How to determine transistor specification range for voltage and current (in relation to design requirements)
You want something like this (note, it could be done with FETs -- but I like BJTs).

You need to supply current to the load, and you want to do that through a transistor.  I'm assuming you want the load to be grounded (you can do this with one transistor if not).

Because you are driving your load from a voltage higher than your processor's VCC, you need voltage amplification.  That requires a transistor stage (Q1).  In addition to assuming that you want the load to be grounded that you don't want to consume great gobs of current when the load is off.  That requires an additional stage that will conduct no current when the load's voltage is 0 (as opposed to a circuit with your topology, which basically requires that the load be shorted).  This assumed requirement is satisfied by Q2, which only conducts when the load is on.

Going backwards from the load:


The load draws 14mA.  A 2N3906 is good for 60V and 100mA, so it's more than adequate for switching 24V and 14mA.
To Q2 in saturation, you need to pull about 1/10th the collector current from the base (you can use less if you use a "super beta" transistor, but why?).
In saturation, Q2's base-emitter voltage will be about 0.7V.


R3 pulls about 1.55mA from the node at the base of Q2
R4 pushes about 0.15mA into it.  The sum is 1.4mA -- good!

R4 is there to make sure that Q2 turns off positively when Q1 goes off.
Q1 needs to pull 1.55mA.  So it needs a base current of 0.155mA (it's the same 1/10th rule).


R1 supplies 0.43mA to the node at the base of Q1
R2 sucks 0.07mA from that node.  That leaves 0.38mA for the base of Q1 -- that's plenty.

R2 is there for the same reason as R4 -- to make sure the transistor turns of promptly.


And that's it, pretty much.  If this were for production, I'd suggest you use pre-biased transistors (basically what you see here, but with the resistors built in).  If it's a one-off, just grab resistors and transistors from your junk drawer.







simulate this circuit &ndash; Schematic created using CircuitLab