Frequency switch to open relay help

Discussion in 'General Electronics Chat' started by DM4N, Apr 17, 2014.

  1. DM4N

    Thread Starter New Member

    Apr 17, 2014
    5
    0
    Hi everyone I stumbled across this forum and i can't believe i have never had this forum in my favourite before! Would have been so handy in earlier projects haha anyway this is my first post.

    Just thought id ask the quick question about a frequency switch circuit.

    I managed to pick up a "Shift light" that connects to a tacho signal wire and when the rpm reaches a certain level the led comes on.

    I was going to make this circuit myself but without the use of the led and use a transistor to open a relay.

    I can buy the shift light circuit made for $30 from aus or $18 from china so i don't think that part is worth me making.

    Here is the info and on the items and my question:
    The led that is on the circuit at the moment has 3v powering it when the desired rpm is reached. That bit is fine but off this led i want to control a transistor to then open the 12v relay.

    The question i am having is what transistor do i need specifically i'm assuming an NPN would be most applicable for what i'm doing then just have the 12v +ve to the relay -ve from relay to the collector then from emitter to the 12v -ve

    If i am wrong please do let me know.
    Also will the transistors base need a +ve supply i'm guessing? so the +Ve on the led to a resistor of some kind then to the base on the transistor?

    Any help would be greatly appreciated cheers :)
     
  2. pwdixon

    Member

    Oct 11, 2012
    488
    56
    Sounds like you want to connect a transistor to the LED and want to switch a relay from that.

    It is possible that the LED is not a DC drive so it may not be directly useful with a relay.

    Also the connection to the LED could be driven either relative to the positive or negative rail and might mean you need an invertor to drive the relay.

    What I'm saying is that you need to know more about the LED output before deciding how to interface to a relay.
     
  3. DM4N

    Thread Starter New Member

    Apr 17, 2014
    5
    0
    Yeah thats exactly right and it doesnt bother me if the led wasn't even there but to complete the circuit i'm assuming it would need to remain there.

    I know the LED pos and neg sides when i had it on i tested either side and found which one was positive so i know its a constant DC feeding the led when it is on.

    Just wondering for the transistor side of things does the base signal need to come from a positive or negative or does that depend on the transistor?
     
  4. pwdixon

    Member

    Oct 11, 2012
    488
    56
    Probably best to leave the LED in circuit if possible then you get a visual indicator of what the relay should be doing.

    If the LED was hanging off the positive rail then one side would always be positive but the other side could be pulsed and you need not notice visually, you need to indicate what the LED control signal actually does, eg. if one side of the LED is on the positive rail and the other side is taken to the negative rail to turn it on. Obviously there must be a limiting resistor somewhere so the LED legs will not a simple one side +ve rail the other side -ve rail. Much more likely one side +ve rail the other side either +ve rail volatge when off and +ve rail minus a couple of volts when the LED is on.

    Really depends on the control method for the LED, choice of transistor type and which connections on the relay you are using ie. NO or NC. I can't give absolute answers to any of these without lots more info on the LED drive in the first place. It would be helpful to have the LED voltages fro both pins when the LED is off and on.
     
  5. DM4N

    Thread Starter New Member

    Apr 17, 2014
    5
    0
    I have one of the circuits on order so i will have to find out when i get it. as i accidentally broke the last one.

    All i know for now is when the led was off it dropped down to 0.25v or something similar but i'll have to wait and see on the new one.

    Cheers :)
     
  6. DM4N

    Thread Starter New Member

    Apr 17, 2014
    5
    0
    Assuming the led on the shift circuit was just a switched positive just open circuit when rpm not reached then then closed circuit when activated would this sort of setup look right?

    [​IMG]
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Does the shift light have a PCB that you can take a photo of? If we can see both sides of the PCB we can see if the LED is driven from the + rail or the gnd rail.

    That info will help a lot!
     
  8. DM4N

    Thread Starter New Member

    Apr 17, 2014
    5
    0
    Hi guys i just got a new circuit i will get some photo's later tonight. Running some tests i found this:

    Multimeter connected to constant +ve rail and otherside to negative leg on led
    Led off:
    approx 8v
    led on
    approx 12v

    Multimeter connected to constant negative rail
    LED off
    12v
    led on
    12v

    So i have found out it is a switched negative but doesnt cut it off completely only drops it down to 8v

    I will get a pic of the circuit top and bottom view and post it
     
Loading...