Transistor Help Please?

Discussion in 'The Projects Forum' started by TedD, Aug 20, 2009.

  1. TedD

    Thread Starter Member

    Aug 20, 2009
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    Hello Forum Members:

    I really need your help. I have a radar detector in my vehicle. When the device detects a radar source, a small LED blinks. This visual cue is too small and I am trying to have the detector light a previously-installed LED cluster that operates on a 12VDC source. The line going to the present LED is providing enough voltage and current to power that small LED but not nearly enough to power my preferred LED cluster. The voltage I am measuring to the small LED is somewhere in the order of 5 volts (it is difficult to measure since the LED only blinks intermittently). My first thought was to install a 5volt relay in place of the small LED and then power my cluster with 12v sent through the relay. When I tried that, there wasn't enough power to close the contacts on the relay. The coil in the relay is rated at 5v and 89mA. Apparently the current going to that small LED must be minute. I read from another post that a transistor could be used as a switch to power a relay and I'd like to try that. My only training in electricity is the 8 credits of Physics I took in college about a billion years ago. I can follow a schematic pretty well however. I am just not sure which transistor type to use (e.g. a NPN, PNP, FET, MOSFET, etc), how it should be wired and whether any resistors have to be added to protect the line from my radar detector. The detector cost my $1000 and I want to make absolutely sure that no current finds its way back into the detector body and blows something out. If you can please afford me some straight forward schematics as to how I may solve this dilemma, it would be genuinely appreciated. Please remember that I am not an electrical engineer (obviously) and I need something simple and uncomplicated. Thank you.:confused:
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What color is the existing LED? If it is red, you could mount a phototransistor next to it and have that drive a MOSFET, which in turn would switch your LED array. How much current does the LED array require? This is physically a little clunky, but it avoids having to electrically modify your radar detector.
    If this sounds interesting, we could probably come up with a schematic and parts list for you.
    Where are you located (so we can recommend a parts vendor)?
     
  3. TedD

    Thread Starter Member

    Aug 20, 2009
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    Thank you for responding. The small LED in question actually is two colors. It is green and red depending on what's going on with the detector. There are three leads to the LED. I isolated the two that are live when radar is being detected and tapped off of those to try to drive my 5v relay. The LED array I'm trying to electrify draws somewhere in the order of 100mA. The small LED is already mounted permenently, so I will leave it in place. I just need to tap off of the leads previously cited and feed that current into the base of a transistor which can then, acting as a switch, power a 12v relay so that I can light my array. I need help with the transistor part of this problem.
     
  4. Ron H

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    What do you want your array to do when the red LED is on? How about when the green one is on?
     
  5. Ron H

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    Can you use surface mount parts?
    Can you switch either end of the LED array? If not, which end is available (high side or low side)?
     
  6. TedD

    Thread Starter Member

    Aug 20, 2009
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    When the red LED blinks, I would like the array to do the same. In essence, I need the array to mimic the small red LED. My array is already mounted as is the red/green detector LED. I'm not sure what you mean by "which end of the array." I have 20 LED's (4 rows of 5) wired in series in the array.
     
  7. Ron H

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    A load such as an LED (array) can be switched on the high (+V) side, or on the low side (ground). Which end do you intend to switch?
     
  8. TedD

    Thread Starter Member

    Aug 20, 2009
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    Since I haven't wired it up yet, I can do either. Unless there is some advantage regarding how it's wired, it makes no difference to me.
     
  9. Ron H

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    Apr 14, 2005
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    How come, when I ask multiple questions, they don't all get addressed? It happens to me all the time.:(

    1. Can you use surface mount parts? (Post #5)

    2. Where are you located? (Post #2)
     
  10. TedD

    Thread Starter Member

    Aug 20, 2009
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    Sorry. The answers are: NO. Rhode Island. Would a MOSFET Relay help me?
     
  11. Ron H

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    Apr 14, 2005
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    Sorry if I sounded cranky.
    A major consideration is the voltage on the red LED when it is lit. Unless it has an integral resistor, it will be around 2V. If the resistor is internal, then your 5V estimate might be correct. It would be good if you could nail this down.
    It's too bad you can't use surface mount. Fairchild has an N-channel MOSFET (FDG410NZ) with guaranteed max Rds(on) of 87mΩ when Vgs=1.8V. This is all you would need, except perhaps a 100Ω resistor in series with the gate. Unfortunately, it is not available in a thru-hole package. Maybe one of the other guys here knows of a suitable device.
    There are other solutions, but they may require more parts, depending on the voltage available at the LED.
     
  12. TedD

    Thread Starter Member

    Aug 20, 2009
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    Thank you all for your input. The solution to my problem is to use a MOSFET Relay. I purchased a Panasonic AQV101 and it works perfectly.
     
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