9vdc boost to 12vdc

Discussion in 'The Projects Forum' started by NT15MCP, Jul 25, 2010.

  1. NT15MCP

    Thread Starter New Member

    Jul 25, 2010
    I'm new here and possibly well outside my general electrical knowledge area so any help would be appreciated. I am attempting a very simple circuit to boost a 9vdc remote on signal to a 12vdc signal for an audio amplifier. I don't know why my factory amp takes only 9vdc to turn on, but I have an aftermarket one that requires 12vdc to turn on and it would be nice if I could use the factory wiring so I don't have to run any new leads anywhere. I thought a comparator op amp would be perfect, but I don't know how to set the circuit up. I have a 12vdc for the +Vcc on the op amp and ground for the reference and -V . Idealy, when the 9vdc signal is applied to the input Vin>Vref the Vout should be +Vcc or 12vdc. I'm not worried about current capacity because it's only a signal line (idealy infinite impedence).

    Now I get to admit how stupid I am. I got a LM339 quad comparator op amp from RadioShack and a little IC board, soldered the chip to the board and then some wire terminals to the chip leads. I then tested it without attempting to hook anything into my car by using three 6vdc batteries. I put 2 in series for a 12vdc +Vcc and connected the - terminal of the third in parallel for a common ground. I connected the common ground to pins 4 and 12 to give a 0vdc ref signal for the op amp. I then connected the 12vdc + to pin 3 for the +Vcc. Now I connect the 6vdc + to pin 5 for the Vin signal and should get 12vdc on pin 2 Vout. Instead I get 0. What am I missing?

    pinout of 339 IC chip is:
    pin 1 OUTPUT 2 (there are 4 comparator op amps on this one chip)
    pin 2 OUTPUT 1
    Pin 3 V+ (+Vcc?)
    Pin 4 INPUT 1 -
    Pin 5 INPUT 1 +
    Pin 6 INPUT 2 -
    Pin 7 INPUT 2 +
    Pin 8 OUTPUT 3
    Pin 9 OUTPUT 4
    Pin 10 GND
    Pin 11 INPUT 4 +
    Pin 12 INPUT 4 -
    Pin 13 INPUT 3 +
    Pin 14 INPUT 3 -

    Also, can anyone double-check me that I'm counting the pins correctly? Looking from top of the chip pin 1 is top left if the U-shape cutout is at the top.

    Thank you for your assistance
  2. tom66

    Senior Member

    May 9, 2009
    You have your (+) and (-) the wrong way - when the (-) is less than the (+) the output goes high.

    The (-) pin should be tied to a voltage divider to give you say 3 volts otherwise when nothing is connected the comparator may pick up noise and switch on and off randomly. Setting it at 3 volts some immunity to this.
  3. SgtWookie


    Jul 17, 2007
    Comparators and operational amplifiers are related, but different.

    In general, comparators are used to indicate if one input is higher than the other; a "yes/no" kind of thing. The outputs are generally at one power rail or the other. They are also not internally compensated, so they make for poor linear amplifiers without providing external compensation.

    Comparators are generally much faster than operational amplifiers when used open loop.

    An important difference to note is that the LM339 has open-collector outputs. This means that the outputs can sink current, but cannot source current. In order to see an output, you need to either use pull-up resistors or a constant current source to provide the current. A generic formula for calculating pullup resistors for LM339 comparators is:
    Rpullup = Vsupply/0.004A
    So, if your supply voltage is 12v, then:
    Rpullup = 12v/0.004 = 3k Ohms.

    If you are amplifying audio signals, you will need to use an operational amplifier.

    However, you really have not given a full problem statement; what kind of signals are you attempting to amplify from 9v to 12v? If for logic input, it may be possible to simply use some transistors & resistors to alter the signal levels.
  4. BMorse

    AAC Fanatic!

    Sep 26, 2009
    So basically you are taking the 9 volt DC source from a receiver to use that to trigger the 12 volt remote on control on an amplifier??

    If so, the easiest way would be with a 9Volt SPDT relay...

    I have included 2 different circuits to accomplish this, but I believe the relay option would be the easiest to implement, I have done it this way several times for a few people....

    And also if the receiver has the option for connecting a powered antenna (Usually a blue wire coming out the back of the receiver) you can use this to remotely activate the amp also (This toggles 12 volts off or on depending if the radio is on of off...)


    B. Morse
  5. NT15MCP

    Thread Starter New Member

    Jul 25, 2010
    Thank you for your help. I believe that I could classify the 12vdc output that I'm attempting to achieve as a 'logic signal', but I don't know that much about the remote on signal to Amplifiers. I didn't even think of a relay option, it is the obvious solution. You guys rock! :D