Can i construct a 2v relay?

Discussion in 'General Electronics Chat' started by cduluk, Aug 2, 2009.

  1. cduluk

    Thread Starter Member

    Aug 2, 2009
    14
    0
    Hey guys/gals-

    I'm new to the forum and hope you can help me on a project I'm working on.

    I have a remote radar detector in my car, which has a little 5mm red LED that blinks whenever it's activated. The only problem is that the LED is small and inconspicuous, and doesn't really grab my attention when it needs to.

    I have also been obsessed with the new "heads up display" (HUD) technology used one some of the newer cars- that displays an image flat on the dashboard that reflects off of the windshield, which gives the image a 'floating' appearance- really gets your attention! Usually used to display speed, etc.

    Like this:

    [​IMG]

    Well i want to create my own simple HUD system using the radar's LED, so i will see an image of the word "RADAR" floating in front of me whenever it's activated.

    Like this:

    [​IMG]

    I would create a small, maybe 2" x 3" x .5" panel that would be illuminated by 20 or so red LED's (ran in parallel or series) covered with a clear transparency with the word "RADAR" spelled out, that would be placed on the top of the dashboard. The light from the LED’s would only appear through the letters.

    Like this:

    [​IMG]

    So i made the unit. I used twenty (20) 5mm red waterclear LED's which each require 1.9-2.1V and <20mA of current.

    Now came the trouble of making it work off of the single LED coming from the radar detector.

    I figured the single LED from the detector was being given 2V or so. So i first wired my HUD unit with all 20 LED's in parallel.

    I removed the red LED from the detector and had the two (+ and -) wires in my hand. I wired them appropriately to the new unit i had made, but the LED's did not light up. I checked again at nighttime, and noticed the LED's only lit up ever so slightly. I guess the current to the single LED is very low, not enough to illuminate 20 LED's.

    So it was clear, i was out of luck! I simply cannot illuminate 20 LED's from a single LED's voltage and current.

    So now i am on the chase to figure out how i can get these 20 LED's to illuminate from a 2V low current source.

    Since then, i have removed the 20 LED's from being in parallel and now wired them in series. Every 5 LED's (4 sets) are wired in series with a 180-ohm resistor. So now i have two wires coming from the unit, that require 12V input. I figure this is the only way i can get them to illuminate correctly.

    So now i need to figure out how i can turn a 2V low current input into 12V with pretty good current.

    My first thought was using a relay. But it seems there’s no such thing as a relay that's activated on 2v.

    I did some research on this site, and found this thread:
    http://forum.allaboutcircuits.com/showthread.php?t=3181

    But i am very confused. I'm afraid i don't know much about using transistors or anything, and I don’t understand what to do.

    Can anyone offer any advice as to what to do? Can someone make a simple schematic for me so I can build a device that might work?

    Thanks!
     
  2. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    145
    Apologies, this thread got erroneously flagged as spam. I have given it a bump up the forum post order.

    Dave
     
  3. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    What you'll need is a MOSFET relay. It is a 4-pin IC chip.

    The "coil" is in effect a LED internally which you can connect directly as a replacement to the existing radar LED.

    The output can control a current of 0.5A which is sufficient to drive a lot of LEDs.

    Take a look at this thread.

    http://forum.allaboutcircuits.com/showthread.php?t=26297
     
  4. bearing01

    Member

    Jul 29, 2009
    14
    0
    sounds like your problem is that the original radar detector LED driver does not have the current capacity to drive 20 times as many as LED's as before. Depending on the original style driver in the detector, if it were a NMOS or NPN type transistor "pull-down" then you could use it's output to pull down the gate voltage on a bunch of PMOS transistors. Those PMOS devices could then provide current to your LED's; use a series resistor as well to each LED to limit current in the devices to prevent burning the PMOS and adjusting brightness. Each LED in this case share a common ground. If the detector has a PMOS or PNP type transisitor "pull-up" driver to the original LED then you can instead take the detector output and tie it to the gates of a bunch of NMOS or NPN transistors and use the transistors as your active grounds for your LED's. You'll want to tie your LED's to the power supply and use a series resistor with each LED to limit current and set the intensity.
     
  5. yourownfree

    Active Member

    Jul 16, 2008
    89
    0
    If each of your leds are bright about 20 ma each(guessing) then that is about .5 amps of current. How about this: take the voltage from the light drive a comparator or op amp then use that to drive a mosfet which supplies all the power you need. Choose one that can handle the current draw and voltage.
    Or use and optocoupler, hook your led power wires up to that, then the output of it can drive a transisor relay, or just drive a transistor to power the load. or find the positive of the led wire run it through a resistor to the base of a npn transistor which has relay attached. You probably have seen those circuits. I didn't calculate the resistor but i would say a starting point would be about 330-470 Ohms. I think about 1K ohms is used for about an input of 5 volts, so thats my reasoning. A trim pot works good for the base resistor. Relay needs a diode attached across it, cathode to the positive side of relay.
    Using a relay will give you what you need too. I think you went to a lot of trouble for nothing. Why dont you just mount the led from the detector where you can see it flash.
     
  6. cduluk

    Thread Starter Member

    Aug 2, 2009
    14
    0
    thanks for your replies guys!

    Can someone guide me further as to what i'd need to buy and how i'd need to wire this circuit up? I don't really understand some of the terminology...

    thanks again!
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    You have a 2V signal that lights one LED.
    You made 4 sets of five 2V LEDs in series and in series with a 180 ohm current limiting rersistor so they all will draw a total of only 84.4mA.

    Then any little NPN transistor can use the current driving the original LED as its base current and the 4 sets of LED/resistor networks can be easily driven from its collector.
    The little NPN transistor becomes your relay.
     
  8. TedD

    Member

    Aug 20, 2009
    13
    0
    Will a Panasonic AQV101 MOSFET relay work for me in this application?
     
  9. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    The AQV101 is rated at 40V 0.7A so is good for your application.

    You can connect up the display unit as shown.

    [​IMG]
     
  10. TedD

    Member

    Aug 20, 2009
    13
    0
    Thank you. Thank you. Thank you. That schematic is exactly what I needed!
     
  11. TedD

    Member

    Aug 20, 2009
    13
    0
    Should the 12V + be attached to pin number 6 or pin 4 as shown in the diagram?
     
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