# How to properly wire a 4.5V LED strip with 12V motors

I'm modding a Nerf gun (Rapidstrike to be exact), and I put a 3S Lipo in there with 1000 mAh. The circuit right now is a rev switch, trigger switch, switch behind the pusher, battery, pusher motors, and flywheel motors. The switches are 21 amp switches and the motors are all 12V motors. I want to put an LED strip in the blaster, however the LED strip is assumed to be rated at 4.5V (I ripped it out of a Harbor Freight light and it took 3 1.5V batteries. There were no resistors inside the light.), and I'm unsure how I can include the strip in the circuit safely. I assumed I could put a resistor in there, however I didn't really know how to calculate what kind of resistance I needed. I'm familiar with Ohm's law, however I have no idea how I'd calculate the current to be able to get the resistance. In addition, I assumed I could just throw a resistor in by the LED strip, but I'm not sure if this is the proper way to do it. I have an image of how I assumed it might work, but please correct me if I'm wrong: If this is correct, how much resistance would I need? If not, how can I wire it properly? I have very little electrical engineering experience, so I figured I'd ask the experts here for guidance.

• you'll probably have to measure the current that the strip normally uses. – Jasen Jan 1 '20 at 9:16 simulate this circuit – Schematic created using CircuitLab

You will need to use a 12V LED strip, just using a series resistor with a 4.5V strip will probably not work. If you use a 12V strip, the resistor can be omitted.

Here is the problem with putting the resistor in series with the LEDS: simulate this circuit

The problem is that you need a drop of 8.1V (12.6 - 4.5V) at R1. In order to calculate the value of the resistor you need to know the current through this resistor. The current will vary with the temperature and aging of the LEDS, so the value is going to be approximate. This may work, if you have a way to measure the current for the LED bar at 4.5V, then you can use ohm's law R=V/I where V=8.1 and I = the current of the led bar in amps to calculate the value for the resistor. The LEDs will tolerate a range of current values so you might get away from this, but if you need whatever you are making to be reliable you might want to just purchase some 12V LED strips or modules and run them directly instead (they are dirt cheap from the usual providers).

• Thanks for your reply. What's the reasoning behind why the 4.5V strip won't work with the resistor in series? – AndreasKralj Jan 1 '20 at 9:49