This is only a preview of the December 2015 issue of Silicon Chip. You can view 37 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "High Visibility 6-Digit LED GPS Clock":
Items relevant to "Check Turntable Speed With This White LED Strobe":
Items relevant to "Speech Timer For Contests & Debates":
Items relevant to "Arduino-Based Fridge Monitor & Data Logger":
Purchase a printed copy of this issue for $10.00. |
By JOHN CLARKE
Check your turntable’s
speed with this
white LED strobe
So you have dragged out the old turntable and are playing vinyl
records again. Good. But how do you know that the turntable
speed is correct? The old way to do it was to use a circular disc
with strobe markings but that does not necessarily work these
days. Why not? Read on.
P
LAYING VINYL records has made
a big comeback in recent years and
many people are resurrecting their old
turntables or buying new ones. But
there are a few hurdles before you get
the optimum result, such as making
sure the cartridge stylus is not worn
out and that your preamplifier provides the correct equalisation.
On a more prosaic note, many turntables which have been out of action
for decades may not necessarily operate at the correct speeds of 33.3, 45
62 Silicon Chip
and 78 RPM. So you need to check that
aspect. How do you do that?
The old tried and true method was
to use a circular card which had stroboscopic markings on it and run the
turntable under mains voltage lighting;
230VAC 50Hz in the case of Australia,
New Zealand and most of Europe or
120VAC 60Hz in the case of the Americas, parts of Japan, Asia etc.
These stroboscopic cards have four
or six bands of markings and depending on the speed selection, one of those
bands would appear to be stationary.
The reason for this was that incandescent or fluorescent lighting had a
strong 100Hz or 120Hz component and
this would act to make the relevant
strobe band on the circular card appear to stop moving.
The same method applies to those
turntables that have strobe markings
on the rim of the platter. But while the
principle is still correct, it does not
work very well in most homes these
days. Why not? Because our political
siliconchip.com.au
masters have deemed that old-fashioned incandescent lights are “wasteful” and “bad for the environment”.
At the same time, fluorescent lighting
in most homes is now out of fashion,
unless it is using those ugly compact
fluorescent lamps (CFLs) with their
unnatural hues and copious electromagnetic interference.
So why can’t these modern lamps
provide the same stroboscopic effect?
The reason is that they run at much
higher frequencies so that any residual
AC component in the light output is
very small. This applies to any lighting
which uses electronic ballasts.
Mind you, even when you are using incandescent or fluorescent lighting powered by 50Hz or 60Hz mains,
the strobing effect is not particularly
strong and it is even weaker with halogen lamps with their much hotter
filaments. We will explain why later
in this article.
Turntable types
Most good turntables are either
belt-driven or direct drive. Cheaper
turntables were driven from an idler
wheel inside the rim of the platter.
The belt-driven types usually have a
small synchronous motor which can
be assumed to be locked to the mains
frequency, provided the belt is not slipping on the motor shaft. This could
happen if the belt is perished, kinked
or hardened. Idler-driven turntables
typically have a shaded pole motor
and they are not so tightly locked to
the mains frequency (and because of
the idler-drive, they are more likely to
produce rumble).
Direct drive turntables should run at
the correct speed but again, that cannot
be taken for granted. Also some direct
drive turntables had or have a variable
speed feature which allows the music
pitch to be shifted over a a range of
about a semitone. Again, how do you
know what is the correct speed setting (unless you have absolute pitch)?
Any substantial speed variation is
liable to cause any music to sound
off-pitch. And if you want to dance
to records and the number of beats
per minute is important, then again,
the turntable speed should be correct.
Our solution has been to design a
white LED stroboscope which produces one millisecond pulses of light
at a very precise 100Hz or 120Hz (ie,
twice the mains frequency). But our
recommendation is to use it at 120Hz
siliconchip.com.au
Turntable Speed Variations
Turntables that rely on a 50Hz or 60Hz mains supply to drive a synchronous
or shielded pole motor may not necessarily run at the correct speed. Typically,
the 50Hz mains frequency can vary between 49.85Hz and 50.15Hz (ie, ±0.15Hz)
over the course of a day. Typically, the mains frequency will be slightly low during
periods of peak power demand and a little high at other times.
