What MOSFET to use?

[xorgmc]

Hi,

I'm trying to build a LED fairy lights running from a Li-Ion battery (which is charged by a solar cell) with a timer (press a button and the LEDs turn on for about 3h). I want to do it completely analog with a capacitor and "a kind of transistor". First I tried using 2 BJTs in a darlington pair, but it didn't work out really good because either the Collector-Emitter resistance was too high (I guess) and the LEDs weren't bright enough when turning the timer on, or the Collector-Emitter resistance was too low and the LEDs were glowing when the timer was turned off (1mA is not a really good stand-by current on a battery). So a relative told me to use a MOSFET which sounded quite a good idea to me as it isn't controlled by current. But the amount of things that seem important when choosing one seem just massive to me 

The operating voltage is of course about 4.2V - 3.3V, and the LED string lights nicely at about 200mA (40 leds in parallel - 5mA per LED) - I got that with a 3.3 Ohms resistor before the leds.

I thought about a circuit similar to this: http://2.bp.blogspot.com/-U_YWj3oGLgs/Ty3ByRJ7cLI/AAAAAAAAAE4/w106YZFBpwE/s1600/off-after-delay-switch-by-mosfet.jpg

Can anyone recommend a MOSFET that is suitable for such applications? Or should I use a completely different method?

[bobaruni]

It could work OK using that method.
You will need a low gate-source threshold voltage VGS N-CH MOSFET and preferably one with reasonable drain-source on resistance RDS(on).
Keep in mind that as the capacitor discharges, the LEDs will eventually dim and the MOSFET will eventually start to dissipate some heat as the drain-source resistance increases as the channel is no longer saturated.

[xorgmc]

It could work OK using that method.
You will need a low gate-source threshold voltage VGS N-CH MOSFET and preferably one with reasonable drain-source on resistance RDS(on).
Keep in mind that as the capacitor discharges, the LEDs will eventually dim and the MOSFET will eventually start to dissipate some heat as the drain-source resistance increases as the channel is no longer saturated.

Like this? http://www.produktinfo.conrad.com/datenblaetter/550000-574999/563625-da-01-en-MOSFET_N_KA_100V_32_ZVNL110A_TO_92_3_DIN.pdf

Is there a formular to calculate the drain current at a given voltage?

[xorgmc]

Or is a TN0604 better? (http://ww1.microchip.com/downloads/en/DeviceDoc/TN0604%20D080813.pdf)

[Chalcogenide]

Take a look at the AO3400. It's logic level rated and can handle a couple of amps of current with <50mOhm RDSon. Super-cheap and small (maybe too small if you are new with soldering).