Lego and Raspberry Pi working together

In a small way, I consider the Raspberry Pi almost like a Lego brick. On its own, it’s an interesting device, but it becomes more useful when combined with other components. What would be more natural then, than combining it with other Lego devices along with a few of my own devising.

In previous posts, I’ve shown how servos can be used to create some pretty whacky Lego devices, but equally I feel that individual bricks are open to hacking. Note… this is pretty gruesome stuff – drilling, cutting and filing Lego bricks. Even something rather similar to “The Kragle” ends up being used!

Rather than steal my son’s Lego, I’ve been using Bricklink as a place to buy specific parts. I’ll try to list their code numbers here so that they’re easy to find on the Bricklink website. Bricklink serves as a market place for people to sell their bricks.

In a follow-up post, I’ll show the current version of my Lego interface.

Lego LEDs

Lego LED

Lego LED in 2×2 brick

I’ve debated at length in my own head how to wire these. Essentially all the device needs is a resistor and LED to be mounted into some kind of transparent brick, or for the LED to be poking out of the top.

A standard 5mm LED can be fitted inside a 2×2 brick if it’s allowed to shine through the top. This does mean that other bricks can’t be stacked with it, but it is a neat solution. These 2×2 bricks are also available in clear (I bought Trans-clear, Trans-Dark blue, Trans-Green, Trans-Red).

To make this, you will need:

  • 2×2 Brick (Part No:3003)
  • 2×2 Plate (Part No: )
  • 0.1″ Female connector cables
  • LED
  • 100 Ω resistor (1/8 Watt is easier to fit in)

Lego switches

Lego tactile switch

Lego tactile switch

These are easily made with a 6mm tactile switch with a long actuator. A blob of hot glue can be used to keep the whole switch central while the rest of the brick and plate is assembled around it.

The switch is connected to a short cable with 0.1″ Female connectors on the end.

To make this, you will need:

  • 2×2 Brick (Part No:3003)
  • 2×2 Plate (Part No: )
  • 0.1″ Female connector cables
  • 6mm tactile switch
4-way tactile switch

4-way tactile switch on a PCB in a Lego frame

I’ve also had success with using 4 tactile switches on a PCB. The spacing between studs is 8mm, therefore it’s a fairly simple job to create a PCB which matches. I’ve linked one side of each switch to a common line to reduce the amount of cables required.


Lego large servo

A Large servo in a Lego framework

Connecting servos into a Lego system remains the biggest challenge. I tried a number of ideas before I settled with two possible solutions.

The micro servos seem to work best glued to a tile. It needs to be something that’s compatible with the plastic of both the servo and plate. Hot glue seems to work well here, although I have had success with some solvent-based cements. Polystyrene cement worked well for a bit, but surprisingly I did have on piece fall apart.

Larger servos require a different approach. I build a framework out of bricks and plates and that seemed to work well, but it does mean that right-angle parts are needed to mount it if the axis of rotation needs to be vertical. The egg-drawing robot used a large servo resting on a tile, with some small axles to centre the spindle within the Lego grid system.

To make a framework that supports a servo on its side, I used:

  • 2 bricks – 1 x 6 (Part no: 3009)
  • 1 plate – 1 x 6 (Part no: 3666)
  • 2 plates – 4 x 6 (Part no: 3032)
  • Optional – Bracket – 2 x 2 with 2 holes (Part no: )

All of the pieces need to be stuck together with solvent and then the servo can be linked into this with hot glue. It’s fairly neat, robust and fits into the Lego system well.

The servo actuator needs a little bit of treatment to make it useful. Small servos can use a Technic axle and pin connector (Part no: 3651) and larger servos will need modifying with a pulley (Part no: 4185) along with some small screws. I used PCB pillars to get the spacing just right.

Lego framework with brackets

Large servo in Lego frame

22mm Pulley for actuator

Servo actuator using a pulley

Lego framework

Largeservo and the Lego framework

Small servo

Small servo on Lego tile with actuator

Mini amplifier – 5v powered stereo modules

PAM8403 stereo amplifier board

PAM8403 stereo amplifier board

I had been looking to create a little monitoring amplifier for the Raspberry Pi for a while, as well as creating an amplifier which our phones could plug into for listening to music. After trawling that well-known internet auction site, I came across these little boards for an absolute bargain price of 99 pence each with free postage and packaging.

The power supply requirements are around 2.5v to 5.5v, which means it is suitable for powering from a USB power supply.

The module is described as a “PAM8403 5V DC Audio Amplifier 2 Channel 3W*2 Volume Control USB Power New Board”. There are a bunch of connections on one side, and a volume control/on-off switch on the other.

