External button interface design help and review

Hi Chris,

For accepting switch inputs, your aim is to have a scheme where each input pin on the Arduino sees either 0V (GND) or 5V. It doesn't matter which one, but the aim is to have it become the opposite selection whenever the button is pressed.

So, for example A0 could be at +5V, but when the button is pressed, it could become 0V. And so on for the other switches and inputs.

A switch on its own can't (generally) work, because it just has metal conductors inside it, and if you short 5V to GND, then lots of current flows and damages the switch.

The resistor is used to reduce current. Think of a series circuit, consisting of (in this order) +5V, Resistor, Switch, GND.

Now if you use a multimeter, stick one end on GND, and the other end at the junction between the resistor and the switch, you'll see that the voltage measured is +5V. But, when the switch is pressed, the voltage measured is 0V because the switch has very low resistance (almost zero ohms) and so the junction voltage becomes close to 0V.

So, if your Arduino input (A0) was connected to that junction, then it too would see +5V normally, until the switch is pressed.

See here for an example. It isn't for an Arduino, so ignore the labels. You would build this circuit for each input, i.e. as many resistors as there are switches.

ok so i did this but I don't really understand what I need to do, ive read your message many times but still haven't understood it could you try to explain it differently sorry I am awful at circuits. when you say "as many resistors as there are switches" i can't figure out how to do that as there is no way I can see to connect ground and +5 and the pin

Hi Chris,

Just build the circuit shown earlier (the one in my previous response), which uses one resistor, and one switch. You can connect the top junction to 5V, and the bottom junction to GND.

The center junction (marked GPIO) goes to A0 on your Arduino.

Test it, run your code, and you'll see that the Arduino code sees the input going low when the switch is pressed.

Now, to replicate this to multiple switches, just repeat the entire circuit. For each repeated circuit, connect the top to 5V, and the bottom to GND. Connect the junction to A1 for the next one, and A2 for the third one, and so on.

Since you're repeating the circuit, you need as many resistors as you have switches.

If you wish to understand the circuit (important, so you feel confident in what you're doing), you can see that when the switch is not pressed, +5V is available at the GPIO pin through the resistor. But, when the switch is pressed, then the switch has less resistance than the resistor. It will be almost zero ohms, so electrically this is the same as the switch being replaced with a wire shorting the connections. That means that GND (0V) is seen at the GPIO connection.

This is known as a potential divider, where the bottom part of the circuit can be seen as a very high (near infinite) resistance when the switch is not pressed, and as a very low (near zero) resistance when the switch is pressed. If you google 'potential divider' it will be explained.

Hi Chris,

Just build the circuit shown earlier (the one in my previous response), which uses one resistor, and one switch. You can connect the top junction to 5V, and the bottom junction to GND.

The center junction (marked GPIO) goes to A0 on your Arduino.

Test it, run your code, and you'll see that the Arduino code sees the input going low when the switch is pressed.

Now, to replicate this to multiple switches, just repeat the entire circuit. For each repeated circuit, connect the top to 5V, and the bottom to GND. Connect the junction to A1 for the next one, and A2 for the third one, and so on.

Since you're repeating the circuit, you need as many resistors as you have switches.

If you wish to understand the circuit (important, so you feel confident in what you're doing), you can see that when the switch is not pressed, +5V is available at the GPIO pin through the resistor. But, when the switch is pressed, then the switch has less resistance than the resistor. It will be almost zero ohms, so electrically this is the same as the switch being replaced with a wire shorting the connections. That means that GND (0V) is seen at the GPIO connection.

This is known as a potential divider, where the bottom part of the circuit can be seen as a very high (near infinite) resistance when the switch is not pressed, and as a very low (near zero) resistance when the switch is pressed. If you google 'potential divider' it will be explained.