Yet another stir plate thread... with a For Sale feeler

After getting frustrated with the speed control limitations of brushless computer fans and just feeling uneasy about the lack of robustness in the speed control methods being implemented by the nice people on this forum I decided to go a step further in making my own stir plate.

Don't get me wrong, I don't think there's anything intrinsically wrong or unsafe with the designs I've seen implemented here. It's just that as an anal retentive electrical engineer I wanted finer control from 0 to full speed with no dead bands in the speed control knob and a robust design that will last me forever.

So I've implemented a very simple PWM (pulse width modulation) speed controller with a brushed DC motor (not a fan). My electronics provide ramp-up control, max speed limitation, and protection of the electronics from voltage spikes created by the motor (back EMF spikes). The brushed motor is a little louder than a brushless fan but the noise is dampened by the enclosure and is completely drowned out by the sound of the stir bar when stirring liquid.

I've made mine and it's working great. I haven't put together a schematic drawing or anything yet, just scribbles on scrap napkins and bits of paper. If people on this board are interested in my design I'll put together a schematic, bill of materials, and build instructions and post them to this thread. If no one's interested, I won't waste the time. So if you want to see it, speak up.

Additionally, if any of you guys (or gals) are really interested I would consider putting together ready to assemble kits of parts for sale. I'm not sure what it would cost yet. I'd just take the cost of all the parts (including a professionally manufactured circuit board and stir-bar) and mark it up a bit for my time and effort assembling the kits and dropping them off to be shipped. This option might be cheaper than getting all the parts yourself because of the volume discounts I might be able to get on the parts if enough people are interested.

I'll try to post pictures sometime in the next couple days but mine looks externally like all the others built in a Radio Shack project box. Inside there's a little circuit board and a bit of PVC pipe to hold the motor in place. On the end of the motor's shaft is a small plastic disk to which rare earth magnets are glued.

Also, in case anyone didn't know a small (too small for a 2L starter?) professionally manufactured stir plate can be purchased on-line for less than $90. But what fun is that? Hanna HI 190M Magnetic Stirrer

Well, I'm not interested in buying/building one as I built a simple 317 based controller, but I'd be interested to hear a quick summary of what you did. I'm just a hobbiest in electronics, and am always eager to see how something is done so I can use it later on.

Like with the spike from the motor you're talking about, I just put in a snubber diode, did you do something more complicated? I can set my max voltage by changing out one resistor that's in parallel with the pot, but it's going to be a whole different ballgame with PWM. Speaking of which, are you using it like a switch mode powersupply, or more as a duty cycle control for the motor?

Sorry for all of the questions, like I said, I love to learn something new.

I've been meaning to do something like this for quite a while, but as I've already got an adjustable-output bench power supply it's easy to just use that, so my motivation has been lacking.

Just out of curiosity, why did you use a brushed motor, instead of just PWM-controlling a fan instead? Not that it's a big deal or anything, but fans are just so convenient because of how easily they can be adapted to make stir plates.

Scimmia. Nope I just put in a diode, just like you did. Super simple solution for a simple problem. Maybe I missed it, very possible, but I didn't see that addition on other designs. The PWM is a duty cycle controller. My design is very simple and I also adjust the max speed using a single resistor. Don't apologize for asking the questions, they're good questions.

Funkenjaeger, I saw your post about closed loop control using a PIC and the speed feedback on the fan. That's definitely a good design and more sophisticated than mine. Mine's all analog. I did try PWM control w/ a fan but it still didn't deliver the results I wanted. Let me elaborate:

At first I thought that the voltage regulator method everybody was using looked super simple, required very few components, and I had all the parts on hand so I gave it a shot with a few different computer fans I had on hand. The minimum speed on my one 120 mm fan was too fast and threw the bar constantly. Additionally, a large portion of the POT rotation created voltages below the minimum operating voltage of the 120mm fan so I had this huge range of uselessness and a small range of POT rotation that just resulted in the fan spinning way too fast. The 80mm fans I had on hand (very old stuff) worked a lot better but were just too slow @ 12 V to get a really vigorous stir going in 2L.

I thought about wiring a resistor in series with the POT to eliminate this dead band but then I decided to quit putzing with the voltage regulator and just move to a PWM.

