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Old 01-22-2011, 02:25 AM   #11
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When I saw the title of this thread I thought "Why would you use PWM for your stirplate, that's just overkill!" but this makes a lot of sense... I'm right there with you on being bugged by losing the lower end of the speed range, and it's really not that much more complicated than the other stirplate builds if you have the slightest experience with electronics.

I was planning on starting my stirplate build this weekend, and I think I've got all the parts (except possibly the MOSFET) on hand already. I'll let you know how it goes.
Thanks man. Yeah, it is super simple, especially compared to some other PWMs I was seeing (quad op-amp chips...really?). Also, you can probably use a JFET or even an NPN (if you add a 100 ohm resistor from collector to ground) if you have either of those.


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Old 01-23-2011, 08:11 PM   #12
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Updated the schematic to make it more accessible.


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Old 01-23-2011, 11:04 PM   #13
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Updated the schematic to make it more accessible.
Oh sure, now that I've got it all breadboarded out...

Got it running though, substituted a 2n2222 for the MOSFET as suggested, and it runs nicely. I really appreciate your build diagram, that'll make it much easier when moving to perfboard; the breadboard gets kinda crowded with jumper wires. Shouldn't 4 (reset) be tied to +9v though?

Also, while the 2n2222 worked for a quick test, it got too hot too quick. I'll probably swap out to the MOSFET for the permanent build.
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Old 01-23-2011, 11:42 PM   #14
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Shouldn't 4 (reset) be tied to +9v though?

Also, while the 2n2222 worked for a quick test, it got too hot too quick. I'll probably swap out to the MOSFET for the permanent build.
Oops, yeah 4 should be tied to Vcc (it works without, but it's safer to be tied due to static/noise).

Sucks about the 2n2222. The MOSFET I have is rated for around 600 mA and stays cool as a cucumber.
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Old 01-23-2011, 11:54 PM   #15
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Hey, I wonder...Does the 2n2222 still heat up if you use it as Q1 just like in the schematic with R3 = 10k (or so), and omitting R4?
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Old 01-24-2011, 01:40 AM   #16
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Oops, yeah 4 should be tied to Vcc (it works without, but it's safer to be tied due to static/noise).
Yup, that's what I thought... Mostly wanted to clear it up for the sake of those who come later.

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Sucks about the 2n2222. The MOSFET I have is rated for around 600 mA and stays cool as a cucumber.
My NPN I thought was rated to 800mA, so really no idea what's going on here. No worries, I need a trip to Radio Shack for perfboard anyways, I'll grab a MOSFET while I'm there. Oh, and I tried throwing R3 in there (10K) and it didn't help. Somehow it actually made the pot stop working (speed was constant) which I couldn't figure out, so I pulled it and put the jumper back and everything worked again.
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Old 01-24-2011, 01:45 AM   #17
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time to chime in.... after "seeing" the actually photos it looks a lot simpler than your pen and paper diagram. probably because i have no background or experience in reading electronic schematics.

can all be had at radio shack, and if so can yo provide part numbers and pricing please?


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Old 01-24-2011, 02:37 AM   #18
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time to chime in.... after "seeing" the actually photos it looks a lot simpler than your pen and paper diagram. probably because i have no background or experience in reading electronic schematics.

can all be had at radio shack, and if so can yo provide part numbers and pricing please?

-=Jason=-
Yeah, pretty simple, and everything can be easily found at Radio Shack. I'm on my phone now, so no part numbers, but they're all simple parts. The 555 is under $4, and everything else is under $1. (The potentiometer may be $2-$3 at RS...) Total under $10, plus some simple soldering work. I'm thinking I may put together a quick video later this week...
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Old 01-24-2011, 02:42 AM   #19
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that would be awesome, because I went through 3 12Volt fans before I found one that wanted to play nice with me.

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Old 01-29-2011, 04:07 AM   #20
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The 555 is under $4, and everything else is under $1. (The potentiometer may be $2-$3 at RS...)
Good god man...555s are like $0.50 or less at mouser.com. Talk about markup.


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