Simple Cheap ATS Final cicuit design & component list


 
R

RobM (YankeeRob)

TVWBB Super Fan
*** Updated with final info ***

This is a real cheap auto drafting circuit that uses a Polder replacement probe to control a DC motor (CPU fan or Guru blower) that mounts onto the front of your smoker and controls airflow to the fire. It’s meant to be used with another thermometer, perhaps your grill thermometer. Basically you take the SCATS thermometer and attach it to the grate of your grill as you want to control cooking temp inside your smoker. Once you get your temp up to where you want it adjust the potentiometer until the fan/blower/LED turns OFF. The smoker should stay close to your desired temp provided it has enough fuel.
Here is the final circuit I came out with:
Scats.png


Parts list for the actual circuit on a breadboard:
Component RadioShack# Name
Q1 276-2072 1RF10 N Channel MOSFET
VR1 276-1771 7812 12VDC Regulator
IC1 276-1712 LM339 Quad Comparator
P1 271-1715 10K Linear Potentiometer
T1 ------ Polder replacement probe **see note**
R1 47K ohm Resistor
R2 33K
R3 4.7M
R4 3.9K
R5 560
LED1 Any green LED
R6 560
LED2 Any red LED
SW1 A SPST switch that can handle maybe 2 amps
M1 DC Fan (CPU Fan, Guru Blower, etc..)


Final Assembly in a project box:
- credit card sized perf board
- 14 pin IC socket to mount the LM339 (keep the IC out when you are soldering!)
- 2 LED holders to make it look pretty (you could just drill a hole and glue them)
- a project box that can fit your circuit board: 3x5 should do it
- 2 DC female connectors - 1 for fan and the other for power brick
- a 3/32 female connector for the probe (which I have not been able to find cheaply so I use alligator clips until I put in a big order for electronics)
- bunch of solid connector wire and solder

Notes: The Polder probe can be found here at this time:
http://www.comforthouse.com/pol362reppro.html

Depending on your pit size you can adjust your blower. A WSM doesn't even need a 5 CFM blower. You can find blowers here:
http://search.digikey.com/scri...ds=5.%20CFM%20blower

- B1 is a DC power source from 12 to 18 VDC. Just about any AC to DC “brick” can do this – just read the back of it!
- In the drawing above, discard the polarity marking on T1
- P1 was chosen for a user temp setting of 200 to 350 degrees F. You can get a full range by shorting R2. This will give lower temps.
- The blower motor bypasses the 12 volt regulator so be sure it can handle the full amount of voltage that you choose for B1.

** I used to work in an electronics shop and could probably build this for the cost of the fan, probe and project box alone. If you know anyone who is an EE tech you could get this done REAL cheap.



How it works in a nutshell:

T1 is the temp probe whose resistance and therefore voltage decreases as the temp increases. The voltage is compared to a user settable voltage by P1. The two voltages are compared by IC1. If the probe voltage is less then the set point (the temp is too hot) then IC1 turns off Q1 and the fan/blower/LED. If the probe voltage is higher then the set point (too cold) then IC1 turns on Q1 and thus the fan/blower/LED.

In the near future I will provide final assembly and some test burns including charts of how this performs.

One last word – Use at your own risk! I put this out here not seeking to profit from it but rather as an alternative for the people who don’t want to shell out for the commercial units. For me this unit is a backup unit I can snap onto my truck battery if all the power fails at a competition. That was my main design goal other than being cheap.

I'll also add that although I'm a little rusty in the EE world as I've been in CS for most of my career so I welcome any design changes.

Enjoy,
rob
 
Looks great Rob!!

Have you plotted the temperature? I would love to see the graph. What range are you getting? 10 degrees?

Sadly, I have not had a chance to work on mine yet. I think I will continue to try an explore the variable voltage/fan speed. Unfortunately it probably would be very difficult to find a small fan with a small breakover voltage. Hmmm, maybe to overcome the breakover voltage, I can discharge a small capacitor. Then the fan slows down.

mark
 
sure do admire folks like you that understand all that stuff. Really wish I had paid more attention during my electronics course while I was a young dumb sailor. That stuff looks foreign to me.
 
Mark,

I plotted the temp to find the optimum value of the 10K pot to be from 210 to 350 degrees. If I set it to say 225 and get my heat gun running so it just barely turns off at 225 it will stay there for a long time. I was monitoring this and needed to take a piss so I left the circuit and the draft that was caused from me moving was enough to set the fan ON. I noted this and once the heat gun brought it back to temp and the fan went off I blew on it and the same thing happened. Once it's set it's pretty sensitive.

