Read a pin

A digital read operation returns a true if the input voltage on the pin is at a high level. It returns false if the input voltage is at the low level. On the Adafruit Circuit Playground Express you read the input value of a pin with ||pins:digital read pin|| block.

let inputValue = pins.A1.digitalRead()

Pin input

A pin has a small range of voltage where it detects the input level as high. The diagram below shows that the A1 pin is at the high level when its input voltage is in the range where the graph shows red.

The second diagram is for the opposite condtion when the input level is low. At some point, as the voltage on pin A1 is at or near 0V, the pin read will return false.

Experiment: Touch high and low

Setup:

1. Connect an alligator clip lead to the A1 pin.

2. Download the following code to the board:

forever(function () {
    if (pins.A1.digitalRead()) {
        light.setAll(0xffff00)
    } else {
        light.setAll(0x007fff)
    }
})

Test: Briefly touch the unclipped end of the alligator lead to both the 3.3V pin and the GND pin.

Result: The pixels show a different color for each input level, high and low.

Switches and buttons

Switches and buttons are the most common ways that people interact with electronic devices. A switch closes or opens a circuit and usually stays in one of those positions for some time. A button will close a circuit momentarily and then open it again when released. Here’s a picture of a switch for a house light and a momentary pushbutton switch for a doorbell.

The following diagram shows how a switch (figure A) or a button (figure B) completes an electrical circuit to turn on or off a light.

In both circuits is light is the load for the power supply in the battery. Current will flow to the light when the circuit is complete. Unlike the circuits above, a switch or a button connected to a pin on the Adafruit Circuit Playground Express isn’t used to supply power to a load like a light. They are used to signal a logic condition of true or false by connecting the pin to a high or low voltage level.

Digital input electronics are designed to use an exrtremely small amount current when connected to a voltage source, like with the switch shown. The purpose is only to detect the voltage level present at the pin and not to use current to power anything.

Experiment: Switch pin input to high

Setup: Connect an alligator clip lead to the the A1 pin. Clip another lead to the 3.3V pin. Connect the other ends of the each lead to a terminal on a light switch.

Download the following code to the board:

pins.A1.setPull(PinPullMode.PullDown)
forever(function () {
    if (pins.A1.digitalRead()) {
        light.setAll(0xffff00)
    } else {
        light.setAll(0x007fff)
    }
})

Test: Turn the light switch on for a short time then turn it off.

Result: The pixels show yellow when the light switch is on and blue when off.

Optional test: If you have a pushbutton to use, connect it to the Adafruit Circuit Playground Express like you did with the light switch. The LEDs should show yellow when you press the button down.

In the experiment above, the ||pins:set pull pin down|| block was used in the ||loops:on start||. This is so that the pin will always read false until the switch is turned on. Using a pin pull is described more in the Input states section.