I built the PWM and tried it with the 120MM fan which improved the situation. That is, I could run the fan at a lower speed but low speed operation was touchy and rather twitchy. The problem with brushless motors (like computer fans) is that they use electronics to switch the magnetic fields as necessary to create a rotating magnetic field (whcih is done mechanically in a brushed motor) and when the voltage drops below the voltage needed to run those electronics (likely 5V in most cases) the fan just stops. Brushless motors are usually meant to be controlled by feeding a desired speed value to the controller or, in the case of a computer fan, to run constantly at one speed preset in the controls. That is, using PWM or reduced voltage to adjust the speed of a computer fan works to a certain extent (by reducing the current through the coils and thus the magnetic field) but that's not generally how they're designed to operate.

I acknowledged that my problems might be solved by getting a different fan. I was going to do just that when it hit me: "Why use the fan at all?" That's when I got a brushed DC motor.

This led to the design I'm using now. The motor is super smooth from 0 RPM to top speed and I can make really fine adjustments. I can turn it on withthe POT turned all the way up and watch the stir bar slowly spin to full speed without worrying about the throw bar getting tossed. Additionally, the plastic disk I made to mount the magnets to the motor made it really easy to get them balanced perfectly.

As far as using a fan in general, I don't really see the motivation. Yes they have a big mounting platform for magnets but it really wasn't a big deal to drill a plastic disk to fit on the shaft of the motor. Additionally as you can see working with the fans I had was kind of a pain.

I want to reemphasize that I'm not dissing anyone else's design. I started from MrSaLTy's design and I really don't mean to imply that I think mine is drastically better than any of those. And surely Funkenjaeger's PIC based closed loop control is much more sophisticated than my open loop PWM. My method may not be for everybody but to me it's simple, elegant, and user friendly. Feel free to take it or leave it.

I'll post a schematic tonight for anyone that's interested.

You should post some pics of your motor/plate/magnet setup. I agree that as a motor, the fan really isn't ideal because of the poor low-speed operation, but at the same time, it's hard to come up with an alternative motor setup that would be anywhere near as quick and easy to set up - not that it's not WORTH extra effort per se, but it's hard to beat building a stir plate from start to finish in like 20 minutes - so if you've got something simple figured out, I'd definitely like to see it.

That's also the main reason I haven't gotten around to finishing the PIC-controlled closed-loop thing, I can't get up the motivation since my current super-simple setup does everything I need already.

I'd be interested in seeing your design though.

CrookedChris said:

Scimmia. Nope I just put in a diode, just like you did. Super simple solution for a simple problem. Maybe I missed it, very possible, but I didn't see that addition on other designs. The PWM is a duty cycle controller. My design is very simple and I also adjust the max speed using a single resistor. Don't apologize for asking the questions, they're good questions.

Click to expand...

You didn't really miss anything, I've just commented in a few of the theads that you should really add that diode for protection. Never posted a new schematic or anything, didn't figure it was worth it.

CrookedChris said:

At first I thought that the voltage regulator method everybody was using looked super simple, required very few components, and I had all the parts on hand so I gave it a shot with a few different computer fans I had on hand. The minimum speed on my one 120 mm fan was too fast and threw the bar constantly. Additionally, a large portion of the POT rotation created voltages below the minimum operating voltage of the 120mm fan so I had this huge range of uselessness and a small range of POT rotation that just resulted in the fan spinning way too fast. The 80mm fans I had on hand (very old stuff) worked a lot better but were just too slow @ 12 V to get a really vigorous stir going in 2L.

Click to expand...

And ironically, I'm using a 92mm fan, and have good control with the voltage topping out at 9V. I do still have the dead spot at the lower end, though.

CrookedChris said:

I thought about wiring a resistor in series with the POT to eliminate this dead band

Click to expand...

I figured I would do that next time I opened the thing up. I found that my fan doesn't kick in until ~2.5V.

CrookedChris, thanks to your inspiration I threw together a simple 555-timer-based PWM fan controller tonight. Essentially just "Nomad's 555 circuit" at this site:
http://www.cpemma.co.uk/pwm.html
but with a bunch of component values tweaked to best suit the stir plate use.

Seems to be working great with my 120mm fan-based stir plate, it covers the full range from barely-spinning to insane-vortex-almost-throwing-stir-bar, with no 'dead zone' in the adjustment, and starts up at any speed without needing a 'boost'.

Is your circuit anything like that one?

Um, why not just buy a stir plate from a local used lab equipment vendor? I wouldn't purchase anything from the place i'm at but there ARE a lot of good ones out there that won't screw their customers!

Melana said:

Um, why not just buy a stir plate from a local used lab equipment vendor? ...

Click to expand...

Why not just buy beer at the corner store?

evanmars said:

Why not just buy beer at the corner store?

Click to expand...

