I’ve considered how to turn a power on indicator LED into an output voltage indicator for an adjustable power supply. All the user would see is the indicator LED. This LED would be full brightness when the PS is on, except when it is indicating the voltage, which is when the LED will dim. The number of long dims is the number of volts – for example there are 5 long dims indicating 5 volts. After the long dims there will be 0 to 3 short dims indicating .00, .25, .50, or .75 volts. I’m calling this a “Dimdicator”.
Since the LM317 three terminal adjustable regulator IC cannot go below 1.25 volts output, the Dimdicator circuit will always indicate at least 1.25 VDC.
The indicator could be an audible beep in addition to, or instead of the Dimdicator. The dimming could be done by an NPN transistor with its emitter connected to the cathode of the LED, assuming the cathode is grounded. The collector would be connected to a low value resistor, possibly 470 or less ohms, with the other end of this resistor connected to the anode of the LED. So every time the transistor turns on, it shunts away some of the current from the LED, causing it to dim.
The base of the transistor will have a higher value resistor, maybe 10K or a little bit more. The other end of this resistor will be connected to the dimming circuit.
The next thing I have to figure out is how to detect the voltage and change it into long and short dims. The simplest way would be to use an 8 pin CPU chip that has one analog input and one digital output. I guess it can easily be only four pins. All the analog sensing and decision making would be done in this chip.
