Simple 2 minute motor timer - SUCH A THING EXIST???

Discussion in 'The Projects Forum' started by sparkism13, May 8, 2011.

  1. sparkism13

    Thread Starter New Member

    May 8, 2011
    7
    0
    Hi, all. I'm new to this forum and I'm hoping someone can point me in the right direction. I'm looking for some type of circuit or chip that will power on a small motor w/fan attached for a certain amount of time (2min) and then shut off.
    The motor will be powered by 2 standard AA alkaline batteries or 2 AA NiMH rechargeables. It will likely be a brushless DC motor or possibly standard brushed motor. I found a couple of 3v motors already but I'm looking for the logic that will kick it on and turn it off after a couple of minutes. Does such a "kit" exist or would I need to build my own circuit? I'm a complete novice in circuitry so hopefully a simple chip or circuit already exists. I can do simple soldering but would like something fairly complete. I will search the site but any suggestions would GREATLY appreciated. Thank you!!
     
    Last edited: May 8, 2011
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    A simple 555 will do that. How much current will this motor need? I assume you want a push button, but there are plenty of other options. Basically what you want is pretty simple.

    [​IMG]

    This is a derivative of the following project, 555 Monostable. This schematic will need modified somewhat, I just pulled this schematic from my library.

    There is one fly in the ointment, you will need 6V. This shouldn't be a major issue, just 2 more AA cells.

    You could build a similar circuit that uses 3V, but the parts will be a lot harder to get.
     
    Last edited: May 8, 2011
  3. sparkism13

    Thread Starter New Member

    May 8, 2011
    7
    0
    Bill, thank you. I've been on this forum for about 3 hours now and I think I'm hooked! I've already discovered your 555 projects but since I'm so new to circuitry, it's going to take some time to digest it all.

    Anyhow, I don't know how much current the motor will draw but I'm assuming no more than 100-150mah (this will be a tiny fan). As far as on/off, I'm looking for a pressure activated option. When I set an object on top of my project, it will kick on the fan and turn off automatically after 1 or 2 minutes. It won't activate again until I lift off the object and place it back down. I want to get to that point and perhaps get "fancier" from there. If it's easier, a push button option would also be ok with an LED indicator that the fan is on for the time specified and then off when finished. I prefer the pressure system but whichever is easier for this novice would be great.

    One thing I don't understand is how to set the 555 timer to achieve different time lengths. Is that DIP switch controlled? Also, where is can I read a good primer on how to read a schematic diagram? Right now it looks like Greek to me. Thanks!
     
  4. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    Rt and Ct are what you change to change the timing. It can be fatal to set Rt too small, but long time constants require high resistances, so that will not be a problem for you.

    I believe Bill just handed you the right circuit for a push-button started, one shot timer. The time is given in his drawing as 1.1 Rt Ct. For instance, 1 million ohms times 10 microfarads times 1.1 = 11 seconds. To get 120 seconds, you will need to increase the capacitor to 109 microfarads. There is no such a capacitor, so you will probably use a 120 microfarad capacitor, check the timing, and adjust the resistor a little bit.

    ps, there are several versions of the 555 chip. The newer ones are better about working at low voltages, carrying higher load current, and accepting a wider range of timing components. I think they are CMOS. CD4538 and TS556 are dual versions (two 555's in one package).
     
  5. sparkism13

    Thread Starter New Member

    May 8, 2011
    7
    0
    Ok, I see. So bill originally stated that I may need to use 6v which would mean 4 batts. Do the CMOS 555 take 3v (2 AA batts)? Would the ones at radio shack suffice. I see they have the 556 timers as well but is there any benefit to using that one or just stick with the CMOS 555?
     
  6. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    The TS556 is rated at 2 to 18 volts. You must read the datasheet when considering any purchase. There are so many variations on this very popular chip that I can't possibly guess what you will find available. I am only naming the TS556 as an example because I happen to have a datasheet for it.

    ps, the IRF510 will not be happy with 2 volts. It needs a higher gate voltage to turn fully on. I expect that voltage is above the 3 volts you wish to use, so you will probably have to change to a bi-polar transistor.
     
    Last edited: May 9, 2011
  7. sparkism13

    Thread Starter New Member

    May 8, 2011
    7
    0
    I think (hope) I might be understanding this. Please tell me if I have listed below what I need.

    -CMOS 555 timer circuit

    -Resistor(s) and Capacitor(s) based on formula for time interval needed

    -core rosin solder?

    -power wires (thin red and black)

    -motor (3v or 6v)
    -perf board (?) to solder everything onto?

    Would anything change if I was to use a brushless motor? Please let me know what else I need to complete. Thanks a bunch.
     
  8. sparkism13

    Thread Starter New Member

    May 8, 2011
    7
    0
    Can I SAFELY assume that if a chips datasheet lists 3v in it's usable range then it should operate at 3v? Just wondering why Bill eluded to it being harder to find parts if I went with 3v.
     
  9. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    If the datasheet says it will work at 2 to 18 volts, that is a form of guarantee that it will safely and dependably work at those voltages.

    It can be very difficult to find a MOSFET that will work at 3 volts.
     
  10. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    You can easily get a CMOS 555. The parts I was thinking about is the transistors. I can design what you want, but any new design may have problems. It is pretty routine, so I'm not too worried.

    You'll note I use protoboards in my work, I would recommend something similar for your project. They are easy to modify and try new things out on. When you have something that works you move it over to a solder type board, and can reuse the protoboard for something else.

    Most of the parts are available from Radio Shack. If you don't live in the United States this may mean nothing, if you do it does. It is the reason most of us put our location in our profile, which is displayed on our posts.

    A CMOS chip will go down to 2V, but it has very little drive. It has so little drive that transistors have problems picking up on it. I'll sketch something up that I think will work and post it in a bit.
     
  11. sparkism13

    Thread Starter New Member

    May 8, 2011
    7
    0
    Wow! I cant beleive how helpful you guys have been already. This is a very neat forum. I live in Georgia in the good ole' USA! I have several Shacks around me so I'll likely be headed there tomorrow. Hopefully they'll have all the components I need. Thanks again.
     
  12. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    [​IMG]

    7555 - TLC555 (Radio Shack equivalent)
    Q1 - NPN (2N2222A or equivalent)
    Q2 - PNP (2N2907A or equivalent)
    CR1 - 1N4001

    For 2 minutes Ct = 470µF, Rt = 240KΩ

    I put the motor max current at 300ma.

    Vcc = 3VDC

    C2, C3, and C4 may be eliminated. They are included because motors tend to be noisy, which can interfere with the circuit, and the capacitors will compensate for. C4 may extend the life of the batteries.
     
    Last edited: May 9, 2011
  13. sparkism13

    Thread Starter New Member

    May 8, 2011
    7
    0
    Thanks again for the drawing, Bill. I spoke with an engineer today about my project and he mentioned that with the low voltage I want to use (3v) the mAh output would be so little that I would need to build an amplifier of some sort to run the motor because I would not be able it run it directly. I don't want to add complexity but how accurate does that sound? Unfortunately, using 2 AA batteries is becoming a requirement now so I will need to do what I have to do to make that work. Thoughts?
     
  14. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    Bill just handed you the circuit. What you have to do to make it work is...build it!
     
  15. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I am assuming you are using a 3V motor. That is a given.

    Of course, I also recommended 6V for the power supply first. Most computer fans use 5V or 12VDC.

    I know 3V motors exist though, getting one is your job. :D
     
Loading...