Rotating Leds System
Harco Kuppens
July 10, 2019
1 Introduction
For the course ”Design of embedded systems” the students learn to program
software on a real-time operating system Xenomai[1]. The students first have to
do several exercises[2] where they stepwise learn to program a real-time program.
In the last exercise everything learned must be proven in practice using a real
world example.
Figure 1: Rotating Leds System writing X
The real world example had the following requirements:
• a physical system which can be used as demo
• the system must be controlled by a Raspberry Pi running the realtime
Xenomai OS
• the system must be controlled by the GPIO pins of the Raspberry Pi
1
• the control must have real-time timing aspects controlling some actuators
using information from interrupts of sensors
• the system must give students experience with real-time programming in
combination with hardware.
• the system must learn the students to read the hardware specs and use
this to correctly control the hardware.
For this real world example we created the ”Rotating Leds System”, shown in
figures 2 and 3 , which full fills all these requirements. This document describes
all the details of the hardware of the ”Rotating Leds System”.
Figure 2: Rotating Leds System
2
Figure 3: Raspberry Pi controlling the Rotating Leds System
2 Controlling the Rotating Leds System
The rotating leds system has a motor driving a wheel on which an array of leds
is mounted. The wheel is turning that fast such that when turning the leds on
and off at the right times you could write letters in the air. In figure 2 you can
see the rotating leds system at rest with the 8 leds array almost at the top of
the wheel. In figures 1 and 3 you see the wheel rotating and the leds controlled
in such a way that the letter X is written. The challenge for the students is to
write the real-time control program for the rotating leds system on the Rasperry
Pi using the Xenomai API to achieve writing this letter X.
D0 = GPIO 2
D1 = GPIO 3
D2 = GPIO 4
D3 = GPIO 17
D4 = GPIO 27
D5 = GPIO 22
D6 = GPIO 10
D7 = GPIO 9
Figure 4: Leds D0-D7 connected to the Raspberry Pi’s GPIO pins
The control system of the rotating leds system contains the following hard-
ware:
• The LEDs are connected to eight GPIO ports on the Raspberry Pi as
3
shown in the figure 2. Note that a high signal (1) means that the LED
will be on; with a low signal (0) it will be off.
• There is an optical switch on the bottom; when the arm passes the optical
switch it interrupts the LED light. This interruption causes the light
sensor within the optical switch to go from a high signal (lots of light) to
a low signal (no light) which is used as output of the optical switch. This
output is connected to GPIO pin 23 of the Raspberry Pi. Because the
light sensor is interrupted only for a small time, the sensor signal will be
like:
-------------------. .-------------------- HIGH(1) = 3.3 Volt
|____|
LOW(0) = 0 Volt
This can be used to measure the time needed to pass the sensor since the
Raspberry Pi can be programmed to give an interrupt when going from
high to low and from low to high.
The passing time and the time of interrupt should be enough to calculate when
to enable/disable leds in such a manner that we can write letters in the air with
the rotating leds system.
3 System construction
3.1 Construction kit
The first well know construction kit is Meccano[4] using steel constructions parts
using sizes based on inches, because it was a British invention. However during
the years plastic construction kits, such as LEGO, became more popular because
it was cheaper and easier to build with.
In recent years because of extrusion process of aluminium, it has become cheaper
to make parts in aluminimum. Therefore modern metallic kits use aluminium
instead of steel. Examples are Makerbeam, Makerblock and Gridbeam. How-
ever plastic is still much cheaper, so for toys plastic kits are most common.
For building something with a real steady construction construction kits like
Meccano are becoming less popular and replaced by these modern aluminium
kits.
Personally I prefer Makeblock[3] because it uses:
• extruded aluminium instead of plastic/steel which is both light and strong
• both a repetitive hole pattern and a the t-slot style system for easy con-
struction
• all kinds of electronic parts as actuators (eg. motors) and sensors
4
• the metric system
For the Rotating Leds System we used the Makeblock Construction kit to
construct the system. To give you an idea of what kind of parts Makeblock
comes with see figure 5 showing the parts of the Makeblock XY-Plotter Robot
Kit.