Well said

And on top of that, even used lab stir plates can sell for something in the neighborhood of $100, and they're usually big, heavy cast-iron things. My stir plate cost me absolutely nothing except a few bucks for the stir bar because I already had parts laying around. Even if you didn't already have parts you could conceivably build one for $20-30, that'd also be much smaller and lighter than the lab ones.

Funkenjaeger mine is also 555 timer based. I have fewer components than are present in "Nomad's 555 circuit". I also have a fixed 5V voltage regulator for the 555 timer and I added 5 big 1000 uF electrolytic capacitors in place of C3 on the control pin. This huge capacitance gives me the ramp up when I turn it on and lowers the top speed of my 3000 RPM motor. I had these capacitors laying around. I just looked up the cost of these big capacitors and let's just say that I think I would have left them out if I didn't already have them on hand.

Melana, I mentioned in my original post that you can get small basic stirrers for cheap but I also agree with evanmars that this is just more fun. I'm an electrical engineer and have always loved messing with electronic bits. So building gadgets like these is as much for fun as it is for utility.

I apologize to anyone who was interested that I didn't get the schematic up last night. I just got too busy packing/preparing for some traveling that I'm doing this weekend. I'll try to get the schematic done and posted today on my lunch. If I can't squeeze it in there I'll try to get it up some time this weekend.

How big of a starter will this stir plate design of yours handle? I have a Hanna that I bought years ago and it struggles with a 1500ml starter. I want one that will handle at least 3000ml and would be willing to build one if the design will handle that volume.

evanmars said:

Why not just buy beer at the corner store?

Click to expand...

I made one that worked fine for 1L and just got the job done on 2L
I then looked on eBay and picked up a big one that I can do 5L on and its would handle one twice that size. I paid $35 for it.

CrookedChris said:

Funkenjaeger mine is also 555 timer based. I have fewer components than are present in "Nomad's 555 circuit". I also have a fixed 5V voltage regulator for the 555 timer

Click to expand...

Out of curiosity, why the voltage regulator? The 555 is good up to like 18V on its own, are you running it from a higher voltage than that?

What range of duty cycle are you getting with your design?

I like the idea of the ramp-up feature, but yeah, 5 big electrolytics doesn't sound too great for size or cost reasons. I think the biggest cap I'm using is 220uF or 330uF and that's just for decoupling, with all other caps much smaller. Wonder if it'd be feasible to do the same ramp-up with more reasonable values. A resistor in series to bump up the time constant perhaps?

I also wonder how well the ramp-up feature would combine with the kick-start of Nomad's circuit... Sounds like some experimentation is in order.

Wow, those are some huge decoupling caps, Funkenjaeger. Is the ESR low enough on them to do the job, or are you using a smaller, faster cap, too?

Personally, I would much rather use regulated power when feeding an IC, most wall worts are pretty noisy, and you should be using a transistor to drive the fan, anyway, not the timer directly.

The schematic of the design I'm using:



I'm using the voltage regulator just to be friendly to the 555 IC. Not absolutely necessary but the voltage supplied by wall warts is usually pretty ugly.

For anyone that wants to build this all 8-pin DIP 555 timers have, as far as I know, the same pin-out. The number I give (NE555DG4) is for the Texas Instruments 555 timer I have. Any 555 timer should work in it's place. The N-channel FET (MTP20N15E) I'm using is, once again, what I had lying around. This one is major overkill (20A, 150V). Any N-channel FET or even an NPN BJT with enough current carrying capacity to support the motor (<400 mA) will work in this configuration. I'd probably go with the lowest cost option were I to build this from scratch.

I'm racking my brain for a better/cheaper way to provide ramp up control. I'll think of something. And of course open to suggestions.

I've seen kick start circuits in a few different 555 PWM circuits but I really don't see the point.

I'm going out of town this weekend but next week I'll take and post photos and hopefully some video of the stirrer in action.

SuperiorBrew, that $35 stirrer is a sweet find. Congrats. maybe I should take a look @ eBay.

jdoiv, I have stirred 2L with this guy but had to add a couple more magnets to make it work. My problem wasn't the motor's power (I was surprised by the torque) but the ability to couple to the stir-bar. Next week when I'm taking photos and video and such I'll throw a 1 gal carboy on it and see what happens.

Scimmia said:

Wow, those are some huge decoupling caps, Funkenjaeger. Is the ESR low enough on them to do the job, or are you using a smaller, faster cap, too?

Click to expand...

