Wednesday, October 26, 2016

107: Controlling a Relay Switch

In the previous post (106), I mentioned using a relay to reduce power drain when operating a stepper motor with the EasyDriver. I covered switches before in post 94. There is yet more at, post 45.

There are a couple confusing things about these relays:

1. These devices are "normally open". Pin settings are backwards:
      GPIO.output(Relay, GPIO.HIGH) # turn OFF
      GPIO.output(Relay, GPIO.LOW) # turn ON

2. On the Raspberry Pi there is a second gotcha. See image:

Note that I used the upper 2 of the 3 relay connectors (explained in previous posts). Yesterday, I connected a relay like the picture above. But instead of switching, it was always ON! I tried it with an Arduino -- worked fine. Then I realized that I had connected my breadboard's 5V rail to VCC on the relay instead of 3.3V. The GPIO pin only does 3.3. A dumb, but easily made mistake. When I changed to the 3.3V rail everything worked.

Note: I used the cheap non-opto-isolated relay in this case because I was only switching 5V/.5amp.

Wednesday, October 12, 2016

106: EasyDriver + Linear Stepper Motor
-- Finally --

I've long lost track of how many failed attempts I've made to get this to work. Here's my successful EasyDriver wiring:
The messy setup

Here's some Python code that works for me.
Note: I had to install PWM -- see

import RPi.GPIO as GPIO
import time, sys
STEP = 22
DIR = 17

sp = GPIO.PWM(STEP, 800)# 800 seems faster than 500 ???

# usage: sudo python direction steps
dir = sys.argv[1]        # f or b -- forward or back
steps = int(sys.argv[2]) # max on my linear screw about 700

def spin(dir, steps):
  GPIO.output(DIR, dir)
  while steps > 0:
    time.sleep(0.005) # smaller delay looses steps ???
    steps -= 1
  return True

if dir == "f":  # i.e., "forward" away from the motor 
  dir = False
  dir = True
spin(dir, steps)
print "Done"

More about the linear motor, see:, post 46.

Not represented in the above code: I added a relay switch for the 5v power to the motor. Otherwise steppers draw current constantly (and the motor housing can get hot). Too bad that the EasyDriver doesn't work like the ULN2003 -- which allows you to turn the 4 motor leads off without an added switch. Re relays: I got 5 single-relay devices on the cheap from Amazon; they lack opto-isolation and have no mounting holes -- but cheap.

Sunday, October 9, 2016

105: RPi Stepper Motor
-- revised sleep time --

After much pain (read: time wasted) I have a half-assed program that runs the cheapo 28BYJ-48 motor (5V DC with ULN2003 Driver Board).

Here's some Python:

import RPi.GPIO as pin, time, sys


Cpin = [6,13,19,26] # to UNL2003 in1, in2, in3, in4; external 5v/GND


# rotate clockwise
Cw = [[1,0,0,0],[1,1,0,0],[0,1,0,0],[0,1,1,0],[0,0,1,0],[0,0,1,1],[0,0,0,1],[1,0,0,1]]
# counterclockwise
Ccw = [[0,0,0,1],[0,0,1,1],[0,0,1,0],[0,1,1,0],[0,1,0,0],[1,1,0,0],[0,0,0,1],[1,0,0,1]]

def motorOff(): # so it stops drawing current
pin.output(Cpin[0], 0)
pin.output(Cpin[1], 0)
pin.output(Cpin[2], 0)
pin.output(Cpin[3], 0)

def moveCw(steps):
for i in range(steps):
for j in range(8):
for k in range(4):
pin.output(Cpin[k], Cw[j][k])
time.sleep(0.002) ### Changed! .001 caused missed steps

def moveCcw(steps):
for i in range(steps):
for j in range(8):
for k in range(4):
pin.output(Cpin[k], Ccw[j][k])
s = int(sys.argv[1])
print s

# Full rotation approx. 128.3. How do you get an exact number?

print "cw"

print "ccw"



The following bits also work for counterclockwise. But which (if either) is better?

I spent even more time trying to get a linear stepper motor with the (so-called) EasyDriver to work. No luck. I have 2 sets of this stuff and both work with an Arduino. See, posts 43 and 46.