insideGadgets

Last week we built an automatic voltage switching circuit and now it’s time to apply this circuit to the Standalone Temperature Logger (SATL).

Let’s bring up the new schematic for the SATL with the voltage switching circuit. As you can see like last week, we separate the grounds of the voltage sources – the 3V battery and USB. Notice that CRTGND is not the same as GND, CRTGND connects to the mostfet drain/schottky diode.

There is a downside to this circuit which is that there is a voltage drop when running from the USB side due to the diode. The USB voltage can vary between 4.75V to 5.25V and the good news is that our ATtiny85 can run fine at 16.5MHz whilst at 4.0V.

If the USB voltage was really 4.75V, we would just make it to 4.05V but we want the best possible voltage even if it was 4.75V. Instead of using just a standard diode we’ll use a schottky diode which can provide a lower voltage drop of 0.3V instead of the 0.7V from standard diodes.

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Today we’ll introduce the N Channel Mosfet and show how we can use it to allow us to switch between voltage sources.

Mosfet explained and example

We can think of a mosfet sort of like a transistor, it has a gate, drain and source compared to the transistor which has base, common and emitter.

With transistors that we want to act as a simple switch it’s all about adjusting the current at the base however with mosfets it’s actually the voltage at the gate that turns them on. Because we don’t lose current when turning the mosfet on, this means that it’s actually more efficient than a transistor.

In datasheets you see it called Vgs and they provide you a minimum voltage that is needed before the mosfet will allow current to flow through the drain and the source.

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