Before I started on this project I was working on something similar that used PWM to control fan speed. I found a cheapo PWM project for like $25 that did this. All you really need to do is slap a diff amp onto the front of the circuit that controls the width of the PWM and it's done. It would have WAY more granularity then this on/off solution but would make the fans hum at the same time. It would also add to the cost.

I think this is probably the cheapest thing you can get for the response it delivers. Don't forget you'll need to add the cost of the project box and connectors to the final cost - at least another $10.

If you do find a fan that can has the responses you want chances are that it will be a very unique fan and hard for everyone to find. The on/off and PWM solutions make the circuit more practical as you don't need to worry about the DC motor characteristics. I should be able to interchange a Guru fan with a CPU fan without any worry in both the on/off and PWM solutions.

I really want to chart this on a smoke but haven't had the time. I've got a BGE that's waiting for a fan mount and need to bottle some homebrew this weekend. Was thinking about doing some ribs using this control. WE'll see what happens. I still need to get it off the breadboard first!!

-rob
 
Really wish I had paid more attention during my electronics course while I was a young dumb sailor. That stuff looks foreign to me
Click to expand...
If you crank your head 90º when you look at the diagram you might see really rough waters.
icon_razz.gif


I built a joystick for my Apple ][e approaching 30 years ago ... not done more than tinker for repairs since.

Any chance you or someone who builds it might post a pictoral step by step?

I can take the list of parts to the electronics supply store.

I have several old PC power supplies and fans kicking around ... one of them would likely work to power the thing, yeah?

Any chance my Maverick probes will work? They have what look like mono mini plugs on the end, just change out the connector or would it be more complicated than that?
 
The diagram might be a little intimidating but there aren't really a lot of components to it. The whole thing could probably fit on a 2"x1" perfboard.

I will take pics of the assembly as I go. I am going to do this in small project box and include connectors for DC in, probe in and fan out. I want a switch to power everything on/off as well.

Just about any DC fan can be used. The challenge is mounting it to your smoker. As far as probes - as long as it has the same resistance characteristics as a Polder probe it should work. 300K at room temp and 1K at 350 degrees (I think) You'll need a ohmmeter and a heat source to measure it. As long as you short R2 it seems any probe whose resistance goes down with heat applied to it should work. I figure the Polder probe is pretty cheap at $10 and chances are you'll need a replacement at sometime so why not get one?

If this becomes something more then a few people want I could probably convince one of those kit manufacturing companies to etch out a PC board and provide the entire project as kit. I don't expect huge demand though.

-rob
 
Rob, your circuit looks prettygood. But I worry if it is prone to oscillation. With a heat gun you are simulating large temp swings where that is not an issue. But in real life, the temp may vary slightly from moment to moment, causing the fan to click on and off (which may shorten its life).

To test, set it to ambient temp so that the fan is just off. Then slightly blow warm air on it with your mouth. See if it cleanly turns on and off rather than stuttering.

I would think a simple solution is a cap across the thermistor. That should provide enough hysteresis to stop the oscillations.
 
Thanks - you are right about that Amir! When i built the next to final circuit I had the output of the comparator triggering transistor and the coil of a relay. When the transition from on to off was made you could hear the relay "buzz". Knowing this would greatly reduce the life of the relay I went with the MOSFET instead.

With the heat gun steady on the probe at 225 if I blow on it you can see the LED blink a few times, the fan jerk a bit then turn ON. This is the oscillation you are talking about. I did read about adding a cap on the front end to reduce "chatter". What size cap do you suggest? The larger the more time it would take to charge but it seems it would also stay in that chatter zone longer as well.

-rob
 
Well, the math is fairly simple. You need to compute the "RC Time Constant." This is the simple multiplication of the resistance by capacitance. That would give you how long it takes for the cap to charge to its final voltage to trigger the comparator.

I don't know the resistance of the thermistor. And even if I did, it would be hard to know without experimentation what the time constant should be. For now, if you can measure the resistance of the thermistor, then maybe a 1 second time constant would be good for now. That means that at most, the fan would tun on or off once a second.

Being lazy though
icon_smile.gif
, I would simply start with a 1 microfarad cap. If the response time is too slow, then make it smaller. And vice versa.

It may also be useful to experiment with much longer delay. Stoker response time is measured in many seconds...
 
Thanks so much. The RC time response is bringing back memories from years ago. I will experiment.

