How to drive a stepper motor with a PIC microcontroller

We've made a few posts about this before, while investigating how stepper motors work, but with our recent CNC drilling machine competition entry, we're getting a few questions once again about how it works. So here's a brief outline of what's happening when you control your stepper motor.

If we were to build our stepper driver from discrete components, it'd look something like this:

Stepper Darlington


The stepper motor has a simple operation. By energising coils in a specific sequence, we can make the motor turn clockwise or anti-clockwise. (energise them out of sequence, however, and the motor simply chatters or makes a buzzing noise!)

What often confuses people is that we're not switching the power rail, but the 0v/ground rail. Let's take a look at the layout above.

Ignore the diodes. We'll come on to those in a minute.
Those funny transistors are basically one transistor feeding the gate of another. It allows teeny-tiny currents to be amplified quite a lot. Basically, it means we can power really beefy loads (motors) with really small signals (microcontroller outputs). Such transistors are often called darlington transistors.

Now, every coil is permanently connected to the power rail. It's only by allowing current to flow through each transistor connecting the 0v/ground wire to ground that the coil can be energised. So when we make PORTB.7 high on our micrcontroller, it allows current to flow from the power rail, through coil 4 and down to ground (through Q1), effectively energising coil four. If we send PORTB.7 low and bring PORTB.6 high, current can now flow from the power rail, through coil 1 and down to ground (through Q2). This energises coil one. This causes the stepper motor to start to rotate anti-clockwise. Triggering the other coils in the correct sequence causes the motor to rotate.

Now, those diodes. What are they for?
When a coil is energised, it creates an electro-magnetic field. When you turn the coil off and the magnetic field collapses, it can create what's known as "back-emf" in the wires. Basically, we could get a spike of voltage - if we leave this hanging around, it could easily zap some of our sensitive components. So we use a diode to direct any harmful back-emf safely to ground.

Luckily, we're using a ULN2003A darlington array.
Unfortunately, this is often were the confusion comes in!

Those free-wheeling diodes shown on the datasheet can cause confusion. Trying to work out how to wire them up without a reference schematic like the one above can hurt your brain, but the trick is to remember that power is always connected and we're switching the 0v rail.

So the common pin is not 0v/ground as would be expected, but the power supply. The internal "free-wheeling" or feedback diodes simply ensure that any back-emf is handled safely.
So to connect a microcontroller to a stepper motor:

Following this logic, we've connected two stepper motors and two servos to our PIC to create a CNC drill controller board:

CNC drill schematic

Multiple touch sensitive inputs

In a previous post we talked about how to use a darlington transistor to create a touch sensitive switch.

For a device with multiple touch-sensitive inputs, it seems obvious that we repeat the single input for each pin we want to use as a touch sensor:



However, for each input our component count increases.
In the example above, we're using PORTB on the microcontroller, which has a built-in pull-up resistor on each input pin. If we need to use a different port, or want to use more than 8 inputs, we're going to have to add a pull-up resistor on each input pin. Also required - though not shown on the diagram above for the sake of simplicity - is a 1M pull-down resistor on the base pin of each darlington transistor.
So for each additional input, we're introducing three extra components.

It's worth noting, at this point, that for each touch sensitive input, we need two pads per input: one pad connected to the base pin on a transistor, and one pad connected to the 5V supply (the user touches and effectively creates a bridge between the 5V supply and the base input pin).

However, we can simplify our design massively by swapping the pads around:
instead of a fixed 5V supply and multiple darlington transistors, we can use a single transistor and multiple, variable 5V supplies. How to do this?
We turn each potential input pin to an OUTPUT and use this output to drive what would otherwise be a fixed 5V supply on the first pad. So each pair of pads that make up a touch-sensitive switch consist of an output pin (from the microcontroller) and a pin connected to the base of the darlington transistor.

a simple PIC/usb device with 16 touch sensitive inputs

Every touch contact would consist of a pair of pins/pads - one going to each of the numbered pads (PAD1, PAD2 etc) and the other a common input (COM_INPUT) so there would be a total of 16 COM_INPUT pads all tied together.

The pseudo-code would go something like this:

• turn off all outputs
• turn on output RA2 (PAD1)
• has RA0 gone low? (yes=finger present across PAD1 + COM_INPUT)
• turn off output RA2
• allow time for input to return high
• turn on output RA3 (PAD2)
• has RA0 gone low? (yes=finger present across PAD2 + COM_INPUT)
• turn off output RA2
• allow time for input to return high
• etc.

