Q. Can these be placed in series with a 9v supply, and if so how many?
A. Hi W Sharkey, thank you for your question. As these have the control chip in the lens they need to each have a 4.5V feed. If you put two in series they do not operate properly. At 20mA and 9V you would need a 225 ohm resistor for each LED, the nearest value is 220 ohm 1/4W carbon which will be fine like 62-0354.