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Need help developing circuit - will pay for help

Discussion in 'Electronic Design' started by ryanm, Jun 10, 2004.

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  1. ryanm

    ryanm Guest

    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 ( 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.

    ryanmAThorsefishDOTnet (do the obvious to email me)
  2. Rich Grise

    Rich Grise Guest

    I'm wondering how you get bipolar outputs from PWM.

    If you already have PWM, just use a switch. If you're controlling AC,
    you'd have to put your switch inside a bridge, if it's just a transistor
    or FET. Or, if you're programming at the level where you can trigger a
    PWM on the power line, then just fire a Triac at the appropriate time;
    that's all that dimmers do.

    If you really simply have a control voltage, then somebody else will have
    to field this one, but I'm sure there's something there.

    If you're a programmer and USENET denizen, you should know by now how
    to operate Just put in as many keywords as you
    want, like: "voltage-controlled dimmer":"voltage-controlled+dimmer"&btnG=Google+Search
    with quotes, and

    Have Fun!
  3. On Thu, 10 Jun 2004 01:00:21 -0500, the renowned "ryanm"

    Can you run the PWM with a very slow cycle, perhaps a couple of
    seconds or less?

    If so, then you can just use a zero-voltage-switching SSR to control
    the heaters.

    There are lots of things that can go wrong with control systems and I
    suggest you add an INDEPENDENT and SIMPLE (viz mechanical)
    high-temperature cutout and ALARM set to a temperature that may not be
    healthy long-term for the animals, but will not hurt them for the time
    it might take you to discover the alarm and fix the problem. I suppose
    the same might be necessary for low temperature. Also use a modest
    amount of heating so that the normal ratio is perhaps 80% on and 20%
    off (we typically use 67% in industrial systems). SSRs generally fail

    Best regards,
    Spehro Pefhany
  4. Rob

    Rob Guest

    Look at kit # K8003 from Velleman. Has 0-10v control voltage so all
    you need to do is level shift your -5 to +5v voltage source.
    If you don't have any luck, come back to me and I'll have a go at
    designing something.
  5. John Fields

    John Fields Guest

    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

    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

    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.
  6. ryanm

    ryanm Guest

    It's possible that I misread the specs on the PWM output, it may run
    from 0 to 5 or 0 to 10 instead for the PWMs. Right now this is all on paper,
    I'm just trying to make sure I can come up with all of the parts before I
    start trying to build it.
    The PWM is only capable of low voltage DC, and the heat source is
    110v/20a AC. I believe that all I have is a control voltage. Of course I
    could build the whole system from scratch to whatever specs I wanted, but
    that would be decidedly outside of my understanding of electronics. The
    premade circuits I can buy take care of the majority of the system,
    including the computer interface which, to me, seems like it would be rather
    difficult to build on my own, so I'll use their parts unless it's simply
    impossible to do what I want with them.
    I did search for "electronically controlled dimmer, but all I got was a
    bunch of house light switches, which aren't what I need. I'll keep
    searching, of course, but I thought this looked like a good place to ask.

  7. ryanm

    ryanm Guest

    What I got from the specs is that I can tell the PWM output to put out
    anywhere from 0v-5v, and what it does is vary the point in the cycle that it
    switches on and off to achieve variable line voltage. It is essentially a
    dimmer circuit in itself, the problem is that it's low voltage DC, and I
    need to control a high voltage AC device.
    So, for example, I could use an AB 700SE SSR (,
    which has a 5v control voltage and can handle switching a 110v/20a load?
    Will that essentially do what I am asking for, which is let me use a 5v
    control voltage to adjust the voltage going to the heat source? I'm not
    familiar with how a solid state relay works, so if I'm way off, tell me.
    I appreciate the concern. My intention was always to overbuild for
    safety. The heat sources would actually run at about 30w-40w, but I want to
    build the circuit to handle 200w so that the heat is negligible. The heat
    sources, which are radiant heat panels specifically designed for reptile
    enclosures, actually have safety measures built in and will not heat to
    dangerous levels, even with inappropriate power applied to them. The heating
    element is completely embedded in a self-extinguishing polypropylene, and is
    UL listed, so the fire hazard from the heat source itself is minimal.

    Of course I planned to burn in this system for several weeks to a month
    on an empty enclosure before I put it anywhere near my animals. The software
    would be logging the whole time so that I could find fluctuations or other
    problems before I use the system on an occupied enclosure. The safety of my
    household and my animals is top priority, of course, and this is just a
    personal project. Like I said in the first post, there is nothing wrong with
    the way I'm doing it now, I just think it would be cool to be able to
    automate it. Thanks for the help, and any further help is appreciated.

  8. ryanm

    ryanm Guest

    This looks to be *exactly* what I need (and it's only $25). Thanks.

  9. nospam

    nospam Guest

    What is the thermal time constant of these heat sources - a few seconds?
    tens of seconds? minutes?