The connections (0.1″ spacing) are:

  • Left and right speaker connections (8Ω)
  • Power (2.5v to 5.5v)
  • Stereo audio input (Left/Right/Ground)

I hunted around the parts bins and soon came across an old usb cable and a 3.5mm stereo audio cable. These took a while to strip and tin as the usb cable had really fine wires. I needed a voltmeter to determine which of the connections would give me 5v. To make things more awkward, the colour codes used didn’t match with any of my internet research.

The internal layout. The amplifier module sits between the two speakers.

The internal layout. The amplifier module sits between the two speakers.

On to the CAM router to produce a case. The first one would be for my wife while she goes into labour, so I opted for something a little more interesting than a black box.

The template was generated from some earlier work (It’s the same size as the servo robot) but equally it could be created from the Automatic Case Designer

I use a lot of foamed PVC sheet with the router as its easy on the cutters, gives a little and makes a case that looks professional. The tabs fit the slots perfectly if a 3mm cutter is used. The case tends to hold itself together purely by friction although a glue gun can be used to permanently join the sides together.

The amplifier module comes without a knob, but searching around I found a 6mm splined shaft soft-touch knob which does the job perfectly.

Finished amplifier in its case

The finished product!

Here we have the finished case. The speaker grilles are drilled using the router into a heart pattern. The cables are USB power and a stereo jack.

One advantage of USB power was that the amplifier can also be used with a USB backup battery to make a portable amplifier. Alternatively, I found that the USB sockets on the Raspberry Pi can also be used.

Eventually, I’ll make a similar circuit for the Dalek so that it’s self-contained.

In conclusion, this little amplifier board is a perfect little problem solver. It might be possible to drive it direct from the 5v supply on the GPIO connector, although a pretty beefy power supply might be needed if the full 3w x 2 is required, given that this equates to over 1Amp. I suspect that the Raspberry Pi might crash if the amplifier is driven at high volumes with a particularly weak power supply.

Another alternative might be to create an internet/network media player. Couple one of these to a Raspberry Pi running Kodi, stick in a USB WiFi dongle to pick up network attached storage, and use a mobile phone app such as Yatse (Android) as a remote control.

Construction of the ScratchGPIO Dalek – Invasion part 2

Here’s the second installment of the ScratchGPIO Dalek – the first part can be found here. My apologies for the poor quality of the photos. Our camera was in use elsewhere so I only had my mobile. I’ve discovered that it can’t cope with low light and close up shots. Perhaps I should have used the Raspberry Pi camera?

MostOfTheBitsHere’s what I’m starting with. I’ve got a servo, some LEDs (red and blue), Chromed LED bezels and a few bits of plastic cut on a CAM router. The Dalek is a plastic container originally for bubble bath,but having removed the bottle it seems that there’s plenty of space for a circuit board, a servo and some odd bits of wiring.

EyeballThe eyeball LED is made from a piece of chromed brass tubing removed from an old telescopic aerial. The LED is soldered to the end of stranded wire and carefully sealed with hot glue. A clip-on LED bezel is used to complete the effect and makes for a realistic eyeball. I would have liked to add on some disks half-way along the stalk, but I didn’t really have time and perhaps it can be easily overlooked?

The head lights are made from blue LEDs soldered to the end of stranded wire and then pushed into some chrome LED bezels that were in my parts stock. At first, the LEDs were difficult to see as they are so directional and the bezel tended to shield the light coming out sideways. The solution? Fill the bezel with hot glue (in fact, overfill it) and then let it cool without disturbance. This created a slightly domed translucent light which can be seen from all angles. For such a simple solution, I’ve been really impressed with the result. And I didn’t burn my fingers. The domed lights are then pushed into the head. The slightly rubbery nature of the head means that I’m unlikely to need to put the nuts on the other side of the bezels. I don’t think I’d succeed either!

BufferboardThe bufferboard is one of my own designs (more information here) that I produced a while ago. I use it as I managed to kill a couple of Raspberry Pi computers while experimenting with servos. I’m not sure whether it’s a faulty servo that did it, but ever since I’ve been quite nervous to work without this board in the way. It also level shifts the outputs to 5v instead of 3.3v, making the LEDs (particularly the blue LEDs) able to run at their full brightness. Having said all that, I’ve used a 330Ω resistor in series.

ServoWheelThis part is a simple three-spoke wheel that fits on top of the servo horn. The outside diameter has been chosen to fit the inside hole under the dalek head. The three spokes gives plenty of space for the wires to pass upwards from the base. I had pondered how to get the servo horn to stay attached as I’ve found hot-glue to be a bit unreliable on these and there wasn’t really room for the M2.5 screws that I usually prefer. Cable ties to the rescue! They hold well, especially as I filed a few little slots in the servo horn. Simple and effective.

