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February 16, 2013
Okay, for those of you that want to use a different color, a smaller LED (like I do) or some other indicator, here is what I found by studying the circuit and the data sheets. The way this works is that the TL431 looks like a 2.3 voltage drop when the voltage is above the reference voltage (se4t by the trimmer). When it enters the range below the reference voltage, the TL431 opens to allow any voltage (an open-circuit). So far so good. Now we need to have an indicator that operates in the voltage range between 2.5v and the reference voltage. The supplied LED starts to light at something like 3 volts. So it works just fine as long as the reference voltage is greater than 3 volts. Note that the intensity of the LED dims as you go from the reference voltage down to the operating voltage (a “turn-on” voltage if you wish.) After looking at several data sheets, I found that most LEDs operate at about 2.0 - 2.2 volts. Since this is below the TL431 minimum voltage, these LEDs are on all the time that a voltage above 2.0 volts is present. The indicator has to have a “turn-on” voltage between 2.5 and the reference voltage desired.
My simple solution was to add a pair of forward-biased 1N4148 diodes in series with the yellow LED which adds about a volt of voltage drop before a current can flow. This simple change allow the use of small (3mm) yellow LEDs in this circuit. Use the discussion above to adapt it to other indicators, just make sure the operating point is between 2.5 and the reference voltage that you want.
about 2 years ago
Just a comment if you ever redo this board – maybe 2 comments. First, it seems like this board is large compared to the components on it, could it have a smaller footprint?
Second, it would be nice to bring IOVDD out to a pin, so that we could tie the GPIO-n pins to the 3.3 volt supply.
My mistake … I created this board for use on my project. I created it before I had the actual part in hand. So I will modify it.
Think I may have blown the VR for AVDD because I did not connect it to input ground. Is that possible?
Also, if I use a larger power for VCC, what is the rating on the power LED resistor? If I use 9 volts in, will that burn out the resistor?
Okay, so I used your Eagle library for this board and discovered that the pins on the edge that are prtpendicular to the other pns are actually 0.4 inches from the other pins, while the library board layout is .035 inches away. Good thing I am not using any of those pins.
You should update your Eagle library to match the actual board (if you haven’t already fixed it.
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