Using a microbit to control a servo motor

Subtitle: Making things move a microbit at a time

In this post I explain how I set up and operated a small servo motor that runs directly from the same 2xAAA battery pack that powers my microbit. I then explain how to operate a servo motor that requires a 5V power supply from the same microbit, using a transistor as a voltage level converter for the control signal.

Introduction

I set up a small servo motor for a project to make an automated guinea pig feeder. Details on how to use this are here. Naturally, mistakes were made. I will detail the hardware setup, explain the control signals for the servo and show where the software can be found.

Hardware: SG51R servo motor

I used a Tower Pro SG51R servo motor, which runs from the same 2xAAA battery pack as the microbit. The battery pack, microbit and servo motor can be seen in the photo below. I slotted the microbit into a Kitronik edge connector.

2xAAA battery pack, microbit in a Kitronik edge connector and a SG51R servo motor

I got my SG51R servo motor from the Pi Hut here.

Why use this servo motor?

The SG51R works using a 2xAAA battery pack as the power supply, while many other servo motors require a higher voltage than 2xAAA batteries produce. This means it can run direct from the same battery pack used to power your microbit.

The manufacturer’s website says that this servo motor requires 4.8V to operate. However, it works fine with the 2.4V or so that we get from the 2xAAA rechargeable battery supply I use to power the microbit. This is great, as it means I don’t need to use a different voltage supply to power the servo motors with.

The servo has three wires for power and control. The black wire is for ground, the red wire for the voltage supply and the orange wire is the control signal.

The wires from the Kitronik edge connector to the servo motor connector can be seen in the photo below. I used D0 as the pin to output the servo motor control pulses on. The labels on the microbit are guides as how to use the microbit for the guinea pig feeder project.

The photo below shows how the wires from the microbit connect to the servo motor plug. Black is ground, red is positive voltage, yellow is the signal, from pin D0 on the microbit.

Wires from the microbit on the left, servo motor connector on the right

A servo typically responds to a control signal of a pulse between 1 ms and 2 ms long within a 20ms period. 180° needs a 1 ms pulse.

So what does that look like? The software I wrote to drive the servo motor is at the end of this article. I put an oscilloscope onto the signal pin. Here’s what it looks like:

Servo motor controller pulses

We see a positive pulse every 20 ms. Remember that these pulses go from ground to the battery voltage (about 2.4 V in this plot). We will revisit this polarity when we talk about driving a 5 V servo motor later in this post. The width of this pulse sets the angle of the servo motor. The pulse must occur within the same 20 ms window, just how much of this window it takes up changes.

From the data sheet for the SG51R:

This servo is pulse controlled. This means that supplying a pulse will move it to a particular angle. In this case 0° is a 1.5 ms pulse, 90° a 2 ms pulse and -90° is a 1ms pulse. This may not be set by your controller by default or give you the full range of movement as such you may wish to try up to 2.25 ms and as low as 0.75ms. Be aware that if you go to far you can break the servo.

Using my ‘scope, I could see that the servo motor responds to pulses with a width from 0.57 ms to 2.58 ms. Remember that the window that these pulses occur in remains as 20 ms wide. Just the width of the control pulse changes to take up more or less of this 20 ms.

Software

Initially I carefully crafted beautifully structured micropython code in classes. Then ran out of memory, so ripped out all of the classes and bludgeoned the code to fit into the memory of the microbit.

I did, of course, find example software elsewhere on the ‘net to get started and apologise that I can’t remember which site I found this on to give proper credit.

The code can be found in the file called feeder.py on my GitHub site at: https://github.com/hardwaremonkey/microbit_projects

5V servo motor MG90D

Maybe you need a more powerful servo motor for your application. In this case, you will probably need one that requires a higher voltage than we get from the battery pack for the microbit. This section explains how to control a servo motor that is running from a separate power supply using a battery-powered microbit.

The mg90d is a servo motor that requires a higher voltage than the microbit can supply to operate. I ran it at 5V from a power supply. So how do we set up our microbit to control this?

I used a 2n2222 transistor to interface the microbit with the servo motor control wire. The control signal from the microbit goes to the base of the transistor. There is a 1K Ohm current limiting resistor in between this signal from the microbit and the base of the transistor. The signal going to the base of the transistor switches the transistor on and off.

The collector of the transistor connects to the 5V power supply through a 100 Ohm resistor.

Remember: We are only using the transistor to change the voltage level of the control signal. The voltage for the servo motor comes directly from the +5V power supply.

When the control signal from the microbit goes high, the transistor turns on. When the control signal goes low, the transistor turns off.

Find the wiring diagram below. The images come from Fritzing, though I made the diagram in Inkscape.

Circuit layout showing how to power and control a 5V servo using a microbit and a 2N222 npn transistor

There is one catch with this circuit – the control signals become inverted. You might ask – why not tap the signal from the emitter of the transistor, not the collector, then the signal would not be inverted. However, doing this produces an output signal going to the servo motor of lower voltage than the signal coming into the base of the transistor. Which defeats the object. There is about a 0.6V drop between the base and emitter of a typical bipolar transistor.

The simple answer is to invert the control signal coming from the microbit. I did this. See the oscilloscope grab below, showing the inverted control signal from the microbit in yellow and the non-inverted control signal going to the servo motor in blue. Note the different voltage levels. The signal from the microbit is around 2.4 V in amplitude and the signal going to the servo is about 5 V in amplitude.

