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| Temperature indicator circuit with LEDs |
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| dazz:
--- Quote from: Hero999 on August 20, 2018, 08:04:31 pm --- --- Quote from: dazz on August 20, 2018, 06:16:05 pm ---The red LED went from 150mA to almost 2A :palm: :-DD. I wasn't measuring the current through the yellow LED, but I could tell it was about to blow up. --- End quote --- The LEDs all have resistors in series with them, so the current shouldn't go that high. On the previous schematic you posted, the red LED has a 100R resistor in series with it, which would limit the current to 90mA, even if the transistor and LED failed short circuit. How hot did you heat it? If it got so hot, it started to overheat and char the resistors, then it's possible they could fail short circuit, but it would start to release smelly fumes, before then. --- End quote --- BTW, I misread the current there. The red LED went from 150uA to almost 2mA, not 150mA & 2A. The yellow one was probably above 20mA |
| Eka:
--- Quote from: james_s on August 20, 2018, 05:41:32 am ---You can get LED bargraph driver ICs like the LM3914 that are meant to do exactly what you want. Internally they have a series of comparators and a voltage reference and can be configured to display in either bar or dot mode. --- End quote --- With the bar graph driver ICs like the LM3914, it is possible in bar graph mode to combine outputs so it increases LED brightness as more outputs add their current to the LED. So, set the LED drive current to 5mA. Group three outputs per LED. When 5 outputs are driven, LED1 will be on full, LED2 will be 2/3 intensity, and LED3 will be off. Unfortunately LED intensities are not linear. Trimming resistors between the outputs and LEDs can help make them look more linear. Also LM3914s can be strung together for more outputs. TI has good application notes for their parts. http://www.ti.com/lit/ds/symlink/lm3914.pdf So I try to buy from them is not outrageous. Another thing to note is the human eye isn't that great at absolute intensity when the background illumination is different. Your display intensity will look very different when the room is lit versus relatively dark. |
| dazz:
--- Quote from: Eka on August 21, 2018, 08:07:48 pm --- --- Quote from: james_s on August 20, 2018, 05:41:32 am ---You can get LED bargraph driver ICs like the LM3914 that are meant to do exactly what you want. Internally they have a series of comparators and a voltage reference and can be configured to display in either bar or dot mode. --- End quote --- With the bar graph driver ICs like the LM3914, it is possible in bar graph mode to combine outputs so it increases LED brightness as more outputs add their current to the LED. So, set the LED drive current to 5mA. Group three outputs per LED. When 5 outputs are driven, LED1 will be on full, LED2 will be 2/3 intensity, and LED3 will be off. Unfortunately LED intensities are not linear. Trimming resistors between the outputs and LEDs can help make them look more linear. Also LM3914s can be strung together for more outputs. TI has good application notes for their parts. http://www.ti.com/lit/ds/symlink/lm3914.pdf So I try to buy from them is not outrageous. Another thing to note is the human eye isn't that great at absolute intensity when the background illumination is different. Your display intensity will look very different when the room is lit versus relatively dark. --- End quote --- I know the gradual intensity thing is far from optimal, but it goes well with the overall shoddiness of my amp build ;D I'll definitely keep those suggestions in mind and will eventually do it "the right way". Thanks for the info, Eka ETA: just to add a video by Dave showcasing the LM3914 |
| dazz:
Let me dig up my thread here to ask another question, if I may. I'm now concerned that this may not work without an input buffer because of the current draw in the voltage dividers. I measured the current draw at the input at 5mA. I don't think the TMP36 sensor can drive that kind of load, right? Here's the datasheet and the circuit again. http://www.analog.com/media/en/technical-documentation/data-sheets/TMP35_36_37.pdf |
| Zero999:
You're right, a buffer is required. The maximum current the output the TMP36 can drive is 50μA. An emitter follower could be added, but that would add an extra voltage drop and the associated temperature coefficient. A unity gain buffer, made with the cheap and cheerful LM358 will be better. |
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