Need help arranging devices to function using a 9V battery

Thread Starter


Joined Dec 1, 2012
So I'm working with a Parallax RFID card reader and a Parallax PIR sensor. They have details posted within the links but I bet the information on the packaging itself may be more accurate:

RFID reader: 5V (according to site: 4.5 to 5.5 V); 10 mA idle, 90 mA active

PIR sensor: 3 - 6 V; (according to site: "Source current up to 12 mA @ 3 V, 23 mA @ 5 V; 130 µA idle, 23 mA active (no load)")

I'm wondering how I can get these to work reliably on a single 9V battery. My understanding is that they will require approximately 7.5V minimum judging by the above information. Am I correct in assuming that some setup with a 9V battery will allow these to function?

Will it be enough to arrange them in parallel or will I need an additional resistor somewhere in the circuit? How can I determine a static setup that will allow everything to work? The part that leaves me unsure is the fact that the current consumption is variable depending on whether or not the devices are active.

I appreciate any help!


Joined Mar 14, 2008
You should be able to operate them from a 9V battery using a 7805 or LM317 regulator to get a regulated 5V. The regulator will automatically keep the voltage constant, independent of the current. Just connect them both in parallel to the regulator output, no resistors needed.


Joined Dec 20, 2007
A 9V alkaline battery is designed for low current. With the high current here then its voltage drops to the dropout input voltage of an ordinary 5V regulator in about 1 hour.

If a low dropout regulator is used then the battery will last for 3.5 hours.

But I don't know how often nor know how long the high current will last so I calculated continuous drain.