Some turntables come with a way to adjust the speed, some with a way to see the speed, but most do not have either. This project is for the person who wants an inexpensive but accurate way to see the speed of their platter on a digital readout. The display is five seven segment LED displays, two whole numbers and three decimal places. The Tachometer cost under $50 USD to build and is quick and easy to build. It is on 2 ¾” square PCB so will fit in most any table internally with a window to see the display and the trigger LED.
The project uses an Arduino Nano to perform the timing counts, calculations and run the display. The Arduino Nano’s clock crystal is neither stable enough nor accurate enough to use for the clock, fortunately the Nano can be configured to accept an external clock as the timing base. I used a 10-Megahertz oscillator that is good for +/- 2.5 PPM (Parts per Million), so within 9,999,975 to 10,000,025 Hertz. That’s a 0.00025% error rate, accurate enough for even the most stable tables. The 10-Megahertz clock output is run through a divider chip with does two divide by 10’s, so the clock is 100 kilohertz to the timer counter.
The Arduino has three timers built in but to make the timing accurate you must shut off the other two timers, timer 0 and 2, so there is no processing overhead to mess up the timing. Timers 0 and 2 are 8-bit timers, too small for counting the 100 kilohertz clock, produces too many overflow interrupts. Timer 1 is a 16-bit timer which is adequate to the task. As you can see from the source code the setup() procedure’s first five lines manipulates the processors registers to set the timers up. The processor uses two interrupt service routines for timing, one internal to count the number of times the counter over flows and the clock pulses, and one external interrupt connected to a Hall Effect sensor which is placed under the rim of the turntable platter and a small button magnet is glued to the underside of the rim of the platter. The magnet is small enough and light enough not to affect the platter balance.
The PCB has a 5-volt regulator on it so you can feed the board 7.5 to 12 volts DC. The optimal voltage in is 9 volts DC. While the regulator can handle higher input voltages the heat dissipation may be too much, 12 volts is pushing the thermal limit of the PCB heat sinking capability.
Note: the cost of the tach is using Blue LED displays, we build one with Red LED displays, a lot cheaper, and it still looks good. The hall effect sensor is around $10 but I designed a small ½” square board with a $0.60 hall effect sensor on it so cut the cost to about $1. So, an overall savings of $15 with Red LEDs and the home-made sensor. I will post the gerbers and part number for the Hall Effect sensor in another post.
This post has the PCB gerbers, the schematic, the BOM, a couple pics, and the code.
Operations: when the tach is powered on it runs through the setup of the Arduino and, before it exits, turns on all the LED segments for 5 seconds, this is a test to show you if you have any issues with your soldering. Then the main code runs, loop(), which looks at the number of overflows of the counter, which being 16 bits can only hold 65535 clock cycles before it trips the overflow flag and resets. If the platter isn’t turning the overflow count will keep going up, but once the number of overflows is over five, long enough to see the platter isn’t turning, it shut off the display. As soon as the platter starts turning the overflow number is reset by the triggering of the hall effect sensor and the display comes on. Whatever speed the turntable is spinning the speed will be displayed whether it is 33 1/3, 45, or 78 rpms. My table is a Denon DP80 built into a heavy plinth and typically displays 33.332 to 33.334 which is extremely stable and what Denon was known for. A friend is working with some Pioneer high end tables with a custom platter and heavy plinth and is seeing similar numbers, once the tables warm up. For grins I stuck a magnet to the side of an old, cheap Girard table and it was varying a lot more than I would like to see in an audiophile table.
The project uses an Arduino Nano to perform the timing counts, calculations and run the display. The Arduino Nano’s clock crystal is neither stable enough nor accurate enough to use for the clock, fortunately the Nano can be configured to accept an external clock as the timing base. I used a 10-Megahertz oscillator that is good for +/- 2.5 PPM (Parts per Million), so within 9,999,975 to 10,000,025 Hertz. That’s a 0.00025% error rate, accurate enough for even the most stable tables. The 10-Megahertz clock output is run through a divider chip with does two divide by 10’s, so the clock is 100 kilohertz to the timer counter.
The Arduino has three timers built in but to make the timing accurate you must shut off the other two timers, timer 0 and 2, so there is no processing overhead to mess up the timing. Timers 0 and 2 are 8-bit timers, too small for counting the 100 kilohertz clock, produces too many overflow interrupts. Timer 1 is a 16-bit timer which is adequate to the task. As you can see from the source code the setup() procedure’s first five lines manipulates the processors registers to set the timers up. The processor uses two interrupt service routines for timing, one internal to count the number of times the counter over flows and the clock pulses, and one external interrupt connected to a Hall Effect sensor which is placed under the rim of the turntable platter and a small button magnet is glued to the underside of the rim of the platter. The magnet is small enough and light enough not to affect the platter balance.
