Grant-

You can also use bipolar transistors to achieve this. The approach depends if you want to switch the supply as Rob suggests with the mosfet or ground as Charles does with the opto.

I will presume switching the supply and for that you will need a PNP transistor like the 2N2907 or the BC557. Connect the emitter to the voltage supply, the collector to the load, and the base to the micro's port via a resistor. The value of the resistor is evaluated as follows. A transisor has a DC gain (Ratio of collector current Ic to base current Ib in linear conditions)sometimes denoted as hFE and sometimes as the greek letter Beta. Its value depends on the current flow through the collector of the transistor Ic.

The data sheet for the 2n2907 can be found here
http://www.onsemi.com/pub/Colla...907A-D.PDF .

You will see that at 10 mA and 150mA the minimum gain is 100. In order to get the minimum collector-emitter voltage, you must saturate the transistor. There are a number of approaches to guaranteeing that a transistor is saturated. Back in the stone age, we were taught that you should guarantee that the current drawn out of the base is 10 times greater than the collector current. So we need to draw ((70/100)*10)=7mA from the base. (The 70 derives from your 70mA requirement.)

The resistor value determines this current (assuming that the micro can sink 7mA). The voltage that drops across the Emitter/Base junction of the transistor is 0.6 to 0.7V- worst case call it 0.7V, so that the voltage on one side of the resistor is 3.3V-0.7V=2.6V

Assuming that the Vout of the micro at 7mA is 0.1V the voltage across the resistor is 2.6V-0.1V=2.5V. By Ohm's Law the resistance value should be less than 2.5/7=0.36K. Choose 332R.

Although the Vcesat from the ON CHARACTERISTICS on page 2 of the data sheet guarantees a maximum of 0.4V across the range at 150mA, you can see from figure 4 that the voltage is closer to 0.1V.

In quantity this solution will cost pennies. If your processor cannot sink 7mA, then use a transistor with a higher gain like the BC557.

You would need to modify this approach if the micro is running at a lower voltage than the supply you are trying to switch.

-Aubrey