Well yes, I have a 0.1uF decoupling cap also... But a single 220uF decoupling cap is far from HUGE, it's hardly bigger than the 555 chip or the power transistor... and the main reason for using it is that I already had it plugged into the power rails on my breadboard, I may very well lower the value when I build a final board, but at the same time, capacitors are cheap and it never hurts to have adequate decoupling when you've got an inductive load (motor) being switched repeatedly.

Scimmia said:

Personally, I would much rather use regulated power when feeding an IC, most wall worts are pretty noisy, and you should be using a transistor to drive the fan, anyway, not the timer directly.

Click to expand...

Well I agree in general, but in this particular application I'm not that worried... The 555 is far from a 'sensitive' IC, and it's being used to drive a fan - if the duty cycle on the PWM isn't perfectly stable, it's really not going to make much difference. I just don't feel the need to add a regulator unless I find it to actually be necessary, and so far it's been fine without.

And of COURSE I'm not driving the fan with the 555 directly, I'm using it to drive the gate of a P-channel power MOSFET that runs the fan. It's what I had laying around, and it's rated to 12 amps, so I think I'm quite safe driving a measly 200mA or so into a computer fan

CrookedChris, your circuit looks pretty nice. Definitely a lot fewer components than what I'm using. I think you may be right about the kick-start... It does seem kind of handy as my stir plate will start up without help even when it's cranked down to the lowest possible speed at which it manages to stay spinning... but then again, it's not like I plan to stir wort that slowly in actual use. Removing the kick-start would save me 3 components - still leaving it a bit more complex than yours, but not too bad.

One thing that sticks out to me in your schematic is that you're using the 555 to drive an NMOS even though you're only providing it with a 5v supply. The threshold voltage on your MOSFET is 2-4V according to the datasheet, so with only 5v on the gate you're not exactly driving it very hard, so depending on the motor it could be saturating, heating and wasting power in the MOSFET. If your motor is small enough that it stays in the ohmic region then it's not a problem, but for flexibility it might be worth it to remedy it (in case someone wanted to use it with a bigger motor) in some way... (of course if the datasheet included a Ids vs. Vds curve it'd be a little more apparent whether it was cause for concern) Adding a couple resistors to bump up the regulated supply voltage or skipping the regulator should help, or you could switch to a drive setup like I'm using - using the DISCH pin of the 555 (open-collector output) to drive the gate of a PMOS with a pull-up resistor to the supply voltage, thus delivering the full 12v to switch the transistor, regardless of the supply voltage of the 555 itself.

Funkenjaeger said:

And of COURSE I'm not driving the fan with the 555 directly, I'm using it to drive the gate of a P-channel power MOSFET that runs the fan. It's what I had laying around, and it's rated to 12 amps, so I think I'm quite safe driving a measly 200mA or so into a computer fan

Click to expand...

Sorry, I should have phrased that better. I wasn't implying that you were using the timer output directly, all I was saying was that the voltage ouput of the timer didn't need to be the full voltage that you're applying to the motor since you're running through a transistor anyway.

Edit, and on the caps, I wasn't meaning big physically, I was meaning large capacity. They're pretty much just extra PS filter caps at that point, aren't they?

Scimmia said:

Edit, and on the caps, I wasn't meaning big physically, I was meaning large capacity. They're pretty much just extra PS filter caps at that point, aren't they?

Click to expand...

Yeah, basically, but it's at the point of load which can help a bit if the connection to the PS isn't great. Not exactly a tremendous necessity, but I'm just a bit over-conservative when it comes to PWM with an inductive load, especially when I've got a parts bin chock full of suitable caps
On the finished board I'll likely drop down to 100uF or smaller, but we'll see how it goes when I probe it with an oscilloscope to find the point of diminishing returns in terms of capacitance vs. supply ripple.

Funkenjaeger said:

Yeah, basically, but it's at the point of load which can help a bit if the connection to the PS isn't great. Not exactly a tremendous necessity, but I'm just a bit over-conservative when it comes to PWM with an inductive load, especially when I've got a parts bin chock full of suitable caps
On the finished board I'll likely drop down to 100uF or smaller, but we'll see how it goes when I probe it with an oscilloscope to find the point of diminishing returns in terms of capacitance vs. supply ripple.

Click to expand...

Makes sense. You're using caps to try to clean up the power, where CrookedChris just used a vreg.

Sorry I haven't gotten photos f my rig up yet. Just been friggin busy. Besides that I can't find the cable to get photos off of my digital camera.