For me it's a blast to try my hand at doing a hardware experiment that I know can easily be solved with a simple micro controller and software. I looked at it as a challenge I would do in 2009 and I appreciate your help. The ultimate goal is making it super cheap for the masses. It's all in fun ;-}

-rob
 
OK, I'll try and make the assembly of this somewhat more clear. I wanted to mount this in a project box and have all the devices plug in with quick connects so I went out and got:

1 project box
2 DC Female connectors (1 for fan and 1 for DC brick)
1 3/32 mono female connector (didn't quite work)

Additionally I wanted to use a IC socket so as to not destroy the IC with heat from the solder gun. I also wanted a switch to turn the thing on/off and a power ON LED as well as the fan ON LED. I used a nice little LED mount for the face.

Here is a pic of most of the parts I used:

Scats1.jpg


Next I needed to come up with a pysical layout of how the parts should be mounted on the circuit board. Here is how I mounted the parts and a block diagram of how it's connected. the diagram does not include the ON/OFF switch or LED.

Scats2.jpg


Here I've mounted the DC connectors, switch and LEDs. At this point it would be a good idea to test so I connected everything up with alligator clips, fired up the heat gun and tested. Everything worked!

Scats3.jpg


Here I've connected all the connectors up, except the probe. The only thing not mounted is the POT.

Scats4.jpg


Here is the final assembled device turned on. I was not able to locate a female 3/32 mono plug for the probe so I connected it with alligator clips for now. It's shown here with a Guru fan which just plugs into the DC female adapter. I have to work on a mount for the PC CPU fan still. The DC brick is an old Compaq laptop brick that puts out 18 volts. I put a blue knob on the 10K pot.


Scats5.jpg


Now the only thing left is to fire it up on a smoker!

More on that later.

-rob
 
I fired it up last Saturday on the WSM using the pit viper blower. I did a 6 hour smoke on 2 slabs of ribs I trimmed to St. Louis cut. I got the pit to 240 then adjusted the pot to the point where the FAN ON led was dim but the blower was off. From there the fan toggled on/off and kept steady within 5 degrees. When I went to foil the ribs at the 3 hour mark I turned it off, foiled, brought it back to 240 and turned the circuit back on. Again it held temps fine. I turned off the circuit at the 5 hour mark to unfoil and glaze then turned it on again. It held for 30 mins then the fan slammed on and it lost temp -- ran out of fuel. No problem - everything was done.

To be honest I was bottling homebrew in the background and didn't keep an eye on it that much. At some point I'll rig up the Stoker on it and get some charts for show and tell.

A 10CFM Pit Viper running at 18VDC on a WSM is WAY TOO MUCH but I didn't have the time to make a mount for my little CPU fan. A less powerful fan would work much better. I still want to test my BGE with this so I think my next step it making a mount for that.

-rob
 
Very promising results.

I will be curious to see how well a traditional propeller type fan works. I have the impression that they do not deal with back-pressure nearly as well as the centrifugal blowers used on both guru and stoker.

The reason the 10CFM Pit Viper is too much is that you are using a higher voltage on the project than the guru would provide?

You might drop in a 12V 7812 style regulator to see if it would provide more appropriate output levels. Driving the reg input from your current output should work.

Have you measured the current drawn by the fan in operation?

Again, good job.
 
RobM,

Great work !!!! I notice that your schematic in your first post does not match the one in your pictures. Looks like you've updated it. Can you post the updated schematic and parts list ?

Thanx -

Alec
 
I've edited the head of the post. All I added was a power on switch and an LED to it for the most part.

When it get warmer out (in Vermont) I'll do some charting on it.

-rob
 
Any news or updates on this?

I just found it, and as i sit here with six (6!) butts on the WSM for tomorrow, I am wishing I had some temperature control.

For ~$10, this controller would work for me. Heck, i could get most of the stuff off the shelf at work.

The only think that kind of concerns me is attaching the blower to the WSM.

I would love to help with the circuit, but my circuits aren't my strong suit.
 
The blower that I used was from the BBQ Guru but you can find any 12 volt system pretty cheap. There is a thread on here about mounting Stokers and Gurus to WSM's. I personally use Guru mounts on the WSM but some have adopted the Stoker type system which basically uses a stainless dog bowl mounted on one of the air intakes. The thread for this is here:

http://tvwbb.infopop.cc/eve/fo...72103/m/34210204/p/1

This simple system works great on a WSM or BGE. I've never tested it on larger smokers though. I keep it as a back up unit that I can snap on a car battery if the power goes out.

-rob
 
Hi Rob, I'm fairly new this and I am interested in a cheap diy atc. I see that you put quite a bit of work into the atc. I hate to bring back an posting but none of the pictures are active. I was wondering if you could provide them to me(assuming you still have them) so I can take a shot at it. I've seen your name in a lot of postings, thank you for all of the assistance you have provided us newbies.
 
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