How does the Arduino 5VDC relay module work? Using SRD-05VDC-SL-C

Question

I was wondering how this 5V relay module works exactly.

I understand how the actual relay circuit (SRD-05VDC-SL-C) works but I'm not totally sure how the signal and 5V inputs work.

My guess would be that the 5V is somehow dropped down to, lets say 4V, and then is combined with the signal voltage (let's say 3.3V from a 3.3V pin from an Arduino.) If the signal input is HIGH then the relay (SRD-05VDC-SL-C) receives a voltage greater than 5V and closes, but if the signal input is LOW then the relay is receiving just the 4V from the VCC and stays open.

If that is the case, how does the relay module do this using diodes, resistors, etc.

If that isn't the case then does anyone know how this thing works?

Answer 1

The arduino controls a npn transistor. The transistor controls the relay. There is no mixing of voltage as you describe. The transistor is current controlled. The relay will get 5V across it, or be disconnected like a switch, if we only look at on/off operation.

The basic schematic is basically this

The transistor acts like a switch.

Answer 2

I would guess the schematic is like this:

^{simulate this circuit – Schematic created using CircuitLab}

The relay coil requires about 90mA so the base current should ideally be roughly 5mA+, meaning for a red LED the input voltage should be 3.3V to 5V for "ON" and more like <1V for "OFF" (there's no emitter-base resistor so leakage should be minimized).

It's a pretty minimalist circuit with a shared supply and no isolation beyond what is provided by the relay.