However I did come up with another design that allows better ramp (up and down, not just power on like the old one I had) rate control for the stirrer speed. It's completely untested so far, just exists on paper. It grew from an application example in the NE555 data sheet (see page 13 of this document). The jist is this:

The first 555 timer controls the PWM frequency of the second. If R1 = 250 ohms and C1 = 1uF this frequency should be around 2.9 kHz (1/(2*0.693*RC)=1/(2*0.693*.000001*250)=2886 Hz). The voltage @ the control pin on the second 555 timer controls the duty cycle, ie: speed of the motor. A simple POT (R4) based voltage divider sets this control voltage and thus the speed of the motor. I added a capacitor and 100k resistor to slow down these voltage changes. If you quickly moved the knob from completely off to full speed it would take about 10 seconds for the motor to reach 60% of full speed and 30 seconds to reach full speed. The same goes for a "full on" to "full off" transition. It can be slowed down even more by increasing either the value of C4 or R3. The op-amp (LT1006) is there to act as a voltage follower and prevent the capacitor C4 from charging much faster via the low internal impedance of the 555 timer.

This design still weakly drives the N-channel FET but I can't use a P-channel because I can't add a pull-up for the p-channel on the discharge pin without screwing up the RC circuit needed on the Threshold pin of the second 555 timer. I could just use a 10V supply for the whole thing instead of a 5V and be fine.

Let it be known that I would never build this circuit unless I already had the parts lying around. When you add up the cost of buying a dual 555 timer, a single source op-amp like the LT1006 (expensive), more board space, etc. it's just not worth it. At this level of complexity it would make much more sense to just implement a PIC micro-controller as Funkenjaeger has recommended. That said, I have all of these parts on hand so I'll probably build one like this in the near future.

And the schematic:

CrookedChris said:

At this level of complexity it would make much more sense to just implement a PIC micro-controller as Funkenjaeger has recommended.

Click to expand...

Why PIC specifically? I've been messing around with AVRs for a while, and a number of people here use Ardunios, which are basically an AVR dev board with their own IDE which sits on top of AVR GCC.

Scimmia, I only mentioned a PIC because that's what I've had the most practical experience with and is what Funkenjaeger mentioned he was using (I think). You're right, just about any microcontroller could be used.

Scimmia said:

Why PIC specifically? I've been messing around with AVRs for a while, and a number of people here use Ardunios, which are basically an AVR dev board with their own IDE which sits on top of AVR GCC.

Click to expand...

Well AVR is fine too, if that's what you're used to. I've used PICs for many years, I don't have any tremendous loyalty but it's what I'm used to and it would take me a lot of time and effort to switch to the AVR camp - if I had it all to do over again, I might have gone the AVR route, because it seems to be more in line with the FOSS movement, and there seems to be a much more active, interested user base. (Practically the only thing remaining that I think PICs have over AVRs is the USB support, and Microchip did a horrible job of documenting it, leaving the users to help each other learn how to use it, that I hardly give them any credit for offering it at all. But I digress...) I think the Arduino board is great for flexibility and bringing microcontrollers to the masses in a way that doesn't involve horrible performance (I'm looking at you, BASIC stamp...), but an Arduino would be complete overkill for a project like this where there is essentially one input (potentiometer) and one output (PWM), and a small 8-pin PIC (or AVR, if you wish) costing about a buck or two would do everything you need.

Absolutely, I wasn't suggesting using something like an Arduino for this project, just pointing out that others here have some experience (and I'll agree on BASIC stamp, WTF were they thinking?). Still being new, I didn't know if there was something specific about PICs that made them more suitable, so I asked the question. The only reason I went with AVR when I started experimenting was GCC.

Made up a few PCB's tonight, after my etching-related failure killed the first batch a few days back. Still far from a perfect fab, especially since I was using big fat 16-mil traces, but they'll work. Should be small enough that I can just glue one to the side of the 120mm fan that I use for my stir plate, making for one compact unit without any bulky enclosure.

With that said, I'll probably start my own thread for this once I get further with it, so I'm not cluttering/hijacking this one any further.

Funkenjaeger, awesome. Would you mind sharing the layout and a parts list. I'd definitely be interested in trying this one out.

CrookedChris said:

Funkenjaeger, awesome. Would you mind sharing the layout and a parts list. I'd definitely be interested in trying this one out.

Click to expand...

EDIT: yeah, don't use the design I previously posted. As expected, I discovered some things that needed fixing after populating the board and testing it.... most importantly, the pinout on the DC power jack (which was in the default eagle libraries) was backwards from the actual jack that I have, which is of course not a good thing. I'll post an updated version once I've finalized things.

I've left the schematic here since it's still an okay reference.