Understanding how Schematic works

Question

i'm starting get into designing some IOT devices for around the house and looking other peoples design on Hackster.io. I've come across several designs that i'm having a hard time understanding. The below picture to me doesn't make sense. My understanding is the the electric flow starts with positive and flows to the negative. On the below picture for example we see GPIO pin 5 (blue wire) connects to resister then flows to the transistor then to the blue wire though the negative side of the led.

I don't know understand how this works since we would need current to flow through the resister then LED. When the developers code execute whatever on GPIO5, isn't this flowing the wrong way? I will go out and say i'm starting at the bottom with basic understand if that.

Answer 1

Here is one channel of your circuit in schematic form.

^{simulate this circuit – Schematic created using CircuitLab}

When the control signal from the micro is high, current flows into the base of Q1 and is limited by R1. This causes current to flow into the top, the collector, of Q1 effectively turning it on. As such current will flow from the power supply through the current limiting resistor R2 and the LED which will light.

When the control signal is pulled to ground, no current will flow through R1 into the base of Q1 so the transistor turns off as does the LED.

Answer 2

So there is a big difference between a bread board and a schematic. The breadboard has internal connections that don't span the columns.

As for what this would look like if it were a schematic, it is fairly simple.

You are using the wired inputs to control the transistor as a switch. This allows for greater current to flow through to the led to give you light from your power supply.

Remember that a picture of a breadboard is not a schematic, it is how the highway system looks when implemented. The schematic is the roadmap, and although the roads layout can be different (breadboard, pcb, etc), you still will get to the destination.

Answer 3

Transistor circuits still contain loops for current to flow through. See here for a diagram that may illustrate this better. In your example, when GPIO 5 is brought high, current will flow out of it, through the resistor, through the base to the emitter in the transistor, to ground. This will cause current to be allowed to flow from the collector to the emitter, making a second loop - current will flow out of the source (the Arduino 5V), through the second resistor, through the LED, through the collector-emitter path in the transistor, to ground.