DalekbaseFitting the servo up inside the base was a bit more tricky. There are quite a few obstructions up inside the base which have to be taken care of. In the end, I cut out the top section carefully and then used added a foamed pvc plate which carries the servo. After nearly exhausting my supply of M3 screws, the servo was mounted and the horn fitted.

TwoPartsOfDalekHere are the final two parts ready for fitting together. The cables will pass down through the wheel into the base where they are then connected to the buffer board with 330Ω resistors. The servo is also connected. I ran a little test routine from Scratch to ensure that the servo can reach both extremes and also find the centre. Once this was done, a few blobs of hot glue ensures that the head and the wheel are joined.

Coming up… programing the Dalek with ScratchGPIO (and Python eventually).

Building a PICAXE controlled rover

P1250840I’ve already shown how a PICAXE can be used to build a simple Robot. I know that this blog is really meant to be aimed at Raspberry Pi users but the techniques would be similar. As a result, I thought it’s worth uploading the photographs that show the robot being constructed. If I can get the technical Lego back from my son, it might be possible to rebuild using the Raspberry Pi, and even extend the functions to make more of it!

The original blog post shows the robot in action.

Chassis parts

The chassis is constructed using a variety of beams and plates along with a couple of gear trains. You’ll need:

  • 8 x 1 beams (4x)
  • 8 x 2 plates (1x)

Chassis parts

Continue reading

Raspberry Pi and L298N motor driver boards

L298 Driver board

A few connections to the Raspberry Pi, a power supply and motors… simple!

A quick search of Ebay revealed these little beauties – L298 motor drivers. There’s a 3-pin terminal block for power (I used the 12v and Ground connection), 4 pins available for connecting to the GPIO and terminal blocks for a pair of DC motors. I didn’t use the 5v connection.

At the end of the 4 pins you will also find the “enable” inputs. This could be used to control the motors from a common PWM signal but I left the jumper links in place so that the motor controllers are always active.

In order to connect the driver board to the Raspberry Pi, a 4-way female-to-female ribbon connector is needed. Also required is a common ground connection and a simple way of doing this is to cut the end off a spare connector cable, strip it and push it into the ground connector for the power supply.

The module specifications state that it can work from a wide range of power supplies(5-35V)  and interfaces easily with the Raspberry Pi. I’ve so far used it with the 9v Fischertechnik motors but my next stage is to connect it to some 4.5v Lego Technic motors. I’ve done similar with the L293 and the PICAXE so I’m not expecting any difference. I haven’t yet found out what the voltage drop across the internal transistors is, so I’ll have to pick my power supply with care.

L298 diagram

These are the basic connections. I left the three jumpers as supplied.

Another use that might not be so obvious is that the module can also drive light bulbs and stepper motors. One board would suit a 2 phase bipolar motor and it would be a straightforward job to write the patterns required to step the motor around. Note, however that this would require four GPIO connections per motor, so it might be convenient to use some sort of port expander if this is the direction chosen.

This particular driver board will probably end up on my disassembled Big Trak. I’ve got a 5v power supply that will run from 7.2v R/C racing packs.

Glockenspiel controller

The component side of the PCB

Top view of the PCB

I managed to find some time to change the Glockenspiel controller from using ULN2803 darlington drivers to a whole bunch of discrete darlington pair transistors (BCX38C). It seemed an ideal way of coping with the huge switch-on surges and spreading the current over a whole load of different devices.

The board is single sided which makes it easy enough to etch and solder up. The power is taken directly into the bottom of the board and the open collectors on the transistors do the switching.

The diodes are 1N4001’s which should help to kill the transient produced when the motor is switched off. It was quite nervewracking to see the spark produced when testing the motors across a power supply, so I guess these motors are really capable of packing a punch. At the bottom, I’ve allowed a space for a polyswitch resettable fuse, although in my version it’s not actually being used.

If I knew a little more, I suppose I should use MOSFETs for the switching, but I don’t and I didn’t. Sorry about that.

The Glockenspiel controller circuit

An example of the basic circuit used. 1 channel shown.

One thing that occurred to me was this little board could be applied to lots of different 12v switching situations. Perhaps a large lighting display using Superflux LEDs wired in bunches.

The board layouts are available for those who may wish to copy it:







Raspberry Pi and a cardboard robot…

This is a little demo of a robot that took a few minutes to make. I had scanned the artwork of a cardboard toy robot kit and then shrunk it to A4. I printed out on thin card (a bit too thin as it happens…) and then shoved in a couple of servos which only just fit.

