Help with a vaiable (3-5V) relay circuit

Discussion in 'The Projects Forum' started by alor, Jul 29, 2012.

  1. alor

    Thread Starter New Member

    Jul 29, 2012

    I need to build a circuit to trip a 12V load on a signal from one of two sources.
    One source is a +5V pulse that can trip a 5V relay on its own but the other source is the output from a IC rated at +3V 20mA (Max).

    I have googled this for a week now and found that I need to use a NPN transistor to boost the IC output so it can trip the relay.

    My problem is that I know next to nothing about transistors and the math behind them. And I have yet to find a tutorial I can grasp.

    One way to build this is to have one input for the 5V source (connected directly to the relay) and another input for the 3V source (connected to the transistor).
    Is it possible to build the circuit to have one input that can handle both sources? This to prevent users from plugging in the 5V source to the 3V input and possible fry the circuit.

    Can some kind soul help me with a schematic and what components to use?
    Thanks in advance.

    Last edited: Jul 29, 2012
  2. Editor1979

    New Member

    Feb 18, 2013
    Hello alor,
    I am including 3 image links that show you how to use a 5-volt (USB) Microprocessor to activate a 12 volt relay which can switch on a 120V light.

    I wrote this example using an Arduino but any microprocessor with a 5-volt output will work. Honestly, once the relay is active, you can make it switch on or off anything you want as long as it's within the tolerance of the relay's specifications. If you are using an Arduino, then this circuit will work using the "Blink" program in Arduino's Example Library.

    I am using this exact circuit (Albeit incredibly more complex) to control the charging and firing of three different 4KVDC impulse generators.

    One is a ring launcher
    One is a thunder generator
    And one crushes quarters to the size of a dime.

    If you need any help with any other circuit problems, education, or help with ideas on any project, feel free to contact me or thousands of other people who are willing to help in The IRC Chatroom at The Geek Group!

    Joining is free!
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