Figure 5: Parts in the Makeblock XY-Plotter Robot Kit
3.2 Explaining the construction
The Rotating Leds System is mainly constructed of the following parts:
1. A rotating axis(fig 6 and 2)) driven by a 700rpm Makeblock motor(fig 7)
on which a plastic gear wheel(fig 10)) is mounted on which the 8 leds array
circuit is mounted(fig 12). The plastic gear wheel is mounted to the axis
using a shaft hub(fig 11).
2. The wires from the rotating led circuit are connected to a slipring(fig 7)
to allow a rotating connection to the rest of the system.
3. The slipring has a plastic axis which is difficult to mount to the steel axis of
the motor. Also because the slipring is a plastic part its axis is not rotating
as straight as a steel axis does. Therefore after some experimentation we
come up with a loose connection between the motor axis and the slipring.
On the slipring axis we slip the 4mm hole of a 62 tooth timing pully
wheel(fig 9) . We use some tape in the connection to make the connection
not to slip.The pully wheel is connected with two screw-able axis to the
plastic wheel connected on the motor axis. This loose connection allows
5
the slipring to wiggle a bit which makes the rotation to happen with less
friction. The slipring is connected with a single screw to a upright standing
beam.
4. The motor is connected via an on/off switch to a separate power supply
using a standard power connector shown in figure 26.
5. To the plastic wheel a strip is attached which when the wheel is rotating
will block the infrared light in the optical switch circuit mounted at the
bottom of the system. In figures 2 and 13. In figure 14 you can see how
we mounted the optical switch circuit.
6. We didn’t made a detailed construction drawing, however using the videos
on the webpage http://www.cs.ru.nl/lab/xenomai/ you can have pretty
good look on the construction.
6
Figure 6: Top view of axis of Rotating Leds System
7
Figure 7: Makeblock 700rpm motor
Figure 8: Slipring
Figure 9: Timing Pulley 62 tooth
Figure 10: Plastic gear 72 tooth
8
Figure 11: Shaft Hub
Figure 12: Leds Circuit board mounted on plastic gear wheel
9
Figure 13: The Leds Circuit board.
Figure 14: Side view of how the Leds Circuit board is mounted.
10
4 Circuits
In this section we describe the electronic circuits within the rotating leds sys-
tem.
4.1 Leds Array Circuit
As led array we used the RTZ 2081 R from MENTOR shown in figure 15 and
which we bought at Reichelt [5].
Figure 15: Led array RTZ 2081 R from MENTOR
The website at Reichelt[5] gives the following specifications:
MEN RTZ 2081R
LED array, 8-way strip from MENTOR
Diameter 2 mm, horizontal.
Typical: 2.25 V/20 mA
Print grid 2.54/5.08 m
Colour: red
Operating voltage : 2,0 3,0 V
Normal current: 20 mA
Using this information we can calculate the value of the resistor which we need
to place in series with this led to have it safely running:
Typical Voltage of led diode: 2.25V
Typical Current of led diode: 20mA
Vcc: 3.3V
Remaining Voltage : 3.3-2.25= 1.05
11
So dissipation of remaining 1.05V on resistor
with a 20mA current gives a resistance of:
R=V/I = 1.05/0,02 = 53 Ohm
We didn’t had any 53 Ohm resistors so we take a slightly larger one of 68 Ohm
giving us the following electronic scheme for each led in the array shown in
figure 16:
R1 = 68
3.3V
LED
Figure 16: Led electronic scheme
The lower pins of the led array shown in figure 15 can be easily bend downwards
so that we can put them through 8 holes of a stripboard. The upper pins we
can easily soldered together and then connected to the ground voltage. This is
shown in figure 17 where the black wire connects the pins to the white wire of
the slipring which is connected to a ground pin on the Raspberry Pi.
12
4.2 Optical Switch circuit
As optical switch we used the OPB625 sensor[6] sold by Farnell.
Figure 19: Optical Switch OPB625
The website at Farnell[6] says:
The OPB625 is a Photologic Slotted Optical Switch with 10K
pull-up (buffered or inverted) output. This printed circuit board
mounting switch consists of an 890nm, infrared light emitting diode
(LED) and a monolithic integrated circuit that incorporates a pho-
todiode, a linear amplifier and a Schmitt trigger on a single silicon
chip. Device is also TTI/LSTTL compatible and can drive up to 10
TTL loads. Suitable for mechanical switch replacement, speed indi-
cation (tachometer), mechanical limit indication and edge sensing.
