How do I get 5V?

[braza]

Hi! It seems that somewhat similar was asked, but the discussions were too deep for my pure programmer mind. I have no sense of circuits, sorry
So I have an SD card that logs data from accelerometer and IR sensor. If powered from USB everything works fine. If powered from accumulator or battery, IR sensor stops working. I've measured voltage between 5V and Gnd - it was like 2.1V.
Is it expected?
What steps should I take next?
Thanks!

[Ben Rose]

Ah- Think of the 5V as an input, it's being supplied by the USB while plugged in. When there's no 5V source attached, you'll see some sort of voltage, but it's not useful as a source, and definitely won't be 5V if the other source is a battery below 5V.

The right spot to power an IR sensor from is VCC(main/processor voltage), but you'll definitely need a sensor that works at your battery voltage.

[braza]

Ah- Think of the 5V as an input, it's being supplied by the USB while plugged in. When there's no 5V source attached, you'll see some sort of voltage, but it's not useful as a source, and definitely won't be 5V if the other source is a battery below 5V.

The right spot to power an IR sensor from is VCC(main/processor voltage), but you'll definitely need a sensor that works at your battery voltage.

Thank you, good stranger