check it's correctly orientated) | |
| Jun 22, 2012 at 3:06 | comment | added | Oli Glaser | Try a higher value, around 10k and see if that makes a difference, if it's still on when it's light then increase further. You might have to experiment with the values a bit to get your required sensitivity, as mentioned above. | |
| Jun 22, 2012 at 3:02 | comment | added | Umer Farooq | I am using 1K resistor | |
| Jun 22, 2012 at 2:59 | comment | added | Umer Farooq | Sir an error occured, The led lights up even when light is on. What do I do? | |
| Jun 21, 2012 at 19:06 | history | edited | Oli Glaser | CC BY-SA 3.0 |
explain the 22k
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| Jun 21, 2012 at 19:04 | vote | accept | Umer Farooq | ||
| Jun 21, 2012 at 18:44 | history | edited | Oli Glaser | CC BY-SA 3.0 |
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| Jun 21, 2012 at 17:53 | history | edited | Oli Glaser | CC BY-SA 3.0 |
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| Jun 21, 2012 at 17:47 | history | edited | Oli Glaser | CC BY-SA 3.0 |
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| Jun 21, 2012 at 17:41 | history | edited | Oli Glaser | CC BY-SA 3.0 |
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| Jun 21, 2012 at 17:00 | history | edited | Oli Glaser | CC BY-SA 3.0 |
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| Jun 21, 2012 at 16:26 | history | answered | Oli Glaser | CC BY-SA 3.0 |