The OPB625 is an optical switch, which is build from a diode outputting infrared
light, and a photological sensor which is sensitive for this infrared light.
The switch is triggered when the light beam from the diode cannot reach the
photologic sensor because it is blocked by some strip in the slot between the
diode and the photologic sensor. The datasheet for the OPB625 optical switch
contains the electronic scheme shown in figure 20 :
Figure 20: Optical Switch OPB625 electronic scheme
The optical switch has an a 10K pull-up which keeps the output Voltage high
14
at Vcc when the switch is not triggered. So the output is Vcc when the switch
is not triggered and is 0V when triggered.
From the datasheet we find:
1. For the Output Photologic sensor in the optical switch the Operating DC
Supply Voltage(VCC) must be in the range 4.5-16V.
2. Input Diode has a Maximal Forward DC Current of 50 mA. The typical
Forward Voltage of the diode is 1.6 V at I=10mA.
The operating voltage is also the maximal output voltage of the output from the
photologic sensor. However the GPIO pins of the Raspberry Pi can at maximum
receive 3.3V as input. So it would be better if we could also use the 3.3V of
the Raspberry Pi to power the sensor instead. Although it was below what the
specifications said, we tested it, and it seems to work alright
When we want to put the optical switch to use in electronic scheme we can
directly connect the Vcc power and GND from the Raspberry Pi to the Vcc and
GND of the photologic sensor, and the OUT of the photologic sensor to a GPIO
input of the Raspberry Pi. However the diode cannot be connected directly to
the Vcc and GND from the Raspberry Pi because its typical Voltage drop is
around 1.6 Volt, and connecting it to 3.3 Volt would burn it out immediately.
So we have to put it in series with an Resistor which dissipates the remaining
voltages:
Typical Voltage of diode: 1.6V
Typical Current of diode: 10mA (max 50mA)
Vcc: 3.3V
Remaining Voltage : 3.3-1.6= 1.7
So dissipation of remaining 1.7V on resistor
with a 10mA current gives a resistance of:
R=V/I = 1.7/0,01 = 170 Ohm
We didn’t had a 170 Ohm resistor but instead took a 230 Ohm resistor which
also works fine with a 8mA current. The electronic scheme of the infrared led
is shown in figure 21:
15
In figures 22 and 23 you can see that we added a 3 pins male Dupont connector
to make this circuit easily connectable.
Figure 23: Side view of Optical Switch circuit
Figure 24: Back view of Optical Switch circuit
In figure 24 we see with the blue line the GND of the diode and the photo-
logic sensor connected, and we see with the red line the Vcc connected to the
photologic sensor also driving the in series connected diode and 210 Ohm resis-
tor.
17
4.2.1 Improvements
Two problems appeared:
1. continuously low signal on sensor caused by external sunlight
2. spurious interrupts
The first problem could be easily solved by lowering the roller blinds in the
room. An alternative solution is to add a small roof on the optical sensor so
that only light from the diode will reach the sensor, but not light from any other
external source.
The source of the second problem was not entirely clear. Several causes could
cause the problem:
1. just enough external sunlight could trigger a low signal, but there is not
enough sunlight to keep it continuously low
2. the power to the sensor is not stable enough, so when having a dip in the
power we get a low trigger in the sensor
3. according to the specifications the sensor needs a 5 Volt power source.
But Because the same voltage is given as high output signal I tested it if
it worked also right with 3.3 Volt because a GPIO input signal may be
maximal 3.3 Volt. After testing I conclude it worked fine. However maybe
it is not fine, causing the spurious interrupts.
Reason 1 seems to be also solved by lowering the rolling blinds, or adding a roof
on the optical sensor.
Reason 2 seems unlikely because the Raspberry Pi has a power regulator chip,
however if you don’t have an adequate power supply it can happen. The Rasp-
berry Pi will both let the red power LED blink and show a lightning bolt in the
top right corner of the screen if the power drops below 4.65V. But we bought
the official Raspberry Pi power supplies and these should be fine. But to be
sure a solution could be to put a capacitor at the power line to the sensor to
prevent any major power drops to the sensor.
Reason 3 seems to most likely cause, because we went outside the specifications.
