"Normally closed" switch: does it use more power than a "Normally open" for a low-power battery-powered ATtiny project?
Indeed a normally closed switch means that a current must flow otherwise you cannot detect when the switch opens (and there is no more current flow).

Fortunately there are several ways to minimize the current consumption.

The simplest way is to use a very high value pull-up (or pull down if you prefer) resistor, a value of a couple of Mega-ohms will reduce the current to a few uA which should not affect battery life much as a microcontroller will also consume a few uA on average when it is in deep sleep mode and only wakes up occasionally to check if it needs to do something.

This is an example schematic:







simulate this circuit &ndash; Schematic created using CircuitLab

It could be that you're using long wires to the switch and then using Mega-ohm pull up/down resistors might not work reliably.

Then another option might be better if it is OK to "poll" the state of the switch, check it every second for example instead of continuously. For that, configure an output
of the micro to become high for a short time every second, connect the switch + pull up resistor to that output. Monitor the voltage across the switch as you would normally do. Then when the output is high, monitor the input.

Here's an example schematic:







simulate this circuit

If this circuit would run on 3.3 V, only 1uA would flow during the polling of the switch. Suppose I poll the switch for 10 ms every second then my duty cycle is only 1% so that would make the average current only 0.01 uA !

Obviously this only works if you do not need to know the state of the switch immediately.

In conclusion I do not think that there is a way to make a normally closed switch use less current than a normally open switch!

However, that isn't really an issue as we can make the design such that the impact of the additional current is negligible.