The PCB has a 5-volt regulator on it so you can feed the board 7.5 to 12 volts DC. The optimal voltage in is 9 volts DC. While the regulator can handle higher input voltages the heat dissipation may be too much, 12 volts is pushing the thermal limit of the PCB heat sinking capability.
Note: the cost of the tach is using Blue LED displays, we build one with Red LED displays, a lot cheaper, and it still looks good. The hall effect sensor is around $10 but I designed a small ½” square board with a $0.60 hall effect sensor on it so cut the cost to about $1. So, an overall savings of $15 with Red LEDs and the home-made sensor. I will post the gerbers and part number for the Hall Effect sensor in another post.
This post has the PCB gerbers, the schematic, the BOM, a couple pics, and the code.
Operations: when the tach is powered on it runs through the setup of the Arduino and, before it exits, turns on all the LED segments for 5 seconds, this is a test to show you if you have any issues with your soldering. Then the main code runs, loop(), which looks at the number of overflows of the counter, which being 16 bits can only hold 65535 clock cycles before it trips the overflow flag and resets. If the platter isn’t turning the overflow count will keep going up, but once the number of overflows is over five, long enough to see the platter isn’t turning, it shut off the display. As soon as the platter starts turning the overflow number is reset by the triggering of the hall effect sensor and the display comes on. Whatever speed the turntable is spinning the speed will be displayed whether it is 33 1/3, 45, or 78 rpms. My table is a Denon DP80 built into a heavy plinth and typically displays 33.332 to 33.334 which is extremely stable and what Denon was known for. A friend is working with some Pioneer high end tables with a custom platter and heavy plinth and is seeing similar numbers, once the tables warm up. For grins I stuck a magnet to the side of an old, cheap Girard table and it was varying a lot more than I would like to see in an audiophile table.
Attachments
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And just for fun, here is a pic of my turntable, uses a Denon DP80 tuntable, SME V arm, Koetsu Rosewood Signature cartridge, Sota Reflex clamp, my tach, a plinth box made with my piece of Zebra wood, made by my friend who owns PBN audio, and his CNC solid aluminum plinth top. Sounds Awesome.
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johnhenryharris:
Thanks for this project! Would you consider updating the BOM? I found issues with the following:
Regards!
Thanks for this project! Would you consider updating the BOM? I found issues with the following:
I couldn't locate the 7-segment display listed in the spreadsheet at either digikey or mouser. Is DSM7TA39106T a suitable replacement?
The linear regulator is out of stock at mouser for the next 3 months. Would MC7805CDTG work as well?
The oscillator (CVT32-10.000) is in very short supply at mouser. Can you suggest any alternatives?
Regards!
The LED display DSM7TA39106T is a good replacement and I actually prefer it over Digikey as they ship directly from the vendor and delays the materials by several days.
The Linear regulator MC7805CDTG is the same part just different material packaging but since you only buy one, or so, it is shipped in an individual package.
The CTS32-10.000 has 29 in stock but is twice as expensive as the new one available, FOX924B-10.000, same foot print and PPM.
So I have updated my BOM with these new parts. Thanks for the questions about the materials, This is always a problem with projects, material going end of life or long lead times.
Good luck.
The Linear regulator MC7805CDTG is the same part just different material packaging but since you only buy one, or so, it is shipped in an individual package.
The CTS32-10.000 has 29 in stock but is twice as expensive as the new one available, FOX924B-10.000, same foot print and PPM.
So I have updated my BOM with these new parts. Thanks for the questions about the materials, This is always a problem with projects, material going end of life or long lead times.
Good luck.
Hi to all,
Johnhenryharris,
In the Bom you are specifying a SMC39LB G/W or DSM7TA39106T with is a 'Common Anode'
but in the 'use' column : Common Cathode, 20 ma 3.2vf SMD
Could you please precise this point?
PS: Great project!
Johnhenryharris,
In the Bom you are specifying a SMC39LB G/W or DSM7TA39106T with is a 'Common Anode'
but in the 'use' column : Common Cathode, 20 ma 3.2vf SMD
Could you please precise this point?
PS: Great project!
Good catch! The display should be Common Cathode, so that display will not work. let me check again for a correct display.
Looking at Mouser I did find 710-157121V12700 which is Green and 710-157121S12700 which is Red, no blue.
Hard to find .39"/10mm height, 7 segment, common Cathode, SMD.
For Blue you have to go to Digikey.
https://www.digikey.com/en/products/detail/american-opto-plus-led/SMC391LB-G-W/12325528
They say they have 36 in stock of the blue ones.
And the updated BOM attached.
Hard to find .39"/10mm height, 7 segment, common Cathode, SMD.
For Blue you have to go to Digikey.
https://www.digikey.com/en/products/detail/american-opto-plus-led/SMC391LB-G-W/12325528
They say they have 36 in stock of the blue ones.
And the updated BOM attached.
johnhenryharris:
Hmm... not to pile on or anything, but if you have a supplier who offers the Arduino Nano 340G for $7.00, please share. Newark charges $23.04 (I think its the same model).
Regards!
Hmm... not to pile on or anything, but if you have a supplier who offers the Arduino Nano 340G for $7.00, please share. Newark charges $23.04 (I think its the same model).
Regards!
I purchase 3 Nano's for $21 from Barnaby Robotics in California, LA area.
also you can get from Flea Bay, you have to be careful as some do not connect to the Arduino IDE but ones with this description do. V3.0 w/340G are good.
https://www.ebay.com/itm/2253393712...xM6K/WIxZKFvZlizwNHmofB+w=|tkp:Bk9SR4r-1eDCYQ
also you can get from Flea Bay, you have to be careful as some do not connect to the Arduino IDE but ones with this description do. V3.0 w/340G are good.
https://www.ebay.com/itm/2253393712...xM6K/WIxZKFvZlizwNHmofB+w=|tkp:Bk9SR4r-1eDCYQ
johnhenryharris:
I would appreciate your guidance: I'm finalizing the BOM for my build and have decided to go with an orange 7-segment display instead of the bright blue unit you proposed. The specs (including the forward voltage) on the orange display (Vishay model VDMO10C0 -- https://www.newark.com/vishay/vdmo10c0/display-seven-segment-10mm-orange/dp/06AC6506) are quite different from the blue display (https://www.aopled.com/AOP_PDFs/SMCA391LB.pdf). Under the circumstances, I suspect that the resistors connected to the displays should be changed, perhaps closer to 300R. Would you mind confirming or correcting my thinking, and also confirm which resistors should be changed?
Thank you,
Scott
I would appreciate your guidance: I'm finalizing the BOM for my build and have decided to go with an orange 7-segment display instead of the bright blue unit you proposed. The specs (including the forward voltage) on the orange display (Vishay model VDMO10C0 -- https://www.newark.com/vishay/vdmo10c0/display-seven-segment-10mm-orange/dp/06AC6506) are quite different from the blue display (https://www.aopled.com/AOP_PDFs/SMCA391LB.pdf). Under the circumstances, I suspect that the resistors connected to the displays should be changed, perhaps closer to 300R. Would you mind confirming or correcting my thinking, and also confirm which resistors should be changed?
Thank you,
Scott
The forward voltage for the orange display is 2v so 5v - 2v = 3volts left. And Max current is 20 ma so 3v/.02 = 150 ohms. A 300 ohm resistor would give you 10 ma which is probably bright enough. You can change all the resistors to the 300 ohms without an issue.
johnhenryharris:
What a pest! I have another dumb question. Mouser and Digikey won't be resupplied with the linear voltage regulator for many months. Mouser does have a few alternatives, and I'd like to confirm with you that NCV7805BDTRKG is a suitable replacement.
Thank you for you patience and consideration.
Regards,
Scott
What a pest! I have another dumb question. Mouser and Digikey won't be resupplied with the linear voltage regulator for many months. Mouser does have a few alternatives, and I'd like to confirm with you that NCV7805BDTRKG is a suitable replacement.
Thank you for you patience and consideration.
Regards,
Scott
Hi Scott, I've been using the STMicro 5V 0.5A L78M05ABDT-TR without issue and have also powered it directly through the USB port on the Nano as well. It works so well that I am considering just runnning a USB cable to it and plugging it into a charging pack when I want to adjust it, instead of trying to find 9V somewhere (or you can use an outboard 9V battery).
I measure the draw over USB and over the 9V input at 40-50mA. Any of these (0.5A to 1A) regs should work, which also includes the onSemi one you listed:
5V TO-252-3 SMD voltage regulators 0.5-1.0A output
Cheers,
Stephen
P.S. johnhenryharris replied in the meantime and I didn't check pinouts (footprint) so anyone reading this, reminder to check pinout when selecting a replacement regulator.
Thanks for sharing this @johnhenryharris !!!
I measure the draw over USB and over the 9V input at 40-50mA. Any of these (0.5A to 1A) regs should work, which also includes the onSemi one you listed:
5V TO-252-3 SMD voltage regulators 0.5-1.0A output
Cheers,
Stephen
P.S. johnhenryharris replied in the meantime and I didn't check pinouts (footprint) so anyone reading this, reminder to check pinout when selecting a replacement regulator.
Thanks for sharing this @johnhenryharris !!!
Interesting, I just looked at Mouser for the regulator MC7805CDTG and it says they have over 4400 in stock.