It depends on the type of darlington transistor you use (different transistors have different response/switching times) and the size of the pull-down resistor on the base pin, but in practice, with a 1M pull-down resistor, we found that 0.5ms (500us) worked well. Using this approach, we were able to poll all pins in under 10ms (0.5*16 = 8ms). At 100 times per second, this was more than responsive enough for our needs!

Until we can use the CNC again....

...the most exciting thing about getting a CNC machine working is the ability to quickly and easily drill our home-etched PCBs. But also, the ability to carve shapes and make enclosures for forthcoming projects is pretty cool too.

We've a few projects in the pipeline, which make use of some pretty simple but powerful underlying technology. After running a few workshops in and around Brighton and receiving a few emails from previous posts on other projects, we're going to document these in their entirety.

We'll be using the 18F2455 and 18F4550 PIC microcontrollers to create USB/HID devices. And we're also incorporating a simple touch-sensitive interface. At the minute, capacitive touch sensors are all the rage. What this basically means is that each touch pad connects to a microcontroller and when the user places their finger over the pad, a simple capacitor is created. The relative capacitance of the pad is compared over time and when the capacitance changes, the microcontroller can detect whether a finger has been placed near or removed from the pad.

The downside of capacitance touch sensing is the need for relatively large pads - or dedicated capacitive sensing hardware.

To keep our project simple - both in writing the firmware (capacitive sensing firmware can be quite convoluted and multiple readings averaged over time to smooth out any rogue analogue readings) and in sourcing the hardware - we're going to use an alternative approach:

A transistor is often used as an electronic switch, but it can also be used as an amplifier. A tiny current onto the base pin of an NPN transistor allows a much larger current to flow through the collector and emitter pins.



Whatever current is directed onto the base pin is amplified onto the collector pin.
By "feeding" the output from one transistor into the base pin of a second transistor, we can amplify the input signal many thousands of times over



In fact, by feeding one transistor into another, even the tiny amount of current that passes over the surface of your skin can be used as a switch. Such transistor pairs are available in a single package, known as a Darlington transistor.

Here's an example of how we can use a darlington transistor as a touch sensitive input device for a PIC microcontroller:

The schematic above uses a darlington transistor, such as a BC517 as a single discrete component. Although a darlington transistor is actually two transistors connected as shown above, we will draw it as a single transistor for simplicity.

Touching the two pads - however large or small they may be - causes the transistor to switch, forcing the current to flow from the input pin to ground. While this may seem counter-intuitive (normally you might expect voltage to flow into an input pin to indicate an input switch) the reason for this should become clear: on a lot of controllers (and had we put this input onto PORTB) you can use internal pull up resistors on the inputs, removing the need for the external resistor as shown in this example. If your controller does not have internal pull-ups, the resistor is there to stop the input pin "floating" when no finger is present on the contacts. The resistor should be quite a high value, say 100K.
Now, when the pads are touched, a tiny current flows from 5v on PAD1, over your finger, onto PAD2 and into the base of the darlington transistor. The transistor amplifies this current, creating a "switching effect" and causes the input pin to go low.
When the finger is removed off the pads, no more current flows into the base pin, the transistor closes the "switch" and no current can flow from the input pin to ground. The pull-up resistor causes the input pin to go high when the pads are not touched.

Although the above gives us a working touch-sensitive switch, we're not quite done. If you try the schematic out, you might find - depending on the type of darlington transistor used - that while the "on" trigger works (i.e. the input goes low immediately after touching the pads) the "off" time can be quite slow (i.e. the input pin remains low for a second or more after removing your finger from the pads).

The reason for this is that the darlington transistor can amplify even the tiniest little current - even residual electrical noise can be used as a trigger; it's a bit like leaving an input pin floating - the base pin of the transistor is so sensitive it can switch on and off almost at random. And like a floating input pin, it can remain active even when the input is removed.



The answer is to put a pull-down resistor on the base pin.
Now, when your finger is removed, any residual current on the base pin has a path to ground, and the switch closes. The size of the resistor determines the response time. If the resistor value is too low, it may stop the transistor switching on at all (the human body has an electric resistance of around 40k-100k so this base resistor needs to be much higher) but too high and the residual current on the base pin may take too long to be pulled to ground, resulting in slow response times. In practice, we found that a resistor with a 1M resistor on the base pin (R2), with a 100K pull-up resistor (R1) on the input pin worked well and gave reasonable response times.