Timeline for Automatic dark detector: base voltage
Current License: CC BY-SA 4.0
21 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| May 21, 2022 at 18:43 | comment | added | jonk | @ElectroNewbie Here are a few I wrote: LDR 1, LDR 2, LDR 3, LDR 4, and LDR 5, in no particular order. | |
| May 21, 2022 at 14:47 | comment | added | ElectroNewbie | @jonk yes, perhaps something more advanced (for me!) like this: electronics.stackexchange.com/a/300505 thank you very much for your time and comments. | |
| May 21, 2022 at 14:34 | comment | added | jonk | @ElectroNewbie You'd want less beta, given that BJT is to be operating in saturation. So yes, more BJTs. But. You will also want hysteresis. | |
| May 21, 2022 at 14:01 | comment | added | ElectroNewbie | @jonk ok, I think it has to be calculated using KVL. (11.42-0.7)/95.23K=0.112mA. So if beta is let’s say 100, we have Ic=11.2mA. If we asume 20mA for good LED light, we would have half brightness. I suppose a second NPN is needed to potentially amplify more the output current, or that there are better ways to do it. | |
| May 21, 2022 at 13:03 | answer | added | jp314 | timeline score: 1 | |
| May 21, 2022 at 12:55 | comment | added | ElectroNewbie | @jonk yes, sorry and thanks for your patience :( Rt=95.23Kohm, so… current is 11.42/95.23K=0.12mA? What voltage do we have on the transistor base? | |
| May 21, 2022 at 12:41 | comment | added | jonk | @ElectroNewbie With 2M and 100k you don't get 95 Ohms. Did you forget the k there? | |
| May 21, 2022 at 12:38 | comment | added | ElectroNewbie | @WhatRoughBeast yes :) by “typical” I meant the first I found. Don’t know if it works; that’s what I am trying to understand. Thank you very much for your comment! | |
| May 21, 2022 at 12:36 | comment | added | ElectroNewbie | @jonk ok, so the Thevenin equivalent (in darkness) is Vt=11.42V and Rt=95.23ohm. So, let’s see if I understood it: I have 11.42V in the base of the transistor but the current will be limited by the resistor to about 120mA, within specs. So it “does not matter” at which voltage we put the base, if the current is within specs? | |
| May 21, 2022 at 12:19 | comment | added | WhatRoughBeast | That better not be a typical dark detector. When the LED is on, the voltages across R1 and R2 will be (very roughly) the same. So the current in R1 (and the LED) will be about 1000 times that of R2. By definition, this requires the transistor to have a gain of about 1000. That won't happen at the best of times, and this isn't the best of times, A BJT being operated as a switch (the term is "in saturation") will have a gain of about 10 to 20. | |
| May 21, 2022 at 11:35 | comment | added | jonk | @user_1818839 Doesn't matter. The only important effect is the Thevenin impedance of the divider. | |
| May 21, 2022 at 11:32 | comment | added | user16324 | @jonk oops, edited comment. | |
| May 21, 2022 at 11:32 | review | Close votes | |||
| Jun 5, 2022 at 3:05 | |||||
| May 21, 2022 at 11:28 | comment | added | jonk | @user_1818839 That's usually pretty small. Ten or twenty Ohms. Or so. The emitter will have some, but usually less than one ohm. | |
| May 21, 2022 at 11:25 | comment | added | user16324 | I don't think you included the base-emitter junction's impedance in your voltage divider equations. | |
| May 21, 2022 at 11:22 | comment | added | ElectroNewbie | @jonk thank you very much for your comment. Yes, definitely I am missing something very basic :) I’m going to research the Thevenin impedance concept: youtu.be/xSRe_4TQbuo?t=656 | |
| May 21, 2022 at 11:20 | history | edited | JRE | CC BY-SA 4.0 |
deleted 51 characters in body
|
| May 21, 2022 at 11:02 | comment | added | jonk | That's not even a good topology for this. I would not even consider it. But to answer your question, you've forgotten the Thevenin impedance of the divider pair, haven't you? | |
| May 21, 2022 at 10:48 | history | edited | ElectroNewbie | CC BY-SA 4.0 |
added 34 characters in body
|
| S May 21, 2022 at 10:39 | review | First questions | |||
| May 21, 2022 at 15:30 | |||||
| S May 21, 2022 at 10:39 | history | asked | ElectroNewbie | CC BY-SA 4.0 |