So instead of using the 3.3V Vcc from the Raspberry Pi, we can use the 5V Vcc
from the Raspberry Pi to power the sensor. But we then need to reduce the
output Voltage of the photologic sensor with an extra resistor. If we look at
figure
we see the photologic sensor’s output pin is connect with a 10k Ohm resistor to
the Vcc (pull up). Thus if we put an 15k Ohm resistor between the photologic
sensor’s output pin and the GND we effectively create a Voltage divider on the
output when the output signal is high(Vcc):
Vcc = 5.0V
18
R_pullup = 10k
R_new = 15k
I = V/R = 5/(10.000+15.000) = 0,2 mA
Vout is the Voltage drop over R_new :
Vout= I * R = 0,2 * 15 = 3 Volt
To the output voltage is 3 Volt when the output is high, and 0 Volt when the
output is low. The voltage divider circuit is shown in figure 25:
5V
10k
15k
Output
MMM MM!MM"MM#MM$MM%MM&MM'MM(MM)MM*MM+MM,MM-MM.MM/MM0MM1MM2MM3MM4MM5MM6MM7MM8MM9MM:MM;MM<MM=MM>MM?MM@MMAMMBMMCMMDMMEMMFMMGMMHMMIMMJMMKMMLMMMMMNMMOMMPMMQMMRMMSMMTMMUMMVMMWMMXMMYMMZMM[MM\MM]MM^MM_MM`MMaMMbMMcMMdMMeMMfMMgMMhMMiMMjMMkMMlMMmMMnMMoMMpMMqMMrMMsMMtMMuMMvMMwMMxMMyMMzMM{MM|MM}MM~MMMM MM!MM"MM#MM$MM%MM&MM'MM(MM)MM*MM+MM,MM-MM.MM/MM0MM1MM2MM3MM4MM5MM6MM7MM8MM9MM:MM;MM<MM=MM>MM?MM@MMAMMBMMCMMDMMEMMFMMGMMHMMIMMJMMKMMLMMMMMNMMOMMPMMQMMRMMSMMTMMUMMVMMWMMXMMYMMZMM[MM\MM]MM^MM_MM`MMaMMbMMcMMdMMeMMfMMgMMhMMiMMjMMkMMlMMmMMnMMoMMpMMqMMrMMsMMtMMuMMvMMwMMxMMyMMzMM{MM|MM}MM~M
3.000 V
Figure 25: voltage divider scheme
And finally powering the 210 Ohm resistor in series in with the diode with 5V
means:
Typical Voltage of diode: 1.6V
Typical Current of diode: 10mA (max 50mA)
Vcc: 5.0V
Remaining Voltage : 5.0-1.6= 3.4
So dissipation of remaining 3.4V on resistor
with a 10mA current gives a resistance of:
R=V/I = 3.4/0,01 = 340 Ohm
However we used a 210 Ohm resistance which
gives a current of:
19
I=V/R=3.4/210=0,016 A (16mA)
A current of 16 mA is ok, because it is still far below the maximum of 50mA
5 Test the circuits
The circuits can be tested in two ways, where the motor is switched off. Down-
load two scripts from the Xenomai lab site[2] at the Documentation page, under
section ”Rotating LEDs System”: led loop.bash and light sensor interrupt.bash:
1. Test the LEDs by using the script led loop.bash. Execute it by bash
led loop.sh. This should turn each led on once, from top to bottom.
2. Similarly, test the light sensor using script light sensor interrupt.bash.
Manually turn the wheel to trigger the light sensor.
6 Connections
We wanted the different circuit boards in the system and the Raspberry Pi itself
to be easy attachable and removable from the system.
6.1 Dupont type Jump wires
Often jump wires are used to connect electronic circuits to the Raspberry Pi
GPIO pins. Jump wire is a collection name for all kind of wires which allows
you to connect electronic components without the need to soldering. They come
with all different type of connectors.
The connector type the rasperry pi gpio pins use is an IDC connector. IDC[9]
stands for Insulation-displacement connector is an electrical connector designed
to be connected to the conductor(s) of an insulated cable by a connection pro-
cess which forces a selectively sharpened blade or blades through the insulation,
bypassing the need to strip the the conductors of insulation before connect-
ing.
