Hi,
Perhaps easier said than done. "Logic Level" is often taken to be (a "TTL" Level of) 5 volts, but the OP is using down to 3.5 volts. However, MOSFETs weren't even available when I was designing logic systems, so I decided to "research" the issue and found
THIS LINK which includes the comment:
"Another thing to beware of in datasheets is Vthresh (threshhold voltage). This is not the gate voltage to turn the device on, its the gate voltage at which it switches fully off (less than a few uA of current, typically). If Vthresh is given as 2..4V range, it cannot be a logic level MOSFET (Vthresh is usually 0.5 to 1V for logic-level MOSFETs).
Another link suggested that the "only"
Through-Hole Logic-Level PMOS FET is the
Fairchild NDP6020P . That does indeed look like a "useful" device (which is why I'm posting its part number), but complete "overkill" for this application ! : The PICaxe here is very unlikely to draw more than 50 mA, so the OP's original proposal of the 2N3906 should be "good enough" (but something like the BC327 has about 4 times lower Vcesat). They just need to use the "rule of thumb" that (to "saturate" a transistor) the base current should be 1/10 times the collector current, e.g. 5 mA. Q4 can easily deliver that, so it just needs a resistor of about (3.5 - 0.7) / 0.005 = 560 ohms.
But as for Q1 and Q2, I still would like to know how much current
they need to be rated for. They possibly
DO need to be Logic Level FETs (however, it does depend if the "LEDs" have series resistors, or if those transistors are being used as current sources).
Cheers, Alan.