That variation would mean that middle C could be as low as 260.841Hz and
as high as 262.411Hz. Whether this is noticeable or not depends on how well
you discern pitch.
Further turntable speed problems can be present if an incorrect-sized pulley
on the motor spindle is used to drive the belt. This could be because you have an
imported turntable that’s been designed to operate from 60Hz instead of 50Hz
(or instead designed to run with 50Hz instead of 60Hz). You may be able to supply the correct voltage for the motor using a transformer but the frequency will
not be correct.
For precision speed from a synchronous motor drive, an electronic driver circuit
could be used to produce a suitable sinewave source for the motor. This could
be a low-powered crystal locked sinewave inverter such as for an uninterruptible
computer supply. Modified sinewave inverters may not be suitable since the square
wave supply may introduce noise into the motor and cartridge pick-up leads.
Why Is This White LED Strobe Necessary?
In the “olden days” the usual method of providing a strobe light source involved
using an in-built Neon discharge lamp powered from the 240VAC 50Hz or 120VAC
60Hz mains supply. The neon would produce light pulses at 100Hz or 120Hz and
this would give a stationary pattern for the set speed.
However, using the mains supply is most unlikely to give a completely steady
strobe pattern when you are using a crystal-controlled direct-drive turntable unless the mains frequency is precisely 50.000Hz or 60.000Hz. Even a slight error will cause the strobe pattern to rotate slightly. Of course, with a belt-driven
synchronous motor turntable, you would never be aware of these speed errors
(unless you build our Turntable Strobe).
to give the most accurate speed indication with a strobe card.
So why is that? Funnily enough, a
lot of strobe cards are not necessarily
accurate and if you want the most accurate speed indications at 33.33 and
45 RPM, you should use a strobe pattern designed for 60Hz operation. Interestingly, as far as 78RPM records are
concerned, it is not possible to get an
absolutely accurate speed indication at
100Hz or 120Hz but 100Hz is the more
accurate, with a speed error of 0.1%.
Because of these issues, we have
also designed a PCB strobe disc that
you can place on your turntable to
check its speed. It is just the right
size to fit on the record label and will
not cover the playing area. Since it
is precisely etched and machined,
it will not have the common fault of
some printed strobe discs which can
be slightly off-centre or the centre hole
is a little over-size.
A turntable rotating at the correct
speed will have one band of the strobe
disc markers remaining stationary. If
the markers drift clockwise, then the
turntable speed is fast and if the markers drift anticlockwise, the turntable
speed is too slow. Any slight wavering
forwards or backwards of the markers
will be due to irregular speed variations and significant variations of this
nature and may be audible as “wow
and flutter”.
What can be done about a turntable
that doesn’t run true to speed? More
information on this is detailed in the
above panel.
Our LED Turntable Strobe is built on
a small PCB that fits into a small plastic
utility box. Alternatively, the PCB can
be installed inside the turntable cabinet and the strobe LED can be mounted
to illuminate strobe markings on the
platter’s rim. It can be powered with
a 9V battery, an external DC supply
or a 5V supply via a USB connector.
