Using one Mosfet to control the gate pin of another Mosfet

Question

I am using Mosfets to power an RGB LED light strip with a Raspberry Pi:

https://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdf

The I though the threshold voltage for the above Mosfet was low enough for the 3.3V supply from the Raspberry Pi to the gate pin but the LEDs are dim (the blue is the dimmest maybe because of different internal resistance in the strip?). I measured the voltage across the drain and source for each of the 3 Mosfets (red, blue, and green) at 7, 6, and 5 volts respectively. I attached the gate pin to the 5V pin on the Raspberry Pi and the lights lit up full brightness. Is it possible to wire up a circuit that uses the Mosfet's I currently have in place at 7, 6, and 5 volts to control the gate pins of three other Mosfets that then power the lights? i.e. I want to open a Mosfet's channel from Drain to Source by providing voltage to it's gain pin from another Mosfet's output. If not possible, is there something I could be doing wrong with the wiring or do I ultimately need to find another Mosfet with even lower threshold voltage? Below is the fritzing diagram I have been using with a 100k pull-down resistor. Power Source is a 12VDC 1A supply

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

Generally speaking there are many reasons why a microcontroller/single board computer GPIO pin might not be suitable to drive the gate of a MOSFET. You may be able to use a single lower power 'logic level' MOSFET as a gate driver, but I don't recommend this. For a low side driver such as this you will have the best results using a MOSFET gate driver IC, which is a device purpose built for exactly this kind of application.

Answer 2

Based on the datasheet, your 3.3V gate drive should be enough. Are you absolutely sure that you are using the MOSFET that goes with that datasheet? Have you verified that you are actually getting 3.3V out of the RPi GPIO?

If you want to use the 12V to pull up the gate drive to the MOSFET, you can add another transistor and some resistors to do that. Here is a circuit that does that with a BJT. You could do it with a MOSFET, but I already had this circuit drawn.

^{simulate this circuit – Schematic created using CircuitLab}

R1 is there so that if the RPi is not connected, the LED is off. Otherwise, it is not necessary. R4's value is chosen high enough to limit the base current of Q1, but low enough so that when the RPi GPIO voltage is low, the base voltage is low enough to turn off Q1.

Edit: Adding a MOSFET version

^{simulate this circuit}