Question about using transistor as switch

Discussion in 'The Projects Forum' started by gotchford, Sep 5, 2013.

  1. gotchford

    Thread Starter New Member

    Sep 5, 2013
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    Hello Everyone,

    This is my first post. I stumbled across All About Circuits while seeking information on various transistor types and how I could use them. Although I have some experience with TTL circuits, I'm more or less a novice when it comes to anything else, circuit-wise. Thus the reading I've been doing. Please pardon the newbie question and feel free to suggest any reading materials on the subject. Now to the point..

    I have a very simple circuit (attached). A battery supplies current through a protective resistor to a series of 5 LEDs. The diagram mistakenly shows 6. There are only 5.

    What I'd like to do is design an augmentation to this circuit that enables me to blink the current flowing through the LED circuit, essentially replacing S1. I recognize that any additional circuitry would require current, most likely imply more battery capacity, and perhaps modify the existing, simple circuitry.

    I've been reading about how Bi-Polar transistors can be used to control the flow of current in a larger circuit. I then found a flasher circuit by Forest Mimms that utilizes a pair of BP transistors buffered by a couple R-C arrangements to achieve current oscillation. I also found pages talking about using Unijunction transistors and MOSFETs as switches.

    I'm lost as to which is most appropriate. Perhaps somebody on the forum can point me in a direction. I'm not against experimentation and was on the hairy edge of just ordering some parts and playing around, but figured I'd ask for advice before wasting money and time.

    Thank you for any insight you can offer.

    -Chris
     
    Last edited: Sep 5, 2013
  2. adam555

    Active Member

    Aug 17, 2013
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  3. pujulde

    Member

    Jul 24, 2013
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    Don't you think about using NE555 or anything like this?
     
  4. wayneh

    Expert

    Sep 9, 2010
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    I think you should start with a single transistor and learn how it works before moving on to a "blinky" circuit.

    If you use a normal NPN transistor (BJT, bi-polar junction transistor), it would replace the switch. Emitter to battery "-". Collector to the resistor.

    To operate the transistor as a switch, you then apply a small current to the base pin. To fully switch on, you need to apply about 10% of the load current onto the base pin. For instance 2mA if 20mA is desired through the LEDs. You can do this by applying battery "+" via a resistor. Calculate the resistor value using ohm's law. The base will be ~0.7V higher than battery "-", although you can ignore this and use zero if the battery voltage is "large" compared to 0.7V.

    Special transistors such as a darlington or a MOSFET can allow using much smaller or essentially no current to operate the switch, and they are often used for this purpose.
     
  5. gotchford

    Thread Starter New Member

    Sep 5, 2013
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    Thank you folks.

    adam555, that circuit animation is really cool, although it shows the current flowing from + to - . What I like about it is how it shows the decay of current flow as the caps charge, as well as the simulated graph of the current flow at the bottom. If you created that, hats off to you. That's rad.

    I'd thought of using a 555 timer, pujulde, but how to wire it up (pin 3, I believe) to the base of a transistor will require more education and experimentation on my part.

    wayneh, thank you for suggesting a BJT. I'm going to start playing around. Got a breadboard and stuff on order. Fun times ahead. Thank you folks!

    As a software guy, I always envied hardware guys. There's so much creative potential in circuitry. Having an understanding of the relationships between components is gold. Cheers to you all.

    -Chris
     
    Last edited: Sep 6, 2013
  6. adam555

    Active Member

    Aug 17, 2013
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    No, I didn't create that; I'm also a beginner. I chose that website because it's really useful for beginners like us to understand what the circuit does at a simple glance. If you want a proper explanation to fully understand how it works, you can also find it here among other similar circuits that you can use for your project: http://www.talkingelectronics.com/projects/200TrCcts/200TrCcts.html#9B and http://www.talkingelectronics.com/projects/FlasherCircuits/Page83FlasherCircuitsP1.html

    Using a 555 is even easier -you'll need less components, and can easily find calculators to tune it, like this one: http://www.ohmslawcalculator.com/555_astable.php-. I just thought you preferred to use transistors instead of ICs.

    About the direction of the current: for me is easier to think about it in that way -from positive to negative- but it doesn't really matter.
     
    Last edited: Sep 6, 2013
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    If you want to learn about electronics, you've found the right place! Did you see the E-book at the top of every page? It's the blue header on every page. If your really wanting to learn electronics read it.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    That's a common convention. It's the flow of "positives" and I think it was established before it was known that electrons are flowing the opposite direction. The plus to minus voltage convention fits with the water flowing downhill analogy.
     
    Last edited: Sep 6, 2013
  9. MrChips

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  10. adam555

    Active Member

    Aug 17, 2013
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    I forgot to post you the index to all the circuits on that website, in which you can find the most basic ones: http://www.falstad.com/circuit/e-index.html

    Also, if you right-click on any example you will see that you can tamper with any circuit and its components; and even start your own simulations from scratch by adding the components yourself.
     
  11. #12

    Expert

    Nov 30, 2010
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    There is a lot to discuss in that.

    Hardware guys are limited by the laws of physics and finances. Just because you can think of it doesn't mean you can build it. Right now, there is a thread about building a cat finder that can SMS a GPS location. (All that for a cat makes my head feel wobbly.) Software people are limited by what the hardware people can build. Again, just because you can write a program doesn't mean a machine somewhere is going to do what you want. Knowing the physical limitations and interactions of the parts that are available would mean quite a lot to software people. When I start telling a programmer how to do a double invert to get the most out of the available voltage, they either glaze over or drool at the possibilities.

    With some programmable chips available for 50 cents right now, the possibilities are ahead of the programmers. Another way to say that is, micro-controllers are presently underutilized. It is going to take a few years before the software catches up with the hardware.

    I did the first semester of Basic and Fortran. Not enough to develop any serious skills, but enough to clue me in to how programming uses the hardware. That's enough for me to guess at the feasibility of any particular scheme. If the programmers knew about transistors, they would have that skill, too.

    So...welcome aboard. You are about to learn skills that other programmers only wish they had.
     
  12. gotchford

    Thread Starter New Member

    Sep 5, 2013
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    Shortbus, thank you, no I hadn't checked that stuff out but now that I have I see immediately that there's a wealth of information here. I'm all over it. To WayneH's point, there's a part that talks about "conventional" flow notation versus actual. This documentation will be very helpful, and eliminate some confusion I've had, I'm confident.

    MrChips that link you sent is fantastic. So much great reading and information. I'm really looking forward to discovering and learning.

    Thank you, #12 (btw is that your number in "The Village"? (Prisoner reference)), and you all, for your links, and words of encouragement. I hope to be able to reciprocate the insight to other members some day.

    -Chris
     
  13. #12

    Expert

    Nov 30, 2010
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    #12 is my QC stamp number. You know, "Inspected by #12"?
     
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