mixing and matching motors and relays for solar tracker

Discussion in 'General Electronics Chat' started by eddie6775, Mar 9, 2013.

  1. eddie6775

    Thread Starter New Member

    Jan 6, 2013
    17
    0
    Hello, everyone, I have a question about my solar tracker circuit. I finally got it to work as far as bench testing goes, i.e. I measure the voltages at each 741 out put and while the led I'm looking at is off it reads @.25 volts, and when it's on I read 3 volts. I chose to use 2n2222 transistors because those are the ones I successfully drove my relay with and I understand they trigger at@1.2 volts. The relays I'll be using are the RS275 005 as the package says it can handle up to 120 volts @12 amps but triggers at only 7 to 9 volts (forgive me if I typed that wrong I'm a newb.) What I'm wondering is: How to pick the appropriate motor. I found a nice 3 to 18 volt gear-head motor at electronics goldmine I was thinking of ordering, but I wonder if I can just go to the junk yard and get an old seat or window motor from a car.

    Hope I asked this question properly, I had NO idea there were so many variables!

    My goal here is to aim a parabolic trough at the sun and just how big I build it depends on this circuit and what it (mainly the relays)can handle.

    Oh....I'm totally open to suggestions as far as the circuit goes. And yes I realize a true comparator would be better but I haven't learned how to use it yet..:)
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,227
    I have several concerns:

    A 741 will not work reliably with a single 6V supply. Use a comparator such as LM339. The only difference between that and the op amp is that the LM339 requires a pull-up resistor from the output to the plus supply (1k to 10k typically). You also should add a small amount of positive feedback for hysteresis to avoid chattering of the output at the set point.

    The diode is series with the base of the transistor will prevent the transistor from turning on (what's the purpose of the diode?). Instead of the diode you should have a resistor in series with the transistor base to limit the current (that wouldn't be needed with the comparator).

    The turn-on voltage of a 2N2222 is about 0.7V, not 1.2V.

    Any reversible motor with the desired operating voltage and torque to move the panels is fine. If a car seat, windshield wiper, or window motor will work, then that may be an inexpensive way to go.
     
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Cant' help with the motor selection, but the diodes D3,D5 should be replaced with 1k current limiting resistors.
     
  4. eddie6775

    Thread Starter New Member

    Jan 6, 2013
    17
    0
    I'll have to do some reading about ?pull up resistors?. I've heard of that before, but positive feed back is a new thing to me. Stumped.

    I'll have to visit uncle Google for that one, lol.

    I'll try it. Now as for the zener diodes (sorry I didn't specify), my thinking was to stop any current until they reached there break down voltage. I used 3.3 thinking that the 2n2222 was supposed to turn on at 1.2 and now I'm aware that it's much less. The idea was to try to make an amp act like a comparator..Oops. Well, I do have a couple of 339s in my box I might as well start learning how to use them!

    I've gotten this far. :)

    Back to the drawing board, This could get interesting at some point this evening!
     
    Last edited: Mar 9, 2013
  5. eddie6775

    Thread Starter New Member

    Jan 6, 2013
    17
    0
  6. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,227
    This should help you understand comparators and hysteresis.

    Edit: Nevermind, I see your reference covers hysteresis also.
     
  7. eddie6775

    Thread Starter New Member

    Jan 6, 2013
    17
    0
    I went out to the garage last night, and set up an lm 339 on a breadboard to try it out. Now I get it...The pull up resistor is what completes the transistor circuit that exists at the out put of a comparator; Which makes sense now that I think about it, because it gives you more control over the i.c.. I spent some time changing resistor values and playing with different voltages last night just to get a feel for what was going on before moving to the next step of my tracker circuit. I'm now temped to get brave and try some bigger transistors and relays just to see what I can do! I was pleasantly surprised at how well this turned out. I read about the positive feed back as well but I didn't experiment with it yet. but I'm sure once I learn it I'll get exited about it too...lol

    Indecently, I gave the motor some more thought and decided to try for the more powerful automotive one as I don't need to mail order it if it burns out. Also I don't see any reason to limit my self in size options as far as a heat collector goes, after all I'm trying to cut down on oil usage. So now what I have to do is find a motor and build the circuit to match. I guess it's time to re do my schematic an post another picture..

    I read the link you sent me too and I think it's starting to click. hysteresis is what keeps my circuit from 'bouncing' on and off erratically?
     
    Last edited: Mar 10, 2013
  8. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,227
    Hysteresis is needed for most comparator applications because of unavoidable noise is any practical circuit. This noise will cause a high gain comparator (as IC comparators are) to rapidly switch on and off when the signal is near the voltage trip point. Thus an amount of hysteresis voltage larger than this noise prevents this oscillation.
     
  9. nickelflipper

    Active Member

    Jun 2, 2010
    280
    35
    I've used car seat motors, great form factor, but are not continuous rated. They can be accommodated by using a 555 timer with say 10-15 minutes off and 20-30 seconds on. But, at some point, the collector will need to be reset to East, and they will get really hot.
     
  10. eddie6775

    Thread Starter New Member

    Jan 6, 2013
    17
    0
    interesting....I've seen the 555timer used this way before but haven't studied it yet.
     
  11. eddie6775

    Thread Starter New Member

    Jan 6, 2013
    17
    0
    Hi crutschow, I haven't had a chance to experiment, but as I recall Saturday night as I was fooling around out in the shop the relay I was turning on and off did act funny at times when I was adjusting the pot that in my original circuit represented the LDRs.The relays sounded almost literally like teeth chattering... But only rite before my test LED turned on. Is this what your referring to? Forgive me, I'm away from home for work during the week so any thing I think of is only theory. I'll read those suggested articles again. I'm kinda slow ya know.
     
  12. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,227
    Yes, that sounds like oscillations at the trip point, which causes the relay to chatter. That's what the hysteresis will prevent.
     
  13. nickelflipper

    Active Member

    Jun 2, 2010
    280
    35
    Even hysteresis on the comparator may not be enough. I used a nfet off the comparator output to energize the coil relay, along with a resistor to match the relay coil rated current. Substituting a capacitor in parallel with the resistor got rid of the chattering. Something in the tens of uf range like a 47uf? should do it.
     
Loading...