The Jumper Wire cables used for the Raspberry Pi are called Dupont wires[7].
This name is derived from the manufacturer Dupont which made these type of
cables and wires. A dupont wire can come as individual line or they can come
in a group of them in a cable. In the latter case the connector of the cable,
called a Dupont connector[8], can come in a male or female version where the
distance between the pins is 0.1 inch or 2.54mm.
20
All current Raspberry Pi boards have a 40-pin GPIO header, which is basicly a
40 pins male IDC Connector where the distance between the pins is 0.1 inch or
2.54mm. A 40 pins female Dupont connector fits on the Raspberry Pi.
So to connect the the different circuit boards in the system and the Raspberry
Pi we use Dupont type of Jump wires. Because the Dupont type IDC connectors
give a good connection, and using a Dupont Crimp Tool[12] we can easily add
Dupont connectors to the wires from a component making the component then
easily attachable and removable[10]. You can easily buy a Dupont crimping
tool[12] and a plastic box[11] containing a set of Dupont Type Pins and Sockets
Set.
6.2 Overview of connectors
Figure 26: A 40 pins female-male Dupont wire connecting Raspberry Pi to
Rotating Leds Systems. Also the 9V power connector for the 700rpm motor is
shown.
We use for the whole system the following connections:
• A 40 pins female-male Dupont wire to connect the Raspberry Pi the ro-
tating leds system shown in figure 26. We can then easily connect the pi
to the system. All circuit boards within the system can then be connected
within the system to the male pins of this Dupont wire.
• In figures 22 and 23 you can see that we added a 3 pins male Dupont con-
nector to the optical switch circuit ot make this circuit easily connectable.
It contains a 3 pins male Dupont connector to connect the Vcc,GND and
21
output signal of the optical switch to 3 pins of the 40 pins male Dupont
connector.(see previous item)
• A 5 and 6 pins female connector, shown in figure 27 to connect together
the 8 output lines for the leds and the GND wire. As you can see in
figure 28 these connectors are connected to the outer wires of the slipring.
The inner wires of the slipring are soldered to the circuit board for the
leds. We didn’t use Dupont connectors here, because this circuit board
is rotating with 700rpm, so with soldering it is better fixed. And also
soldering takes less space and therefore making the circuit board smaller.
Figure 27: Connectors from the Leds Array Circuit board to the Raspberry Pi
22
Figure 28: Dupont connectors attached to slipring’s outer wires; inner wires
connected to led-array circuit board. Some wires are unused.
6.3 Overview of GPIO pin connections to internal wires
of Rotating Leds System
6.3.1 Leds Array Circuit
gpio phys.pin led number color of wire
2 3 D0 black
3 5 D1 brown
4 7 D2 red
17 11 D3 orange
27 13 D4 yellow
22 15 D5 green
10 19 D6 blue
9 21 D7 violet
ground 34 white
We use the resistor color coding for our led number to color mapping. From
the D2 to the D8 resistor the resistor coding matches the rainbow color coding
used for the 40 pins Dupont wire.
6.3.2 Optical Switch Circuit
The circuit pins are label right,middle and left where we use the perspective of
standing right in front of the rotating leds system as in figure 2.
23
For the original circuit using the 3.3 volt power supply.
gpio phys.pin circuit pin color of wire
Vcc(3.3V) 1 left red
23 14 middle brown output signal
ground 16 right black
For the improved circuit using the 5 volt power supply.
gpio phys.pin circuit pin color of wire
Vcc(5V) 2 left red
23 14 middle brown output signal
ground 16 right black
24
7 Parts
Below we specify all the parts you need to build the rotating leds system. In
this listing we didn’t include wires, a soldering iron, a block of wood and wood
screws. We assume you already have them.
Winkelwagen
Totale prijs van order
Subtotaal goederen:
€ 1,80
Levering:
€ 0,00
BTW:
€ 0,38
Totaal:
€ 2,18
*Berekend op basis van uw voorkeuren of geselecteerde
verzendmethode
Transmissive Photo Interrupter, 10K Pullup,
Through Hole, 4.826 mm, 1.52 mm, 10 mA, 3 V
Fabrikant:
TT ELECTRONICS / OPTEK TECHNOLOGY
Beschikbaarheid:
355
Op voorraad
€ 1,80
Artikelnr.