Circuit description
Fig.1 shows the circuit and it is
based on a PIC12F675 microcontroller
(IC1). The microcontroller vastly simDecember 2015 63
S1
D1 1N4004
CON1
A
9-12V
DC IN
1N4004
REG1 78L05
K
OUT
IN
A
GND
100 µF
470 µF
GND
1N5819
16V
16V
+
78L05
K
IN
OUT
K
A
D2 1N5819
9V
A
–
+5V
100nF
1k
1
2
3
4
5
USB
MICRO‘B’
SOCKET
K
4
CON3
2
X1
4.0MHz
33pF
33pF
3
1
Vdd
GP3/MC
GP5
68Ω
A
GP1
6
LED1
(WHITE)
GP2
C
470Ω
5
Vss
8
EXTERNAL
LED
λ
CON2
K
7
IC1
PIC12F675 GP0
GP4
68Ω
B
JP1
IN: 120Hz
OUT: 100Hz
Q1
BC337
E
BC 33 7
LED
SC
20 1 5
TURNTABLE STROBOSCOPE
K
A
B
E
C
Fig.1: the circuit is based on a PIC12F675 microcontroller (IC1), with 4MHz crystal X1 used as the reference clock. Pin
7 of IC1 drives transistor Q1 to flash white LED1 while jumper JP1 sets the strobe frequency to 120Hz or 100Hz.
plifies the circuit, compared to using
a separate crystal oscillator and dividers. In addition, the microcontroller
makes it easy to incorporate 100Hz
and 120Hz operation.
IC1 uses a 4MHz crystal as the reference clock for its program to run the
strobe. The un-calibrated accuracy of
the crystal (typically 50ppm) is sufficiently accurate for the strobe. IC1
internally divides the 4MHz frequency
by four, so that the program runs at
1MHz. Single clock instructions of the
program are therefore 1μs in duration.
As already noted, the strobe LED
is driven with 1ms pulses and this
gives a duty cycle of 10% at 100Hz or
12% at 120Hz. This will ensure that
the strobe disc markings appear quite
sharp. Longer pulse durations will
cause noticeable blurring of the strobe
pattern as the markings move further
during the on-period. This is a distinct
advantage of our LED strobe compared
to the light from an incandescent lamp
powered from a 50Hz or 60Hz mains
supply, with the resultant display being quite indistinct by comparison.
Designing The Strobe Disc
We have designed our strobe disc to suit 120Hz operation for 33.33 RPM and
45 RPM. We have also provided a strobe band for 78 RPM at 120Hz but it will
produce a speed error of -0.325%. To counter that, we have also provided a 78
RPM strobe band for 100Hz operation and this will have a speed error of -0.1%
(close but no cigar). Mind you, precision speed setting at 78 RPM is not so important because most records from that era were not cut at a precise 78 RPM.
Note that there are lots of strobe disc patterns that can be down-loaded from
the internet but most are incorrect. They may be correct at one speed, say 45
RPM, but incorrect at 33.3 RPM or 78 RPM. As an example, some patterns are
designed for 33 RPM, not the correct value of 33.33 RPM.
If you already have a strobe disc, how do you check that the pattern is correct?
It’s a simple calculation. Just multiply the strobe frequency (100Hz or 120Hz) by
60 to convert to pulses per minute. Then divide the turntable speed in RPM into
this number. So 33.33 RPM requires 100 x 60 ÷ 33.33333 or 180 bars for a 100Hz
strobe or 216 bars for 120Hz. It’s not possible to obtain a correct pattern for 45
RPM at 50Hz, since the number of bars is not an integral number; it is 133.333.
So any card with 133 bars is doomed to error.
If you want to be sure of the result, use our strobe disc.
64 Silicon Chip
The white LED (LED1) is driven via
transistor Q1 and a 68Ω resistor connected to the +5V supply rail. Q1 is
switched on and off by the GP0 output of IC1, using a 470Ω base resistor.
The LED is driven at a nominal current
of 29mA, assuming a 3V drop across
the LED.
Connector CON2, a 3.5mm jack
socket, is provided so that an external
LED can be connected.
We have provided several options
for the power supply: a 9V battery, a
9-12V DC plugpack via CON1 or 5V
via a micro-USB “B” socket. If using
a 9V battery or a DC supply via CON1,
the 78L05 3-terminal regulator (REG1)
provides 5V to the micro. Alternatively, if you are using a 5V USB supply,
this is fed to the micro via Schottky
diode D2. If you intend using a USB
power source exclusively, you can
omit the other supply components
such as CON1, D1, switch S1, REG1
and the 100μF capacitor.
