Timeline for Basic circuit to keep LED either on or off depending on night/day
Current License: CC BY-SA 3.0
13 events
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| Jun 11, 2020 at 15:10 | history | edited | CommunityBot |
Commonmark migration
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| Jan 11, 2013 at 14:02 | comment | added | user16324 | Olin, while your concerns have some validity, let me add one thing. The intent in this answer was to help the questioner find the right general direction rather than sell a complete solution. He responded with an improved circuit addressing the biggest issue. I say : good for him, despite the value error. He determines the spec; if this version fails to meet it, he can work towards your more complex version, step by step. | |
| Jan 11, 2013 at 13:50 | comment | added | Olin Lathrop | No, I shouldn't have to read the comments. I see now that you mentioned R3 should be 47 kOhms, but that's not what the schematic says. Also, you can only decrease R1 so far before the LED current becomes limited by the gain of the transistor. With 47 kOhm for R3 and R2 completely off, you get 94 uA base current. At 100 gain that supports 9.4 mA LED current. That could be quite bright, but you are also loosing the threshold then, and there is still no snap action as the OP asked for. Basically, this circuit does not meet the specs. | |
| Jan 11, 2013 at 13:42 | comment | added | user16324 | @Olin : read comments re value of R3. You are correct that 2.2K is wrong on the schematic, but I didn't put the schematic up there. A high efficiency LED will be bright enough for some purposes at 1.5ma; if it isn't, the OP can reduce R1 to fix that. | |
| Jan 11, 2013 at 13:38 | comment | added | Olin Lathrop | This circuit won't work. The OP said that R2 will be about 3 kOhm when light. That is still much higher than this circuit requires to turn off the transistor and therfore the LED. Also, the LED will be quite dim since it will get less than 1.5 mA. | |
| Jan 11, 2013 at 9:41 | comment | added | user16324 | Good job on the schematic! I updated the answer to call the new resistor R3 to match the schematic. Note that its value should be 47K or around that figure. @Dave T: good idea on adding hysteresis (snap action). | |
| Jan 11, 2013 at 9:37 | history | edited | user16324 | CC BY-SA 3.0 |
edited body
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| S Jan 11, 2013 at 8:47 | history | suggested | Pangolin | CC BY-SA 3.0 |
Added a touchup to answer.
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| Jan 11, 2013 at 8:06 | review | Suggested edits | |||
| S Jan 11, 2013 at 8:47 | |||||
| Jan 11, 2013 at 8:05 | vote | accept | Pangolin | ||
| Jan 10, 2013 at 23:49 | history | edited | user16324 | CC BY-SA 3.0 |
added 54 characters in body
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| Jan 10, 2013 at 23:46 | comment | added | Dave Tweed | You can add a snap-action (hysteresis) feature to this circuit, too. Add a PNP transistor with its emitter to +5V. Connect a 100K resistor between the base and the junction of R1 and LED2. Connect another resistor from the collector to the base of the NPN. The value of this last resistor will determine the amount of hysteresis. Start with 100K and experiment from there. | |
| Jan 10, 2013 at 22:55 | history | answered | user16324 | CC BY-SA 3.0 |