Ah, accurate digital readouts? Now you are getting specific enough to design with.The various experiments used voltages between 2 and 6 volts and current between 2 and 6 milliamps. One experiment cited "2.4 milliamps at less than 3 volts." So adjustable voltage and amperage within the above specified ranges would be best if there was a way to insure accurate digital readouts during adjustments.
Thanks