For those interested in the effects of
the strobe flash length, you can select
a 2ms flash duration by tying pin 6 of
IC1 to pin 8 using a short piece of wire
under the PCB. This will set the strobe
to flash for 2ms but it will still run at
100Hz or 120Hz, as selected with JP1.
This change needs to be done while
power is off. A return to a 1ms flash
duration will only occur when pin 6
is disconnected from pin 8 with power
switched off and on again.
siliconchip.com.au
TOP OF CASE (NO LID)
12mm
12mm
+
13mm
B
A
+
A = 5mm dia.
B =- 6mm dia.
TOP OF CASE (NO LID)
24mm
10mm
5mm
C
+
+
E
C = 6mm dia.
D = 9 x 5mm
E = 5 x 9mm
10mm
D
9mm
+
Fig.2: the two end-panel drilling
templates. They can either be
copied or downloaded as PDF files
from the SILICON CHIP website.
The program checks the GP2 input
level and produces the 100Hz strobe
signal when this input is high at 5V.
It produces a 120Hz signal when the
input is low. The GP2 input is pulled
high via an internal pull-up resistor in
IC1 when JP1 is out and is pulled low
when jumper shunt JP1 is inserted. The
jumper setting can be altered while
the strobe is operating and the strobe
frequency will change immediately.
Drilling the case
The Turntsable Strobe is housed in
a UB5 plastic utility box (83 x 54 x
31mm) with holes cut in one end for
the LED and the external LED socket (if
fitted) and in the other end for the on/
off switch, the DC socket and microUSB socket.
It’s necessary to drill and cut the
case before installing any parts on
the PCB. There are a few options here,
though. First, if you will be running
the unit from battery power only, then
there’s no need to cut holes in the case
for the DC socket and the micro-USB
socket and these two parts can be left
off the PCB. Alternatively, if you will
be supplying power via the DC socket
or micro-USB socket only, then the
battery and on/off switch can be left
out and there’s no need to cut a hole
for the switch.
You could also leave out either the
DC socket or the micro-USB socket,
depending on the external supply.
At the other end of the case, you can
leave out the 3.5mm jack socket if you
don’t intend using an external LED.
By the way, the micro-USB input
siliconchip.com.au
does not have to connect to the USB
port on a computer. Any USB output
from a 5V plugpack or power board
can be used to supply power. Some
modern turntables even include a USB
port on the turntable plinth.
The first job with the case is to remove the internal ribs on each side
and this can be done using a small pair
of sidecutters. You can then finish off
by using a sharp chisel to remove any
remaining rib material.
The next step is to use the PCB as a
template to mark out its three mounting
holes in the case. That’s done with the
PCB sitting inside the case and pushed
hard against two of the side pillars (see
photo). The PCB is then removed and
the mounting holes drilled to 3mm.
Countersink these holes on the outside of the case using an oversize drill.
You now have to cut and drill the
holes in the end panel and that’s done
using the templates shown in Fig.2.
These templates can either be copied
from the magazine or downloaded in
PDF format from the SILICON CHIP website and printed out.
Once you have the templates, cut
them to size and attach them to the
end panels using adhesive tape. Be
sure to attach the correct template to
its panel – the template with the two
circular holes must go on the end that
matches the LED end of the PCB.
It’s now just a matter of drilling
and cutting the holes in the panels as
required. The square cut-outs for the
micro-USB socket and switch S1 can
be made by drilling a series of small
holes in a row, then joining them and
filing to the required shape.
Note that it’s a good idea to always
use a 1mm pilot drill to start the holes
(to ensure precise location) and then
enlarge them to the required size using successively larger drills.
PCB assembly
All parts (except the battery) are
mounted on a PCB coded 04101161
and measuring 79 x 31mm. Fig.3 shows
the parts layout. Begin by soldering
the surface mount micro-USB socket
(if used) to the underside of the PCB,
then flip the board over and install the
resistors on the top side. Table 1 shows
the resistor colour codes but it’s also
a good idea to check each one using
a digital multimeter before soldering
it into place.
