# Trouble understanding circuit + quick question

Hello! I'm new to electronics, or more specifically, physical electronics. I'm a developer, but I'm not a technician. I recently received the Student Kit in order to begin my electronic adventure; thus far, I've grasped the most of what is mentioned in the courses, but I've run into two challenges.

I'm at the "TRAFFIC SIGNALS" class and I'm having problems understanding the circuit:
1 - The resistors are here to supply enough energy to the LEDs to turn on but not too much that they burn out, so why are they behind the LEDs?
2 - In addition, the pull-down resistors article, which describes the method of pull-down resistors, shows that the current flow is from GND to digital pin 2, isn't it meant to be the other way around?
3 - Another thing about that circuit is that the lesson states that the top-pins for the button ALWAYS communicate EVEN IF THE BUTTON IS NOT PRESSED, so why isn't the current flowing through them when the button is not pressed when the current appears to be going directly to GND to Pin 2 without passing through the top-pins of the button?

Diagram schematic : https://i.imgur.com/j7ej2MO.png
Arduino schematic : https://i.imgur.com/v2WgrmG.png

Last issue or question should I say is why do we measure everything in volts? The lessons always asks me to measure the voltage between two points, shouldn't we be measuring the current instead?

I'd be grateful if someone could answer those questions and explain how the circuit works; I realize it's a lot, but I still don't get it after reading, analyzing, and verifying everything on my own.

P.S. I can supply links to other schematics, but I'm only allowed to provide two because I'm a new user.

Well now, some basic electronics here. The LED is not "able" to regulate its own current flow. A resistor controls the current flow in proportion to the voltage across it - "Ohms Law". So given that the LED operates with a quite specific voltage across it and you are providing a certain voltage across the resistor and LED in series, the resistor will have the applied voltage less the LED voltage across it, so will define the current.

For the green LED, we may estimate the LED will have 3 V across it, given that the Arduino provides (approximately) 5 V, the resistor will see the difference of 2 V and Ohms Law says that 2 V across a 220 Ohm resistor will result in about 9.1 mA which is an appropriate current for an indicator LED and for an Arduino output to provide.

An electrical circuit is a closed loop, the same current flows through all parts of a loop, so it does not matter in which order those components are arranged. The direction the current flows does matter however. Can't see which article this might be, but we do not get too concerned about which way current flows through resistors. Diodes such as LEDs; it does matter.

Just to confuse you, or at least mention, we (with experience) generally have the button connected to ground and the resistor to "pull up" to the 5 V. This imply means that the Arduino see HIGH when the button is not pressed, and LOW when it is. You simply write the code to suit that.

Not sure what you mean here by "top pins". If the button is not pressed, then no current flows because the Arduino input pin neither sources or sinks any current - it only "sees" voltage.

If no current flows, then using Ohms Law again, there will be no voltage across the resistor, so the Arduino input pin will effectively connected to whatever is on the other end of the resistor, whether that is Vcc or ground.

Voltage between two points is measured in Volts. To measure current (Amps or fractions thereof) in a wire, you have to break the connection and re-connect it through a current meter.

Because the order of components in a series circuit can not change the series current value. It's because there is only one possible current path, thus the current through each individual component must be the same.

So, although in your stated case, unlimited current is presented to the LED, it's still limited by the resistor since it has to return to ground through it.

I could invoke Kirchoff's current law to further justify it, but it might confuse you.

We don't. We measure what needs to be measured, using the appropriate connections to make that measurement.

If you want to measure current, you certainly can, but it's a different connection.

Okay, but given that an LED has an input and output pin/leg, cathode (-) and anode (+), why is the resistor connected to the cathode pin when it should be regulating the flow before the anode pin in order for the LED to function properly?

This is the article : https://studentkit.arduino.cc/resources?id=18990536

This doesn't completely address my issue; I understand what voltage is; it's essentially the 'pressure' between two places. But why would we need to measure voltages between two points?

By the way, thank you for assisting me! Because it does not matter in which order they are. They are in series and so the current in both LED and resistor will be the same..

In order to know which way the wind blows you will need to measure the pressure in two places.... Then the wind will blow from the place with highest pressure to the place with lowest pressure. Same with current. I flows from place with highest voltage to place with lowest voltage.
Measuring current is unhandy, you have to take your circuit apart. Voltage can be measured without disturbing the circuit. Another thing: when you measure voltage, the resistance of your meter is close to infinity, therefore you will not easily cause any damage. When you measure current the resistance of your meter is close to zero. If you make a mistake, current will go to infinity, you will blow your circuit, your power supply and or your meter... If you measure voltage and happen to know the resistance, you can easily calculate the current. In your case you could measure the voltage over the 220 ohm and calculate the current through the resistor, which will be equal to the current through your LED.

Because a measurement of voltage is known as a potential difference. So you need two points to measure a difference.

If you just connect one probe of a voltmeter you can never get any reading at all.

Normally you measure between ground and a point. Where ground is the common ground of the circuit. The voltage point you measure can be either above or below this reference ground. This determines the sign of the reading of the meter.

If you get connect the positive of your meter to ground then any positive voltage will read as a negative voltage.

It doesn't. That is correct. Anode goes toward the positive supply, whether through a resistor or not. Correspondingly, the cathode goes toward the negative supply, whether through a resistor or not. In that regard it is symmetrical, there is no "input" and "output"; both terminals are an "input" to the LED. Since the current - as I have explained - flows in a loop, it does not matter where the resistor is in the loop, as long as it is there.

Sorry, but that link goes nowhere. Apparently it is locked to a transaction you have made with Arduino.cc. Very few of us here would have such a kit so it means nothing to us.

Well, that relates to how you might have it on the breadboard, but is not a good description of the circuit.

I find it much simpler to ignore that connection and just wire tack switches like that across the diagonal. That is use only two connections from one corner to the other. This way the switch always works no matter what 90˚ rotation you have mounting the switch.

I think your imagining that current starts flowing at one place and then this movement progressses along the circuit encountering components one-by-one. It doesn't work like this at all.

The current flows everywhere at once in lockstep, like water in a pipe - you can place a valve anywhere in a loop of pipe to stop the flow everywhere.

Charge cannot just build up somewhere in the circuit (for if it does so the voltage will rise to enormous values, like in a Van-der-Graff generator). In low voltage circuits the charge has to move accurately(*) in concert everywhere in the circuit loop.

Note that the standard analogy of electricity to water flow in pipes is good precisely because water is incompressible.

(*) For instance even a local build up of 1µA for 1ms in a typical circuit with a few pF of stray capacitance would create ~100V potential - you'd notice this(!) and the potential would simply prevent the charge build up before it happened. This is why we use Kirchoff's current law in most circumstances. Look up Kirchoff's laws and in particular how they provide simple equations for series and parallel circuits.

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