Inverted controller signal from microbit is yellow, signal from transistor emitter to servo is blue

I put the micropython code needed to generate both a non-inverted and an inverted pulse below so you can see the simple trick needed to invert the pulse.

   def set_ms_pulse(self, ms_on):
        # pin.write_analog(1023) is constant on pulse
        # (1023 * ms_on / 20) gives a pulse of length ms_on
        self.pin.write_analog(1023 * ms_on / 20)

    def set_ms_pulse_invert(self, ms_on):
        ''' Gives inverted logic servo control pulse. '''
        self.pin.write_analog(1023 - 1023 * (ms_on / 20))

This worked to operate the 5V servo motor. The transistor allows us to drive the servo motor using the output of a battery powered microbit.

Another component that allows us to drive a device requiring a voltage that is not compatible the part that generates the control signal is an opto isolator. This is a story for another day.

As always, comments are welcome.

200V piezo crystal driver

Spoiler alert: I got the driving circuitry to behave as planned, but I could not feel the crystal vibrate.

The idea is to use the tiny crystals used in piezoelectric motors to create an array of dots that can be made to vibrate under the area of a finger tip. This will be used to create a tactile display that can be felt. The initial aim would be an array of these crystals under an area the size of your fingertip. This would enable things like a Braille or Moon display that can be felt under your finger and that updates real time.

These crystals are designed to resonate at around 40 kHz. Now, you won’t feel something vibrating at this high a frequency. Your touch is sensitive to vibrations of around 10-100 Hz. So the crystals need to be driven with a frequency of around 40 kHz, which is then switched on and off (modulated in engineering speak) at around 30 Hz. All of this with a voltage amplitude of around 100 V RMS (200 V peak to peak for a square wave). How hard could that be? Errrr….

Well, I built a 100 V power supply in another post here. However, I used some Rohde & Schwarz HMP4040 adjustable power supplies I found in the lab as I could get over 200V by daisy chaining the outputs in serial. I read the supply’s manual online to check that the outputs could be connected like this safely. I ended up with around a 220 V peak to peak waves, modulated at around 30 Hz.

How to create a 40-42 kHz output signal which is then switched on and off at about 30-50 Hz? I used an N-channel MOSFET (FET) to switch a low voltage signal, with the gate of the transistor operated by an operational amplifier (op amp). The op amp input comes from an external microcontroller. I thought of lashing something up to create this input using a BBC micro:bit. Then I hosed money at the problem until it went away and bought an Analog Discovery 2 gizmo with a built in waveform generator. The software for the Analog Discovery allows for signals to be modulated, so I could easily create got the driving signal I was after. There are a range of YouTube videos to get you started with the Analog Discovery 2.

Top tip. Run the Analog Discovery from a laptop and disconnect the laptop from the mains when connecting the Analog Discovery 2 to your circuit. I measured the potential difference between the ground on the Analog Discovery 2 and my circuit and it was around 0.06V with the laptop connected to mains power. When running on battery, the potential difference was a magnitude lower. This means there is less chance of a ‘ground loop’ cooking off your laptop when you connect the ground of the Analog Discover 2 waveform generator to the ground of your circuit board.

I simulated the circuit using the Falstad and ltspice simulators. Simulate twice, build once as my Grandma used to say. I tried the qucs simulator as well, but could not get it to ‘converge’ with my design. Probably something I’m doing wrong.

Falstad is not as accurate as ltspice but is more interactive. Falstad runs through the browser. I found a downloadable version of Falstad called Circuit Simulator here, which seems to load the CPU less than running the browser version. I’m grateful for the all of the simulators being made available for us to use for free.

A screen grab from Falstad/Circuit Simulator can be seen below. I use a CA3140 CMOS op-amp as I found a few of them in the lab and they are fit for purpose. The FET is a 450V rated N-channel SSN1N45B (farnell p/n 2454128). This FET can handle the voltage that I need to use and can be switched on and off with a reasonably low voltage swing to the gate.

Falstad simulation of the piezo driver. The piezo crystal is represented by the 20F capacitor.

An example ‘scope grab from a Tektronix TBS1104 is shown below. The dense bursts of signal are the 40kHz driver, the larger gaps show that this is being switched on and off at around 30Hz. The orange trace is the signal from the Analog Discovery 2 board used as the input to the non-inverting port of the op-amp. The output from this op-amp switches the FET on and off. The green trace is the voltage at the high side of the piezo crystal. In this display, the driving voltage is 212V peak to peak, which is 106V RMS for a square wave. The signal frequency on the ‘scope is shown as 6.250kHz, with a question mark, as the modulation of the 40kHz with the 30Hz signal confuses the ‘scope’s frequency measurement.

piezo driver signal, 40kHz moudulated at 30Hz

The ‘scope grab below shows a close up of the gate driver where I try 41kHz as driving frequency for the piezo crystal. We can see that the FET gate is being driven with a 7.6V peak to peak square wave, which enables the 226V high voltage rail to be switched to generate the piezo driver signal. I tried a few frequencies to try and get the crystal to resonate. The gate of the FET needs around 7.5V peak to peak to ‘open’ the FET enough for the full 226V to switch through it. With a lower high voltage supply, a lower FET gate voltage is needed. I spent a few years trying to study physics, so did at one time have a good understanding of all the semiconductor shenanigans that go on inside the transistor. That was a long time ago.

We can see that when the FET gate goes high, the piezo driving voltage goes low. This is as the FET is opened by the gate going high, which connects the drain to source to ground through the 300Ohm resistor. This pulls the voltage low. When the gate signal is low, the FET is closed, so the high voltage rail is measured at the piezo crystal.

piezo driving signal, 41kHz

I messed around with some transformers with limited result. I could wind one of my own, but would rather use something off the shelf. I tried a few from coilcraft but without success. You need to be careful with transformers, as their impedance changes with frequency, meaning you can end up putting more current through them than their windings are designed for if you’re not careful, as shown by the imitable Electroboom.