Re: Need help developing circuit - will pay for help

From: John Fields (jfields_at_austininstruments.com)
Date: 06/10/04 

Date: Thu, 10 Jun 2004 12:41:10 -0500

On Thu, 10 Jun 2004 01:00:21 -0500, "ryanm"
<ryanm@fatchicksinpartyhats.com> wrote:

> 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. 

---
It sounds like your +/- 5V control signal isn't PWM at all, but a
steady, controllable variable voltage coming out of a 10bit DAC,
resulting in the 1024 ~ 9.8mV steps.
IMO, what you'll need to do is to shift that output voltage so it goes
from, say, 0V to 10V and then use that voltage to control the firing
angle of a TRIAC.  Since you have 1024 discrete steps available, and
you'll want to turn the TRIAC on somewhere between 0° and 180° in each
mains half-cycle, with 1024 steps you ought to be able to get
180°/1024 ~ 0.2° resolution, which is ridiculously fine, but it'll
work.  Basically, what you'll want to do is set it up so that 0V = no
output pulse to the TRIAC gate, 9.8mV = an output pulse at a little
before 180° and 10V = an output pulse just after the mains
zero-crossing. 
So, what you'll need is a zero-crossing detector and a pulse generator
with a voltage-variable delay which will put the TRIAC gate-driving
pulse wherever you want it between 0° and 180° after the zero
crossing.
If you like, I can design something for you and supply you with
documentation and a working unit on perfboard which you can copy to
get as many units as you want.  
I'm pretty sure you're right about the $5 worth of parts, and if
you're interested, email me with what you have in mind as a
"reasonable" fee and maybe we can work something out.
    
-- 
John Fields

Relevant Pages

  • Need help developing circuit - will pay for help
    ... circuit, but instead of a pot to adjust the voltage sent to the heat source, ... the least of which is that I also want to control light and humidity, ...
    (sci.electronics.design)
  • Re: how to get this PWM DC motor control to handle more power
    ... The LM324 Quad op-Amp limits Voltage to 32V ... I would think many many mosfets working together ... original circuit, but want it to handle more power. ... "Applied Control Theory for Embedded Systems" gives you just what it says. ...
    (sci.electronics.basics)
  • Re: MRC Ampack upgrade: Attn electronics froods
    ... control on many of my locomotives. ... pulse-power switch to cut out one diode. ... calculations are correct, the circuit should work. ... I'd put a ceramic capacitor across the voltage supply as close to the ...
    (rec.models.railroad)
  • Re: Need help with optoisolator theory, CTR, If ,???
    ... The voltage between the tool and work piece is ... the best CTR is going to occur when IF is around 10ma. ... I put together a test circuit to try to map the 0-170 VDC to the 0-12 ... Do you need to implement control loops in software? ...
    (sci.electronics.basics)
  • Re: My Stove
    ... >law, and trying to get a feeling for how current, voltage, and power ... >generate heat rapidly and heat the element. ... The resistance of the copper wire from the control ... and the voltage at low heat registered at 240 ...
    (sci.electronics.basics)