snek-balloon

Sat, 20 Apr 2019 21:59:59 -0700
tags/snek

Snek and a Balloon

(you can click on the picture to watch the model in action)

This represents the scale of projects we typically do in our Lego class. A motor, a couple of sensors and some simple logic. Here's the Snek program driving the balloon:

# Make the balloon go up and down

motor = (D9, D8)
top = A0
bottom = A1

def up():
    talkto(motor)
    listento(top)
    setpower(0.5)
    setright()
    on()
    while read() < .8:
        pass
    off()

def down():
    talkto(motor)
    listento(bottom)
    setpower(0.3)
    setleft()
    on()
    while read() < .8:
        pass
    off()

def play():
    while True:
        up()
        time.sleep(10)
        down()
        time.sleep(10)

play()

Light Sensors and Lights

I like to use light sensors with robotics and wanted to make some new ones sensitive to visible light. I found these Vishay TEPT5600 photo-transistors which are sensitive to most visible light. Using those with a 470kΩ resistor generated a good range of outputs for indoor lights.

For a light source, I'm using Cree 5mm green LEDs with a 5V power supply and a 100Ω current limiting resistor, these draw about 20mA and generate a lot of light.

For both of these, I take a 1x4 brick and drill a 3/16" hole in one end and a 1/8" in the other. The 5mm device fits snugly in the larger hole and the wires thread out through the other hole. The 1/8W resistor is tucked inside the brick.

You can see both of these in action in the example — the white bricks contain LEDs and the red bricks contain light sensors. The path between the light and sensor is interrupted by the model, allowing the program to determine when the motion has completed.

Motors

This model uses the Circuit Cube motors that I discovered last month. There's one visible near the middle of the Ferris wheel and another one just behind the Duemilanove board driving the balloon mechanism.

These motors have a built-in gear reduction and so they offer low speed and high torque. For Lego modelers, that's a pretty useful combination as the older non-geared motors always involved a long gear-train or worm gear to provide a reasonable balance of speed and power. For the balloon, there is a string wrapping around an axle driven directly by one of these motors.

Simple Models with Simple Programs

This class uses Lego to build simple mechanical devices that are controlled with simple computer programs. The Snek environment is the latest in a long history of computer systems which started with Lego Logo on the Apple ][ over twenty years ago.