Follow with diodes D1 & D2, making sure that the 1N5819 is used for
Parts List
1 PCB, code 04101161, 79 x
31mm
1 set of turntable templates (see
text)
1 UB5 case, 83 x 54 x 31mm
1 4MHz crystal (X1)
1 DIL8 IC socket
1 SPDT vertical slider switch
(Altronics S 2071) (S1)
1 2-way header (2.5mm pin
spacing) (JP1)
1 pin header shunt
3 6.3mm tapped Nylon stand-offs
3 M3 x 5mm countersink head
screws
3 M3 x 5mm machine screws
1 Micro-USB type B socket
(CON3) (FCI 101035940001LF) (au.element14.com
– Part No. 2293752)
1 PCB-mount DC socket (CON1)*
1 9V battery*
1 9V battery snap connector*
Semiconductors
1 PIC12F675-I/P microcontroller
programmed with 0410116A.
hex (IC1)
1 78L05 regulator (REG1)*
1 5mm white LED (LED1)
1 BC337 NPN transistor (Q1)
1 1N4004 diode (D1)*
1 IN5819 Schottky diode (D2)
Optional external LED parts
1 5mm white LED
1 switched stereo 3.5mm PCBmount jack socket (CON2)
1 mono 3.5mm jack plug
1 length of single cored shielded
cable
1 100mm length of heatshrink
tubing (1mm and 5mm)
Capacitors
1 470µF 16V PC electrolytic
1 100µF 16V PC electrolytic*
1 100nF MKT polyester
2 33pF ceramic
Resistors (0.25W, 1%)
1 1kΩ
1 470Ω
2 68Ω
*Note: omit DC socket CON1,
diode D1, switch S1, the 100µF
capacitor, regulator REG1, the
9V battery and the battery snap
connector if the unit is to be
exclusively powered via the micro
USB socket.
December 2015 65
REG1 78L05
100 µF100nF
470 µF
+
33pF
D2
CON1
IC1
PIC12
F675
33pF
4MHz
4004
9V
1k
X1
D1
+
Turntable Strobe
470Ω
Q1 BC337
JP1
5819
CON3
68Ω
+
68Ω
S1
JP1 out 100Hz
JP1 in 120Hz
04101161
© 2016 revB
LED1 A
WHITE
CON2 K
MICRO
USB-B
T
S
+
R
+
FROM 9V BATTERY CLIP
Fig.3: follow these two parts layout diagrams and the photos below to assemble the PCB. The micro-USB socket (CON3)
should be soldered to the underside of the PCB first, after which the remaing parts are installed on the top side.
Left: inside the
completed unit.
The battery and
switch S1 can be
omitted if the unit
is to be powered
only via the DC
socket or microUSB connector.
Similarly, CON2
can be left out
if you won’t be
using an external
LED.
D2. Make sure also that D1 & D2 are
correctly orientated. The DIL8 socket
can be then installed, followed by the
100nF capacitor and the two 33pF ceramic types.
Crystal X1, transistor Q1 and REG1
are next on the list but don’t get Q1 &
REG1 mixed up. The two electrolytic
capacitors can then go in, along with
the 2-way pin header (the header’s
shorter pins go into the PCB). Once the
header is in place, install the jumper
shunt (ie, to short the pins) so that the
unit will operate at 120Hz.
As explained earlier, DC socket
CON1, jack socket CON2 and switch
S1 are optional. CON1 is required if
you are using a 9-12V DC plugpack (ie,
one with no USB output) to power the
unit, CON2 if you are using an external
Table 1: Resistor Colour Codes
o
o
o
o
No.
1
1
2
66 Silicon Chip
Value
1kΩ
470Ω
68Ω
4-Band Code (1%)
brown black red brown
yellow violet brown brown
blue grey black brown
LED and S1 if you are using battery
power. If you are using a DC plugpack
to power the unit (via CON1) but will
not be fitting a battery, switch S1 can
be replaced by a wire link.
