Signals between devices

A signal is a message carried by electricity. A digital signal has only two levels.

An input pin listens for a signal. digitalRead(2) gives 1 if pin 2 is HIGH, and 0 if it is LOW.

Button A goes to pin 2. Button B goes to pin 3. Each LED shows one gate's answer.

• AND: on only when both buttons are pressed.
• OR: on when at least one button is pressed.
• XOR: on when the buttons are different.

void loop() {
  int a = digitalRead(A_PIN);      // 1 = pressed, 0 = not pressed
  int b = digitalRead(B_PIN);

  digitalWrite(AND_LED, a && b);   // AND: both
  digitalWrite(OR_LED,  a || b);   // OR: at least one
  digitalWrite(XOR_LED, a != b);   // XOR: different
  delay(200);
}

Stage 2 of the build uses no code at all. A 74HC08 chip holds four AND gates, made of transistors.

A 74HC32 chip holds OR gates. A 74HC86 chip holds XOR gates. Their pins are the same, so you can swap them.

Two micro:bits can send signals to each other by radio. Both must use the same group number, like a channel.

from microbit import *
import radio

radio.on()
radio.config(group=7)        # both boards use group 7

while True:
    if button_a.was_pressed():
        radio.send("A")      # send a message
    message = radio.receive()
    if message == "A":       # a message arrived
        display.show(Image.YES)
        sleep(500)
        display.clear()

1. Put the same program on both boards.
2. Press A on one board. It sends the message "A".
3. The other board checks for messages every time round the loop.
4. When "A" arrives, it shows a tick for half a second.