Which transistor?

I want to switch power (3.3V) on and off to an nRF24L01 transceiver using a Teensy 4.1 GPIO pin and, I guess, a transistor. But which transistor? and how to connect it?

… it’s because the nRF24L01 can occasionally lock up, and must be power cycled to get it going again.

by mmessiter
October 19, 2022

From the Teensy 4.1 specs:

"The recommended maximum for external 3.3V usage is 250mA."

and from the nRF24L01:

"Operating current: 250mA (maximum)".

So, if the nRF24L01 is the only device plugged to your Teensy, you should be able to go with connecting a GPIO pin of the Teensy to the Vcc of the nRF24L01, directly, and turning it on and off by sending a high or a low value on the said GPIO pin, from the Teensy. No transistor as a switch requires.

If you don't like to work your component at their limit, you can add a transistor as a switch, indeed. I suggest to use a MOSFET, NMOS or PMOS. Things to check: it should be able to handle the 250 mA and its Vgs(th), in absolute value, should be a little as possible, but definitively less than the 3.3 volt that the Teensy will deliver to the gate pin of the MOSFET.

That excludes the 2N7000 which is only good up to 200 mA, as example. An IRF9520, a little bit more expensive, but, well, some will do nicely and some won't do at all, given that its Vgs(th) is between 2V and ... 4V. Manufacturing tolerances.

Part of the problem is that the best transistors, with low Vgs(th), in absolute value since PMOS are denoted with a negative sign, are more and more expensive. You have to check what is available around your localisation.

Now to connect the MOSFET? its gate pin is wired to the GPIO pin of the Teensy to be used to turn on and off the NRF24L01.

With an NMOS: its source pin to the ground. Its drain pin to the ground pin of the nRF24L01. The Vcc of the nRF24L01 to the positive side of the external 3.3 DC source. The GPIO pin (of the Teensy) connected to the gate of the NMOS must be at a logic low to turn off the transistor (and the nRFL01), and to be at a logic high to turn on the transistor (and the nRFL01).

With a PMOS: its source pin to the positive side of the external 3.3 Volt. Its gate to the Teensy. The PMOS drain pin to the Vcc pin of the nRF24L01. The ground pin of the nRF24L01 to the ground. The GPIO pin of the Teensy should be a low (0 Volt) to turn ON the transistor (and the nRF24L01), which will probably occur by default, at bootup of the whole assembly. And a high value (close to 3.3V) to turn it OFF.

by vanderghast
October 21, 2022

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