LED1 is installed by first bending its
leads down by 90° exactly 10mm from
its plastic body. Make sure that it is
correctly orientated before doing this
though (the anode lead is the longer
of the two). The LED is them mounted
with its leads 4mm above the PCB (use
a 4mm thick spacer to set this height),
so that the centre of its lens lines up
with the adjacent jack socket.
The last part to connect is the battery
snap. Feed its leads through the stress
relief holes as shown in Fig.3 before
soldering them to the PCB.
If you intend using an externally
connected LED, this can be now wired
to a length of single-core shielded
cable. Connect the centre lead to the
LED’s anode and the shield wire to
Table
2: Capacitor Codes
Value µF Value IEC Code EIA Code
100nF 0.1µF
100n
104
33pF NA
33p
33
5-Band Code (1%)
brown black black brown brown
yellow violet black black brown
blue grey black gold brown
siliconchip.com.au
These two views show the completed unit. Note that some of
the holes in the end panels can be omitted, depending on the
options chosen when you build the unit (see text).
the cathode. The other end of the cable is then terminated in a 3.5mm jack
plug, with the centre lead going to the
tip contact and the shield to the outer
sleeve contact.
Final assembly
Now for the final assembly. First, attach three M3 x 6.3mm tapped Nylon
stand-offs to the PCB mounting holes
and secure them using M3 x 5mm machine screws. The PCB assembly is
then installed by angling it down into
the case so that LED1 and CON2 pass
through their respective holes, then
squeezing the sides of the case together
and pushing the other end of the PCB
down until the switch and micro-USB
socket go into their panel cut-outs.
The PCB is then secured in position
using three M3 x 5mm countersinkhead screws which go through the
base and into the stand-offs. Once it’s
in place, fit the battery snap to the battery and slide the battery into the case
as shown in the photo.
Fig.4: this screen grab
shows the waveform at
the GP0 output, pin 1,
of IC1. In this case, the
circuit is set for 100Hz
operation (JP1 out). The
LED is lit for 1ms at a
10% duty cycle. Ignore
the error of the displayed
100.032Hz which is
because the oscilloscope
frequency calibration is
not particularly precise.
Dataflex/Datapol Labels
(1) For Dataflex labels, go to:
www.blanklabels.com.au/index.
php?main_page=product_info&
cPath=49_60&products_id=335
(2) For Datapol labels go to: www.
blanklabels.com.au/index.php?
main_page=product_info&cPath
=49_55&products_id=326
SILICON
CHIP
siliconchip.com.au
Testing
Now for the smoke test. Apply
power and check that there is 5V
(4.85-5.15V) between pins 1 and 8 of
IC’s socket (or 4.5-5.2V if using USB
power). If this is correct, switch off
and install IC1 (watch its orientation),
then reapply power and check that the
LED lights.
If it does, then your Turntable Strobe
is working and you can attach the lid
which now becomes the base of the
unit. If you now move the unit rapidly
from side-to-side with the LED viewed
side-on (ie, not looking directly into
the lens), it should be seen to light in
several different positions. That indicates that the LED is being flashed
siliconchip.com.au
on and off. By contrast, if you look
directly at the LED when it is stationary, it will appear to be continuously
lit due to its 120Hz flash rate.
Finally, if you have made up an external LED cable, plug it in and check
that its LED also operates.
Front panel label
The front-panel label is available in
PDF format on the SILICON CHIP website. It’s just a matter of downloading
it and printing it out onto an A4 sized
synthetic Dataflex or Dataplex sticky
label (see panel). This label is then
attached to the top of case (ie, not the
lid), as shown in the photos.
Turntable
Strobe
Fig.4: the front panel label can be
downloaded as a PDF file from the
SILICON CHIP website.
Alternatively, you can print out a paper label and attach this using doublesided tape. That’s it – your Turntable
SC
Strobe is ready for use.
December 2015 67
|