Fabrikant / beschrijving
Ordercode
Artikelnummer
fabrikant
Prijs per
stuk
Hoeveelheid
Artikelprijs
1
491342
OPB625
€ 1,80
Verwijderen
RoHS
1
Farnell element14 file:///Volumes/cup/doc/help/realtime/led_motion_clock/rotating_clock/build_rotating_clock/par...
1 of 1 04/07/2019, 17:45
Figure 29: Parts ordered at https://nl.farnell.com/
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Figure 30: Parts ordered at https://www.reichelt.nl/
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Sub-Total (Ex. VAT):
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Total:
€ 187.40
Product Name
Model
Quantity
Unit Price
Total
DC Motor - 25mm - 9V/700RPM
MB-80032
1
€ 15.95
€ 15.95
Slip Ring with Flange - 22mm diameter, 12 wires, max 240V @ 2A
KW-2355
1
€ 21.95
€ 21.95
Timing Pulley - 62T - Blue - 4-pack
MB-83008
1
€ 9.15
€ 9.15
DC Motor Bracket B - 25mm - 2-pack
MB-61802
1
€ 5.75
€ 5.75
Plastic Gear 72T (Pair)
MB-83454
1
€ 1.95
€ 1.95
Plate 7x9 B - Blue - 2-pack
MB-61210
1
€ 4.60
€ 4.60
Hardware Robot Pack - Stainless Steel Screws
MB-95017
1
€ 27.95
€ 27.95
Small Fourway Socket Wrench
MB-70020
1
€ 2.95
€ 2.95
Beam 0412-044 - Blue - 4-Pack
MB-60703
1
€ 3.10
€ 3.10
Crimping plier - 0.08-0.5mm - 20-28 AWG
KW-1985
1
€ 19.95
€ 19.95
Dupont Type Pins & Sockets Set - 310 pieces
KW-1938
1
€ 9.95
€ 9.95
Bracket 3x3 - Blue - 4-pack
MB-61500
1
€ 7.95
€ 7.95
Plate O1 - Blue - 2-pack
Not in stock.
***
MB-61220
1
€ 3.45
€ 3.45
Beam 0412-076 - Blue - 4-Pack
MB-60707
1
€ 4.15
€ 4.15
Slide Beam 0824-064 - Blue - Pair
MB-60014
1
€ 5.25
€ 5.25
Beam 0808-120 - Blue - 4-pack
MB-60528
1
€ 10.50
€ 10.50
40 pins Rainbow GPIO cable extender male/female - 20cm
KW-1857
1
€ 5.95
€ 5.95
Terminal block to 2.1mm DC barrel jack - Female
C-DC21FT
1
€ 2.50
€ 2.50
Power Adapter 9V/2.5A - 22W - 2.1mm DC plug
HNP24-090L6
1
€ 13.95
€ 13.95
Prototyping Board - 12x18cm - 2.54mm pitch
KW-1921
1
€ 5.95
€ 5.95
Shaft Hub - 4mm - 2-pack MB-84740
1
€ 4.50
€ 4.50
Figure 31: Parts ordered at http://kiwi-electronics.nl/
26
References
[1] https://xenomai.org/ and https://en.wikipedia.org/wiki/Xenomai
[2] http://www.cs.ru.nl/lab/xenomai/
[3] https://www.makeblock.com/ and https://en.wikipedia.org/wiki/
Makeblock
[4] https://en.wikipedia.org/wiki/Meccano
[5] https://www.reichelt.nl/8-voudige-led-array-rood-2mm-liggend-men-rtz-2081r-p6283
[6] https://nl.farnell.com/tt-electronics-optek-technology/
opb625/photo-interrupter-transmissive/dp/491342
[7] https://en.wikipedia.org/wiki/DuPont_wire
[8] https://en.wikipedia.org/wiki/DuPont_connector
[9] https://en.wikipedia.org/wiki/Insulation-displacement_
connector
[10] https://www.instructables.com/id/Fitting-Dupont-Connectors/
[11] https://www.kiwi-electronics.nl/dupont-type-pins-sockets-set-310-pieces
[12] https://www.kiwi-electronics.nl/krimptang-0-08-0-5mm-20-28awg
27
