Three volts three, and nothing else

The whole bill of materials

Both boards are the same short list. On the microSD board: one socket, six 22 Ω resistors, five 10 kΩ resistors, one 5.1 kΩ resistor, a 10 µF and a 100 nF capacitor, and a red LED. The full-size board is the same with two more 10 kΩ resistors, for the two switches its socket has.

That is the list. There is no regulator on it and no level shifter on it, and those two absences are the only thing on this page.

An SD card is a 3.3 V part. Its supply range is 2.7 V to 3.6 V, and a signal above its supply is out of spec on any input. With nothing in the wires, your board's logic level is the card's logic level.

Why the board is built this way

Because the parts that would protect you also get in the way. A regulator drops a volt and needs current it cannot always supply on a burst; a buffer adds propagation delay and a direction problem, because the data lines go both ways. Leaving both out is what lets this board run 4-bit mode at 40 MHz — which the buffered modules cannot do, because their buffers only pass signals one way and only cover the four wires SPI uses.

It is a board for a 3.3 V host, and on a 3.3 V host it is the simplest thing that can work.

The LED

The LED is on whenever the board has 3.3 V. It is not connected to a GPIO, it does not blink on card activity, and it cannot be turned off. If it is dark, check the 3V3 wire before you check anything else.