Why does this active-low relay work when floating?

Question

I have the following circuit:

^{simulate this circuit – Schematic created using CircuitLab}

When I digitalWrite(HIGH) the GPIO pin, the transistor closes and thus the relay's input is grounded, causing it to switch.

However on digitalWrite(LOW), the transistor opens, so the IN pin is floating, yet the relay switches.

I thought the operation was working on strokes of luck. But when I connect 5 V to pull it up like this, it won't switch when I make the GPIO pin high. I thought the connection to ground would cause it to be pulled down.

^{simulate this circuit}

(The module looks like the one shown in this question.)

Answer 1

I found out why it is working.

For anyone encountering this, please read Justme's answer first.

---

Here is a typical active-low trigger relay module circuit:

^{simulate this circuit – Schematic created using CircuitLab}

When the input is floating, the base-collector junction in the PNP transistor, which would be forward biased, acts as a pull-up.

Thus, the relay switches.

---

In case of NPN, the base-emitter junction would be forward biased, and thus act as a pull-down.

Another way to understand it is this: leakage current from base to emitter will bring the base potential down to emitter potential, which is low. Hence, the transistor will be off. (Refer Neil_UK's answer to Transistor with floating base)

Base to emitter leakage current is encountered during any basic BJT device physics study. Emitter injection efficiency (γ) signifies that, since this leakage current is responsible for recombination in the emitter, reducing the efficiency.

Answer 2

You don't have a relay, you have a relay module with some other electronics as well in addition to the relay, and that part is unkown. So we don't know why it works like that. You need to show the schematics of the whole module before someone can say why it works like that.

But it does not matter as the problem may be elsewhere.

Your first schematic has an error that there is no base resistor on transistor. It may have damaged the MCU output pin or transistor.

The second schematic has even greater error. In addition to not having a base resistor, turning on the transistor will try to short circuit 5V to ground directly. The transistor may be damaged even more now.

So if the transistor is damaged it may not work correctly any more and can drive the relay module input to some state regardless of what the GPIO pin does. But the short circuit needs to be removed first.