Re: Need help developing circuit - will pay for help

From: N. Thornton (bigcat_at_meeow.co.uk)
Date: 06/10/04 

Date: 10 Jun 2004 15:24:55 -0700

"ryanm" <ryanm@fatchicksinpartyhats.com> wrote in message news:<10cfu5ihpebkc40@corp.supernews.com>...
> Ok, to spare you the boring details, I keep and breed reptiles. At
> present, the heat, light, and humidity for each of my numerous enclosures
> are controlled manually and monitored using old-fashioned, analog gauges.
> There is nothing wrong with the system I have, except that it's a lot of
> work. What I would like to do is develop a set of computer controls to
> automate much of this. I've have the majority of it worked out, thanks to a
> small company I found that sells prefabricated, chainable circuits
> (including USB interfaces) that all contain a CPU with an easy to use API
> for all the major programming languages (www.makingthings.com). They take
> care of the vast majority of the parts I need to build the system I want,
> but there is one part that they don't make (and they want an arm-and-a-leg
> to make it for me), and I need to figure out how to build it.
>
> Basically what I need is an electronically controlled dimmer circuit to
> make a proportional thermostat. It is essentially a standard light dimmer
> circuit, but instead of a pot to adjust the voltage sent to the heat source,
> there needs to be some circuit that returns a resistance directly
> proportional to the input voltage. In case I said that wrong, let me say it
> like a 4 year old: I have this other circuit with a PWM output that is
> capable of sending from -5v to +5v in 1024 discrete steps, and what I want
> to do is make that output control a rheostat (dimmer) that my enclosure's
> heat source will be connected to.
>
> I can follow a wiring diagram and even a schematic if it's simple. The
> reason I'm using these prefabricated circuits is because I am, obviously,
> not an EE and have no training in electronics. Everything I know about
> electronics I learned by jacking around in the back of old guitar amps,
> which contained no ICs or PCBs (I can also only work on cars that have a
> carburetor). I'm a software developer by trade, so these prebuilt circuits
> make it simple for me to wire a temp sensor to an input on the little module
> with a CPU on it, plug the USB port into my computer, and then use the
> provided API to sample it at pretty much whatever rate I want. So all that's
> left is to be able to adjust the voltage going to a 110v/20a/<200w heat
> source. There are a number of reasons why I don't simply put a relay on the
> heat source (like the HVAC in your house), but they would require
> long-winded descriptions of reptile husbandry requirements that I'm sure you
> don't care about. There are also a number of good reasons why I don't simply
> buy a proportional thermostat that is intended for reptile enclosures, not
> the least of which is that I also want to control light and humidity, and I
> want to have logging of average temps and humidity, alerts in case temps
> rise or drop to dangerous levels, etc.
>
> I would be willing to pay (a reasonable price) if someone could provide
> me with a schematic and a parts list (and a wiring diagram if possible), but
> a schematic I paid for I would expect to be over-engineered for fire safety,
> etc (since I'm only building a few of these, I can afford to over-engineer
> it to ensure that it doesn't start any fires or kill my animals). I feel
> pretty confident that this is a simple circuit, since it's effectively a
> modified light dimmer, and that it should be about $5 worth of parts and
> less than an hour to solder together. I may be wrong about that, because
> like I said, I don't know exactly what's involved, but I'm sure there's an
> IC that can replace the pot in a standard light dimmer circuit so that I can
> adjust the resistance by varying the input voltage, I just don't have any
> idea how to find out what, if any, IC would do the job.
>
> Any help, pointers to urls, or sanity checks are appreciated. Let me
> know if you need more detail or if I left out anything major. Oh, and the
> standard disclaimer for these groups: I'm not a student trying to get you to
> do my homework, I'm not trying to get someone else to do my job, this is a
> hobby project and I have hit the limits of my experience. I myself hang out
> in programming groups and email lists and play the geek with all the answers
> for the newbs, so I figured there must be someone similar in here who can
> either point me in the right direction or who would be willing to do it for
> a reasonable price. Thanks.
>
> ryanm
> ryanmAThorsefishDOTnet (do the obvious to email me)

This isnt so hard to do, although I wouldnt tackle it in precisely the
same way you suggest. These online offers dont usually work out
though, usually theyre just carrots for ideas and discussion. You seem
serious, but:
What are you willing to offer?
How is it going to be set up so you do pay but dont get bitten?
If you can set up one of these offers so it works, well done. These
offers tend not to get taken up by pros because of these problems.

BTW Any potential designer will need to know whether youre on 120v or
240v. What type of lighting you use may also be relevant.

Regards, NT

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