Wrong! Unless you choose to drive the white at a higher current. The voltage-drop across the white is higher than the red, but the current drain is determined the value of the series resistor. Ignoring Vsat, R=(Vsupply-Vled)/Iled sets the current drain no matter what LED you choose. Even if you stacked two LEDs in series.
Unless you are considering the relative inefficiencies of white vs red, selecting the correct value for the series resistir will keep the current drain exactly the same.
Well I had in mind essentially what Fred said when I talked about "outside the box". My design is not to your stated requirements - Fred's is. I just figured you might be able to use something a bit different than what you stated originally.
But aside from that, never dismiss Fred's circuits or ideas. His stuff is *good*.
Keeping the current drain exactly the same was not the point. Same *performance* was the point. Should be obvious. Two keys: when battery voltage drops below ~ 3.2 the white LED won't glow at all, while the red will. And he needs to lower the value of the 1K series resistor if he uses a white, which means the current drain will be higher.
Perhaps more subtle: you dismiss the *critical* factor (by assumption perhaps) when you talk about ignoring Vsat. The NPN may saturate initially, but it quickly falls out of saturation and Vce then becomes the dominant factor in determining current. The base of the NPN needs to be driven harder to keep Vce to ~2.8 volts or less for a white LED. For the red LED, Vce can be as much as 4.2 volts, which means less base drive is needed. The cap will discharge to the point where the white LED quits before it discharges low enough for the red LED to quit. Take 1 mA as an example. For 1 mA through the LED, which happens in about 2 minutes in this circuit, the red LED drops ~1.8 volts; the 1K drops 1 volt, which means the transistor must drop 3.2 volts. With the same ce drop and a white LED, the LED would not be glowing at all, since it needs a higher voltage. You can't get the same performance from the two LEDs in this circuit, regardless of what series resistance you use - unless you put in such a high value resistor that neither white nor red glows, and you call that equal performance. The white is going to stop glowing sooner than the red because of Vf.
It should be stated that the comparison made is based on nominal Vf of ~ 3.2 for a white LED and ~ 1.8 for a red LED. However, at the very low currents this circuit will produce through the LED as the cap discharges, Vf changes dramatically. For example, I measured a white LED Vf at 2.47 volts with current < 100 uA. Red LEDS also exhibit a delta Vf.
My prior reply seems to imply that there is a linear comparison between circuit operation with a white vs a red LED. That was not intended. I do not know how the delta Vf compares between the red and white LEDs, only that it exists for both, and that Vf for the white is greater than Vf for red for all currents from 0 to max.
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