Which Mosfet/transistor? (3V input)

Hi everybody,

I’m looking for a Mosfet/transistor that can be enabled with a lower power signal (TTL, 3.3v). This Mosfet/transistor should allow an Arduino to be turned on/off (also 3.3v). Can anybody recommend a Mosfet/transistor I could use?

(fyi: powering the Arduino directly by the 3.3v TTL signal doesn’t work, it’s not enough to power the Arduino. The 3.3v TTL signal is the output of a PIR motion sensor running standalone.)

Thanks in advance,
Jan

Hi! I'm not an expert but I think the answer depends on what you have connected to your Arduino. Maybe you could measure the total current drawn by your Arduino setup.

If you are powering just the Arduino, basically any general purpose NPN transistor should work. For example, the popular 2N2222 transistor can drive loads up to 500mA which is plenty more than required by the Arduino alone.

I guess you will want to switch the +3.3V power line and not the ground line, in which case a P-channel mosfet would be best. I suggest this: http://uk.farnell.com/jsp/search/productdetail.jsp?sku=1864586. It has Rds(on) specified down to 2.5V.

Alternatively, if your Arduino doesn't have a separate power-hungry USB-to-serial converter on board, why not leave it permanently powered and put it into power-down mode when the PIR output is not active?

Thanks dc42! I'm using an Arduino Pro Mini, so it doesn't have USB-to-serial but it does have a regulator which consumes power (even when bypassed).

I tried the 2N2222 transistor, but it doesn't work as expected. I connected the output of the PIR sensor to the middle pin of the transistor. Then the + of my power source and the VIN of the Arduino to the remaining pins. Strangely the transistor only passes like 2.8v (instead of the 3.9v I use as the power source).

So the next step is to try the TSM2311CX, does it work the same as the transistor?

Thanks! Jan

You have to be more specific than:

Then the + of my power source and the VIN of the Arduino to the remaining pins.

In any case, if you try to high-side switch power using an NPN transistor, the base must be able to go a few volts higher than the voltage you are trying to switch so as to overdrive the transistor to get a lower VCE, or it’ll drop at least 0.6V, or more. The 2N2222 isn’t that well suited to being a switching transistor.

A PNP transistor will make a decent high side switch. Overdriven, it should have only a few tens of mV dropped across the collector to emitter. But you’ll need an NPN driven from the PIR output through a resistor, then the NPN collector pulls the base of the PNP low, also through a resistor.

Do you need a schematic?

polymorph: Do you need a schematic?

Thanks for the explanation polumorph! I'm a bit lost with the 2 transistors, so a schematic would be great! :-) Appreciate the help!