I am a total electronics newby (besides the odd solder repair) - I'm trying to create a robot with my son out of an old tower PC and he'd like to use the fans for eyes. I've seen that a fan can be run off a battery so could buy a case with a switch so he could turn them on and off, but can anyone show me a super simple diagram of how I connect 2 fans to 1 switch and preferably 9v batteries (limited space inside robots head!)?

1 fan label says 12v 0.10A and had 2 cables, sadly in his excitement he removed the other label and all cables! I am OK with soldering cables back and figure I'll need to be soldering in a switch anyway.

Better still - if we can also add a coloured LED to the centre of each fan that would be awesome. What LED spec would we need so they don't burn out??

Any help gratefully received - he's only 8 and busting to make this work Thanks

  • \$\begingroup\$ For low power and low RPM you can wire in series \$\endgroup\$ Feb 21, 2021 at 0:01

1 Answer 1


This circuit should work assuming the fans are 12V. If the unmarked fan is 5V, it might not like having 9V applied. 12V fans won't run at full speed but this will make them safer for little fingers. I'm guessing that is a good thing. Notice that there is a resistor added to your list of parts... You need that if you use an LED. You can repeat the LED part of the circuit in parallel to run more than one if you want. Make sure you use a fresh battery since the 12V fans may not self start at 9V. I tried this with a bench power supply on an old Dell 12V fan, and it didn't start every time at 9.0V but did at 9.3V. A fresh 9V battery should be around 9.6V.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ I just tested an old Intel case fan and it starts below 5V and increases fairly linear with current up to 14V for 1.5W and 0.5W @ 9V worse bearings that are aged might be worse. \$\endgroup\$ Feb 20, 2021 at 23:10
  • \$\begingroup\$ Just a note about 9V batteries, they're the devil. Cost is very high per unit energy and they're basically meant for fairly small loads. You should consider using a battery pack made of AA batteries to reduce running cost. \$\endgroup\$
    – K H
    Feb 21, 2021 at 6:00

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