Connect with us

Need a super simple 3v DC timer circuit

Discussion in 'General Electronics Discussion' started by supak111, Jun 6, 2015.

Scroll to continue with content
  1. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    327
    4
    Apr 29, 2012
    Hey everyone I'm running a small 3V DC motor 50ma on a regular on/off switch. I would like to replace a switch with a super simple timer circuit to stop the motor after 2-4 minutes so that I don't have to go back and turn it OFF every day. I forget sometimes and it kills my batteries. I would like this timer circuit to be super super simple. Device I'm controlling is cheap and nothing fancy so I don't care if the circuit is designed bad as long as it does the job and I don't have to waste 3 hours making the circuit.

    Would adding a NPN transistor with a large cap on the gate do the trick? If I add a momentary switch from battery to cap/gate, cap would fill up and when I let go of the momentary switch the transistor should conduct until the cap discharges right?

    How big of a cap would I need to run a 3v 50ma motor for 4 minutes? Is it even possible? I have bc337-25 transistors sitting around.

    PS. dc motor doesn't need to run at constant speed the whole 3 minutes, just as long as it runs for around that much time. I was looking to buy something but there is nothing out there that is small and cheap.
     
  2. davenn

    davenn Moderator

    13,648
    1,884
    Sep 5, 2009
    do some googling on 555 timer circuits :)
     
  3. Colin Mitchell

    Colin Mitchell

    1,417
    312
    Aug 31, 2014
    You will need 3 stages. Use BC338 in place of BD679 and motor goes in collector of BC338. Use 100u Remove diode and 1M and put switch in place of 1M. Remove globe.

    LightExtender.gif
     
    Last edited: Jun 6, 2015
  4. Martaine2005

    Martaine2005

    2,930
    799
    May 12, 2015
    Personally, I thought making something was all part of the fun. Not wasting time at all.
    I would definitely go the 555 timer route.
    There is a wealth of info out there on these very useful chips.
     
  5. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    327
    4
    Apr 29, 2012
    No I like making electronics, I just didn't want to invest too much time in this particular project. What the simplest way to make a time with a 555 chip
     
  6. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    327
    4
    Apr 29, 2012
    Wait 555 timers are 4.5V-18v, I only have 3v max
     
  7. Martaine2005

    Martaine2005

    2,930
    799
    May 12, 2015
    [​IMG]

    This example I found in 15 seconds on google. It's here..http://electronicsclub.info/p_timer.htm
    The buzzer can be replaced for your motor. Although a transistor might be required from pin 3 to drive the motor.

    But my point is, a little research and reading can find the the answers to just about anything.
    BTW, the above circuit is an adjustable timer circuit from 1 min to 10 minutes..
     
  8. Martaine2005

    Martaine2005

    2,930
    799
    May 12, 2015
    That's the input voltages they can be powered by.

    You said ,you are running a 3v 50ma motor. Not that you have fixed 3volts powering a motor.
     
  9. poor mystic

    poor mystic

    1,066
    31
    Apr 8, 2011
    Hi
    You can get CMOS 555's which run as low as 1.5V.
    Google "CMOS 555" and you'll see... the part number is LMC555
     
    Martaine2005 likes this.
  10. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

    327
    4
    Apr 29, 2012
    Oh cool. I will try to make something with LMC555 then.
     
  11. Colin Mitchell

    Colin Mitchell

    1,417
    312
    Aug 31, 2014
    You will need a buffer transistor as the chip will only deliver a few milliamp.
     
  12. poor mystic

    poor mystic

    1,066
    31
    Apr 8, 2011
  13. Colin Mitchell

    Colin Mitchell

    1,417
    312
    Aug 31, 2014
    A MOSFET needs at least 1.8v
     
  14. poor mystic

    poor mystic

    1,066
    31
    Apr 8, 2011
    :)
    OK, but that need not be subtracted from the supply voltage. The full supply voltage is still available to the load, with a MOSFET.
    With a bipolar transistor, 0.6V is taken up by the base-emitter junction, which reduces the voltage available o drive the load.

    Also, the statement that a MOSFET needs at least 1.8V isn't correct.
     
    Last edited: Jun 7, 2015
  15. Colin Mitchell

    Colin Mitchell

    1,417
    312
    Aug 31, 2014
    "With a bipolar transistor, 0.6V is taken up by the base-emitter junction, which reduces the voltage available o drive the load." What rubbish.
    The collector-emitter voltage can be as low as 0.3v to 0.5v and some ZTX transistors can be as low as 0.05v.
     
  16. Arouse1973

    Arouse1973 Adam

    5,164
    1,081
    Dec 18, 2013
    I think Poor Mystic was talking about a common collector buffer where approx. 0.6 Volts is lost due to the Vbe junction.
    Adam
     
  17. Arouse1973

    Arouse1973 Adam

    5,164
    1,081
    Dec 18, 2013
    It depends on the MOSFET. MOSFET turn on voltage is specified at generally 250 uA Ids. Some can have a turn on voltage as low as 0.2 Volts but you will only be able to draw 250 uA at this voltage. It's always best to look at the data sheet for the specific Vgs versus Ids of that MOSTFET.
    Thanks
    Adam
     
  18. Colin Mitchell

    Colin Mitchell

    1,417
    312
    Aug 31, 2014
    "I think Poor Mystic was talking about a common collector buffer where approx. 0.6 Volts is lost due to the Vbe junction."
    0.6v doesn't even apply to an emitter-follower because the LMC555 does not go rail-to-rail and you lose another 0.3v.
    You don't think of using an emitter-follower when you cannot afford to lose any voltage.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-