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555 timer help

  1. Apr 18, 2006 #1
    Hi everybody,

    I have set up a 555 monostable timer to run a solenoid for 0.05 seconds - very rapid on then off, moving the solenoid about 3 mm. I have found that it is not producing quite enough force for what I need and was wondering if I put 6 vdc solenoid in place of the 12vdc solenoid (12vdc circuit) this would double the force??

    Is the LM555 timer able to momenatrily produce an output of 500mA for the 6vdc solenoid?

    Any help would be greatly appreciated
     
  2. jcsd
  3. Apr 18, 2006 #2

    berkeman

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    Staff: Mentor

    Check the specs on the datasheet for the 555 -- I'm fairly certain that you need to add some kind of driver component to drive your coil. And the lower the voltage rating of the coil, the higher the requred current. You should be able to just use a 2N3904 or 2N2222 NPN transistor as the pulldown buffer element between the 555 and the coil. Be sure to put in the traditional catch diode (1N4148 or equivalent) across the coil, or you'll blow up the transistor.

    EDIT -- like this: http://www.naisweb.com/e/relaye/mech_eng/mech_eng_rti/idaevbx.html
     
  4. Apr 19, 2006 #3
    Thanks berkeman, I'll give it a go and let you know how i get on
     
  5. Apr 19, 2006 #4
    Just wondering would i need to put a 1k resistor in front of the transistor?
     
  6. Apr 20, 2006 #5

    berkeman

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    Staff: Mentor

    When you drive the base of an NPN transistor in that configuration, you are basically driving a current in through the base-emitter junction in order to turn on a much higher current that flows from collector-to-emitter. The B-E junction looks like a diode, so you will only get about 0.7V across it when you drive it on. In order to limit the current that you drive in, yes, you will usually put a series resistor between your drive and the base. The size of the resistor is chosen to be sure that the transistor is driven hard enough to saturate, or at least to support whatever C-E current you need. So if I knew that the current gain of the transistor (Beta) was about 100 at the currents where I was going to be running it, and if I wanted 100mA of C-E current, then I would size the base resistor to be able to drive 1mA into the B-E junction. If my drive source was 5V, then the resistor would be about (5V - 0.7V) / 1mA = 4.3kOhm. If you're doing a more careful sizing of the resistor, you'd take temperature and tolerances and other stuff into account. As a practical matter, a little overdrive on the base current doesn't hurt anything, at least not unless the switching frequency (transistor recovery from saturation) is important, like in DC-DC converter design.

    BTW, check out the book "The Art of Electronics" by Horowitz and Hill. It's a great basic electronics book, with lots of practical examples. Read it cover-to-cover, and you will have a very good understanding of circuits like the one you are asking about.
     
  7. Apr 20, 2006 #6
    thats fantastic, thanks again for your excellent advice.
     
  8. May 19, 2006 #7
    i remember using a 555 timer IC to program the current flow to a led and a buzzer. I used it in a circuit constructed to be fit in a door of a fridge, so that if the fridge is left open for about 30 seconds or more the led and buzzer comes on and when the door is shut both the led and the buzzer stops.

    -jake
     
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