There’s a simple python program outputting random PWM signals via ServoBlaster.

It’s nothing special, but it might give some inspiration for someone to take further. Perhaps it could really be a Dalek, Wall-E or “Buddy” from Q Pootle 5.

In addition, I’ve created a new version of my ServoBlaster buffer board using a 74HC541 this time. At some stage I’ll post an artwork and circuit diagram.

PICAXE controlled LEGO rover

This little LEGO rover will scurry around wherever there's space!

A LEGO rover controlled with a PICAXE motor control board

Some time ago I put together a little LEGO rover and added a PICAXE motor control board. A little bit of programming goes a long way and although the motors are slightly different speeds, this thing wanders around quite happily.

Two large lever-arm microswitches take care of sensing obstructions. If it hits these, the rover is programmed to reverse and turn away. The overall effect is quite effective for such simple programming. I also modified a 2×2 brick with a 220Ω resistor and a green superflux LED. By drilling out the internal pillar, there’s enough space to hide the superflux LED inside with its resistor. I then connected it to one of the MOSFET outputs connected to output 0. I’ve also put a 2×2 clear brick on top and this is great for spreading the light out.

The overall power supply is 6x AA Alkaline cells. I will eventually change these for NiMH but they’re in use around the house somewhere. Once the volt-drop through the L293 integrated circuits have been taken into account, it’s about the right voltage for powering the 4.5v LEGO motors.

The simple algorithm for this rover is:

go forwards
if left sensor touched, then stop, flash the LED, reverse and then rotate right
if right sensor touched, then stop, flash the LED, reverse and then rotate left
go to start

The bells are ringing in my head.

Sometimes, when things are going smoothly, it’s easy to lose track of time. This is compounded if the school bells aren’t working correctly (Excavator + Buried Cables might have something to do with it). Replacing cables is expensive, disruptive and time-consuming. Here’s where the Raspberry Pi might come in handy – particularly if it has the ability to use “Network Time”.

It’s also a bit of fun. I’ve got an RGB Piranha LED as well as a set of speakers. Ideally, I’d connect up that doorbell sat on my desk so that it sounds more realistic, but the “Ring Ring” spoken by espeak is a better talking and teaching point. Perhaps I should even get the head connected up. In addition, I’ve used figlet to make the time visible, although I’d really recommend running this headless to reduce power consumption.

The LED lights up green in the last 5 minutes of a lesson, and then goes red when the bell should be going. It repeats each message a couple of times to hammer the point home. If I really wanted to use that doorbell, then it’s a simple job to use the PiFace relay where the bell push-switch would be fitted.

Using this program with mplayer or omxplayer might make an interesting clock radio with mp3 files playing instead. Perhaps use mpd to play streaming radio. So many ideas, so little time…

#!/usr/bin/env python
import time, RPi.GPIO as GPIO
import os
GPIO.setup(11, GPIO.OUT)
GPIO.setup(12, GPIO.OUT)
GPIO.setup(13, GPIO.OUT)
GPIO.setup(7, GPIO.IN)
shortDelay = 2
while True:
         GPIO.output(11, GPIO.LOW)
         GPIO.output(12, GPIO.LOW)
         GPIO.output(13, GPIO.LOW)
         os.system("figlet "+str(current_time))
         if current_time in bell_warning:
                 print("Five minute warning for the bell")
                 GPIO.output(11, GPIO.HIGH)
                 os.system('espeak "the bell will go off in five minutes"')
                 GPIO.output(11, GPIO.LOW)
         if current_time in bell_times:
                 print("Brrrring, Brrrring, Brrrring....")
                 GPIO.output(12, GPIO.HIGH)
                 os.system('espeak "Ring Ring. Ring Ring. Ring Ring"')
                 GPIO.output(12, GPIO.LOW)

RGB LEDs driven with ServoBlaster


By changing a few options on the command line, it’s possible to get ServoBlaster to output PWM signals from 0-100%. By using three channels and connecting these to an RGB LED it’s possible to produce a wide spectrum of colours. Each channel can be given a value, so for instance Red=100%, Green=0% and Blue=100% gives a rather pleasing purple/magenta colour. The circuit is rather simple, although my RGB LED was common-anode, which meant that using a ULN2803 darlington driver seemed a better option compared to a bunch of transistors.

This RGB might become a useful status LED, a sunset simulator or perhaps an interesting weather display, where colours represent the coming weather. There’s plenty of spare capacity for more LEDs in that darlington driver!

The RGB LED can be found here: