An update and simplification of the original MC-2100 PWM Circuit
The circuit. Left connector goes to MC-2100 (GND, 12V, Signal). Right connector goes to speed control pot.
After discussion with Terry in the comments of previous posts, we came to the conclusion that the MC-2100 wasn’t expecting a full 5V signal at the control signal input (blue wire). As shown in the MC-2100 schematic, the optoisolator on the input (U1) has a 22ohm resistor (R2) in series with it. Applying 5V to the blue wire results in 170mA flowing through the optoisolator, higher than its 50mA rating.
This lead to the conclusion that there must be another resistor in the circuit at the dash panel end of the blue wire. Upon inspection, there is a 240ohm resistor in series with the control signal output on the treadmill’s dash panel PCB. With the series resistance at 262ohms, the current through U1 is now 15mA, a much better current level for reliable operation.
Taking things a step further, it’s possible for the circuit to operate on higher voltage if a larger series resistor is used. This allows us to eliminate the 5V regulator from the circuit, as well as the transistor from the output, which was mistakenly added to allow the higher output current the circuit required without the current limiting resistor.
Here’s the resulting circuit (keep reading, not done yet!):
MC-2100 PWM Driver v6 – No Filter
Because the LM393 comparator only sinks current, the current limiting resistors end up acting as pull-up resistors. This also means that the LM393 must sink the full output current, otherwise the optoisolator won’t turn off.
The limiting current of the LM393 is 20mA and the typical voltage drop is 0.15V, so choosing resistors that satisfy the 50mA requirement of the optoisolator may not satisfy the requirement of the LM393. The calculation for current through either component is as follows:
(Supply Voltage – Component Voltage Drop) / (Series Resistance) = Current through component
So for example, a 560ohm + 22 ohm resistor pair would result in (12V – 1.2V)/582ohm = 18.6mA through the optoisolator, an acceptable value. The same 560ohm resistor (with no additional resistor) would result in (12V – 0.15V)/560ohm = 21.1mA through the LM393, higher than the rated current of the comparator. If this value were used, the comparator would try to pull the output to ground, but wouldn’t be able to fully turn off the optoisolator. In fact that’s exactly what I observed after testing the circuit as described, leading to understanding this concept, and revising the circuit.
I ended up just sticking another resistor in the circuit (the flying resistor in the picture at the top) rather than removing and replacing the 560ohm resistor, resulting in the 560+580ohm pair shown above. This combination resulted in 10.3 mA through the LM393, and 9.3mA through the optoisolator when driven at 12V. If I were to start over, I’d replace two resistors (R6 and R7) with a single 1kohm resistor and the circuit would still work fine.
With the current draw of the circuit figured out, the MC-2100 would reliably respond when I’d initially send it a signal. However, it was still susceptible to noise from the motor drive voltage (when the motor would turn on, the motor PWM frequency showed up in the oscilloscope trace of the blue wire signal). This would shut the MC-2100 back down, as noted previously. I ended up adding a low pass filter to the output signal, which seems to have solved the problem.
The final revision is shown here:
MC-2100 PWM Driver v6
Here are links to a parts list, and the Eagle schematic file.
Alex, 
Arduino Compatible MC-2100 Controller and Lathe Tachometer | Sons of Invention 8:03 pm on May 22, 2013 Permalink |
[…] An update and simplification of the original MC-2100 PWM Circuit […]
John Tappan 9:38 am on February 3, 2015 Permalink |
Guys,
This is really interesting stuff, and although I don’t understand any of the technical jargon, I am hoping I can replicate your controller for my MC-2100. I’m sure I’ll have many questions throughout the process, although your detail and explanation is top-notch. I lam a bit confused at the beginning, though. Is the “original MC-2100 PWM circuit” incorporated into the Arduino compatible “MC-2100 Driver V 6” or are they two separate components of the overall controller. I guess I’m asking if the 555 timer circuit is necessary in additon to the Arduino controller. Thanks.
schoolie 3:04 pm on February 9, 2015 Permalink |
The two circuits are independent, and produce the same basic function. The arduino based circuit is just a fancier way of doing it that provides a digital speed readout among other things. The simplest approach is to build the circuit in this post, and ignore the arduino stuff entirely. Good luck!
Doug 12:27 pm on October 30, 2015 Permalink |
Hi and thanks for the great article and your work on the MC2100.
I have your circuit constructed on a breadboard. I have double checked everything twice and made sure all values are correct. I checked continuity between all points to make sure the breadboard contacts were ok.
The red led on the MC2100 comes on and stays solid, which means it’s not receiving a pulse from the external circuit. I don’t have a scope to check to see if I am getting a proper square wave. You don’t show any DC voltages on your schematic, so I am not sure what voltages I should be reading on the external circuit?
The previous owner said the treadmill control panel got wet and it stopped working properly. I tested the Motor and it works. The DC2100 board looks to be in good shape. I am wondering if the U1 opto is bad on my MC2100?
I am at a standstill now until I figure out what to do next
Any help would be appreciated
Thanks, Doug
Here’s a link to a pic of my setup
Doug 1:30 pm on October 30, 2015 Permalink |
I measured voltages on my external board and added them to your circuit diagram
Doug 12:07 pm on October 31, 2015 Permalink |
Update.
I was able to get Terry’s dual 555 timer circuit to work with my MC2100.
Details on my build are here.
https://sonsofinvention.wordpress.com/2013/05/21/an-alternative-mc-2100-dashboard-replacement-circuit/
schoolie 10:15 am on November 2, 2015 Permalink |
It probably doesn’t matter now that you’ve got Terry’s circuit working, but was the 1.2 to 1.4v at the output measure with the blue wire plugged in to the MC2100? Did you adjust the R1 pot?
Glad you got it working either way, thanks for sharing!
Brian
Doug 10:25 am on November 2, 2015 Permalink |
Hi Brian, I don’t remember now. That was a few days ago. I did play with both pots for quite a while until I gave up and built Terry’s circuit. I am not sure why I could not get your circuit to work properly?
I set up a build page here for other people and there is a link to a you tube video of my build in action
Check out my finished build here.
http://el34world.com/Misc/Cnc/TreadmillMotor1.htm
Doug 10:27 am on November 2, 2015 Permalink |
And by the way Brian, thanks for posting your blog. I would have just chucked the MC-2100 board in the trash and bought a MC-60 if it was not for finding your info 🙂
Here’s my you tube video link for those that are interested.
Bryan 3:54 pm on November 17, 2015 Permalink |
I have a quick question – I’ve got the NE555/LM393 circuit built that works, but anytime I disconnect the power to the unit, it takes quite a bit of fiddling with the trimmer pot to get the motor to run again. I seemed to remember reading something about this issue, but I can’t recall where in the various threads on this topic. For most of the time, I can leave it plugged in and leave the main power circuit on, and just use a switch in line with the rheostat. Is there a way to get the unit to restart reliably when the power is completely turned off?
Bryan 8:37 pm on November 17, 2015 Permalink |
Correction: it’s been a while since I worked on this project, and had forgotten that I couldn’t get the 555/393 circuit to work, so built the twin 555 circuit (just like Doug). Going to try the momentary contact and see if that solves my issue.
Bryan 10:07 pm on November 17, 2015 Permalink |
Well… I seem to have solved my own problem just by reading a little more carefully – Doug’s solution works well for me. I’ve added an on/off switch in line with the hot side of the speed controller so I won’t normally have to turn it off at the circuit breaker.
Doug 6:42 am on November 18, 2015 Permalink |
Cool, glad that worked out for you!
JD Combs 5:30 pm on November 13, 2016 Permalink |
I realize this is an older post but I am just getting into building a treadmill motor powered 2×72″ belt grinder using a MC-2100 control from the tread mill I scavenged. I found the V6 interface circuit and was in the process of bread-boarding it but noted what looks like a discrepancy in the schematic. I am really old school electronics, late 60s Navy electronics, but it appears to me that a component is missing from the “A” side of the LM393. Either that or it is intentional “grounded” out of the circuit so as not to be used. Pins 1 thru 3 all go to ground effectively grounding pin 4 of the “B” side. Is this correct?
schoolie 9:38 pm on November 13, 2016 Permalink |
You’re reading it correctly, the A side of the LM393 isn’t used in this circuit. The LM393 has two independent comparator circuits, and we only need one here, so the three pins associated with circuit A are just tied to ground. Pin 4 is the main ground reference that is used by both circuits. Good luck!
Alex 3:10 am on January 6, 2017 Permalink |
I am trying to build this to see if I can get this treadmill back to use and I seem to be getting the same results as Doug did when he tried to build it. Similar voltage at R5 and R9. Parts list has R1 as the trimpot and R5 as potentiometer is this correct? If so around what value am I looking for on R1 when adjusting it?