    If it is more than a few seconds just use a simple solid state relay module
    and do your own PWM in software. Even a PC running windoze should manage
    100ms resolution of the switching point which in a 10 second time frame
    would give 1% control resolution. Reliability may be a concern but is your
    USB voltage generator much safer?

    With an additional time delay relay you could probably incorporate a
    hardware watchdog which forces the heaters off should the PWM signal stop

    Lighting if it is mains powered is a more difficult problem. If it is or
    can be dc powered then a voltage controlled dimmer is not so difficult.
  10. Sorry, it wouldn't work very well- this kind of SSR should be fed only
    5V for "on" and 0V for "off".

    It's possible to adapt the PWM output to an SSR with an LM324 and a
    handful of R's and C's. Maybe someone feels like draing it up, or
    perhaps you should take JF up on his offer.

    Best regards,
    Spehro Pefhany
  11. N. Thornton

    N. Thornton Guest

    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
  12. John Fields

    John Fields Guest

    Interesting... a pronouncement of knowingingness followed by an
    admonition of sorts, but with nothing substantive offered as proof.
    An opinion followed by another opinion seemingly introduced to
    validate the first as fact...
  13. Rich Grise

    Rich Grise Guest

    I'd give my eye teeth to see that on a scope. This description sounds
    tantalizingly like a signal that you could just connect to the gate of
    a triac through about a 220 ohm resistor, and have phase control already.

    Is that possible? (to get a screenshot)? Or you could even get a small
    speaker, or a headphones, and listen to it through about a 100-470
    resistor and maybe a small cap. You'll be able to hear if it's pwm,
    and at what freq, or if it's 60 hz.

    Good Luck!
  14. Rich Grise

    Rich Grise Guest

    Don't worry - If I can get this one question answered, and it's the answer
    I think it is, I'll design the circuit for you for free, and you can have
    John Fields write up the contract for going into production or whatever.

    What, exactly, is the signal we're dealing with here? Sometimes it
    sounds like a 5V PWM, that merely _averages_ to 0-5V, sometimes it
    sounds like a plain DC level - which of these is it? You keep referring
    to "the PWM output." This is such a tease - it makes it sound like you
    don't have to design a freaking thing! If it is a 5V PWM pulse, and
    happens to start at the zero-cross of the line freq, then just use
    it to gate on a transistor or MOSFET that's wired directly between
    the DC terminals of a bridge, and the AC terminals go in series with
    the heater. Well, it might have to be optoisolated. But it's incredibly
    simple to do, if you really have a "PWM output."

    Please, if only to humor me, see if you can find out what the exact
    signal is that we are dealing with here. :)

  15. ryanm

    ryanm Guest

    From what I've been able to glean from their documentation, it is really
    a PWM output. They say that it is achieved by turning a FET switch on and
    off "many times per second", but they don't specify if it starts at the
    zero-cross or not. So, from what I understand it is a 5v PWM output.

    You can check out their documentaton if you want, in case I missed
    something, but I don't think they would be calling it a PWM output if it


  16. ryanm

    ryanm Guest

    So everyone knows, Rob pointed out a Velleman kit that will do pretty
    much exactly what I want (K8003 or the newer K8064), and it's under $25. I
    sent a link to it back to the company that makes those other prefab circits
    and they said they might just go ahead and make a few cheap for me, already
    tested and matched to the other components I was planning to buy.

    Thanks, everyone, for the help. While I realize that you guys don't need
    these kinds of things, I think the stuff available at
    is pretty cool and opens up some hardware to those of us who are usually
    software-only types. There are other companies producing kits (like
    Velleman) but these guys are really less about cookie-cutter projects than
    they are about offering a bunch of parts that can be easily networked and
    connected to a computer for control. Their stuff is mostly used for
    interactive art projects and stuff like that, but it seemed to be a good fit
    for what I'm trying to do. Thanks again.

  17. Paul Burke

    Paul Burke Guest

    Ah yes, I remember a colleague who "fixed" his new girlfriend's tropical
    fish tank thermostat- the relationship finished the following morning,
    she didn't like fish soup for breakfast.

    Paul Burke
  18. N. Thornton

    N. Thornton Guest

    hmm, hopefully youll get out of bed the other side tomorrow :)

    Regards, NT
  19. N. Thornton

    N. Thornton Guest

    no mains isolation.

    Regards, NT
  20. John Fields

    John Fields Guest

    Geez, Rich, when did you you sign on as our business manager???

    Actually, since I didn't get the design job :-( and I've still got
    the circuit in my head, :) I'll go ahead and post the design to abse
    over the weekend on the chance that it might help somebody out,
    altruistic guy that I am!^)

    How about if we turn it into a little contest and you post your
    circuit too? Just for grins, let's blow off mains isolation...
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