What do I need?
I am told that I need some transitors. I have a 12v powerpack and 2 x12v motors. How do I set this up so that I don't blow-up the RPi and that I can control the motors through the GPIO?
I have a gertboard on the way but I would like to see if I can do it without it first.
I have little knowlegde about transitors and electronics but I do know how to build circuits.
If all you want to do is turn a small DC motor on and off, all you need is Gert van Loo's circuit in this thread's 7th post: http://www.raspberrypi.org/phpBB3/viewtopic.php?f=44&t=13473. The motor is driven by an NPN Darlington Pair (http://en.wikipedia.org/wiki/Darlington_pair), which is a great way to convert a small current into a really big current. The three diodes in series are for people are afraid the Darlington pair might accidentally conduct from collector to base. Gert says they're not necessary, but with 12V present I'd worry about framboise flambée and put them in.
The diode next to the motor coil is critically important. It's called a flyback diode (http://en.wikipedia.org/wiki/Flyback_diode). When you turn on the transistor, the flyback diode is reverse-biased and has no effect. However, when you turn off the transistor, the magnetic field in the motor will cause current to continue to flow for a while. If that current can't go anywhere (because the transistor is off), it will charge the node to a very high voltage and turn the transistor into a fuse and possibly destroy anything connected to the transistor. It's how spark plugs work. The flyback diode gives the current somewhere safe to go to prevent Spitzensparken. The diode should have a current rating higher than the current through the motor.
Gert doesn't give a value for the resistor between the GPIO and the base of the Darlington pair. I'd start with 1 KOhm which gives you 3.3 mA. But then, I usually start with 1K 
Here is the schematic. JP1 is meant to be the motor. I should mention that the transistor logically invert their signals.
When the GPIO is high, the chip input is low. So from the Pi's perspective, they are active when they are zero.
You need two power supplies. The solarbotics board has an on board regulator to provide the +5 volts.
The Pi only has one PWM output, so you could do PWM in software but get the motor going fast first.
Make sure the motor fits within the chip's power output max or just buy extra chips.
The 2N2222A2N2222A is cheap. Buy extra. I live in the US so I would get them here. http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_38236_-1.
Or use Newark/element 14, or Digikey, or Mauser.
 |
Ken Seymour wrote:
The schematic is for TI's L293D chip, which was posted above in a link from Adafruit.
It is a dual H-Bridge motor driver.
The E on the end of the part number is from the Eagle part library, but the pinout is the same.
How can I get PWM working on the motors? Can you please give me instructions on software and/or extra hardware required (input device) for PWM to work.
Thanks for all your help so far.
Hardware such as what gert used in his video (the slider at 3:40):
