Need to switch 5V from 3.3V Trinket Pro

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
I am using an Adafruit Trinket Pro 3.3V and I need to use one of it's digital outputs to apply 5V power to another board that only draws 30ma. It needs to be positive logic so that a high on the trinket pin applies 5V to the second board. The 5V source I have is regulated and filtered. I would like the switch circuit to use a few components as possible. An IC would be okay if it can work with 5V.

What would a switch like that look like?
 

Dodgydave

Joined Jun 22, 2012
9,206
If your trinket gives out 3.3v and you want to switch 5v, the you need an npn transistor to pull the load down to switch it on, or use two transistors to switch the positive supply side.
 

GopherT

Joined Nov 23, 2012
8,012
I am using an Adafruit Trinket Pro 3.3V and I need to use one of it's digital outputs to apply 5V power to another board that only draws 30ma. It needs to be positive logic so that a high on the trinket pin applies 5V to the second board. The 5V source I have is regulated and filtered. I would like the switch circuit to use a few components as possible. An IC would be okay if it can work with 5V.

What would a switch like that look like?
 

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
It seems this would be easier and use fewer components if I reverse the logic and use a single PNP transistor, like this. I will just need to set the Trinket output pin low when I want power to the other board.

I think this will work.
upload_2016-3-7_11-30-29.png
 

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Alec_t

Joined Sep 17, 2013
11,406
With that setup, the PNP base will never go above 3.3V (if it even reaches that), so the PNP will be permanently switched on.
 

AnalogKid

Joined Aug 1, 2013
8,488

Thread Starter

PickyBiker

Joined Aug 18, 2015
88
I'm not sure how that should be calculated. I looked at the datasheet for the 2N2907 to see if I could determine the maximum base current flow but I didn't see it. I admit I don't know what a lot of the specs mean.

A guess would be that 670 ohms should drop that to a little over 5ma. Is that large enough?
 

AnalogKid

Joined Aug 1, 2013
8,488
The old rule of thumb is that the base current should be 10% of the collector current for hard saturation. 50 years later, 20 or 30 to 1 are ok if you need to conserve battery power. Anything from 470 to 1K is fine, with 10K to the emitter for turnoff.

ak
 
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