5 V digital to turn on 24 V lights via relay, Help needed

Discussion in 'The Projects Forum' started by DeminJanu, Oct 26, 2010.

  1. DeminJanu

    Thread Starter New Member

    Oct 26, 2010

    I apologize if this question is answered elsewhere on the forums, and thank you in advance for any suggestions and help.

    I have a mixer board in our radio station that has a "Tally" out - it will output 5 Volts DC when the microphone is on, and 0V when it is off. (max current = 30mA - actually the manual says "do not draw more than 30mA"!)

    I would like to use this 5V signal to turn on some light bulbs using some relays already wired in our wiring closet. I think the relays have 12V coils (~100mA to throw if I remember correctly), 4-poles, and they are set to connect the lightbulbs to -12V and +12V (providing a total voltage across the lightbulbs of 24V) from a nearby supply. We confirmed that the bulbs do light when the relay is flipped - now I just need to flip the relay using this digital signal (30mA is too small, and I didn't want to try to fear of burning out the microphone's mixing board module).

    How can I make the light bulb turn on with the 24 VDC source when the Tally signal turns on to 5 Volts?

    Any help would be greatly appreciated! Either simple transistor/relay/amp circuits I can build, or a part I can just purchase. I fact, I'm not even sure of what I'm looking for here - I was looking at "DC to DC converters" for a while, but am not sure if that's what I need. It originally sounded to me like I just need a single transistor circuit (I'd have to use some schematic to bias it up), but figured a commercial option may provide more protections to the board circuitry...

    fyi, I can use the nearby +/- 12V DC and +/- 5V DC supplies to boost the 5V signal.

    Last edited: Oct 26, 2010
  2. n1ist

    Senior Member

    Mar 8, 2009
    You can either use a single transistor (N-FET or NPN) and some resistors or a small relay to go between your board and the existing relay. Just make sure you have a flyback diode across the relay coils.
  3. SgtWookie


    Jul 17, 2007
    Does this 5v "mic on" signal share a common ground with the other power supply/supplies already? If not, you should probably keep them isolated from each other. Attempting to put a common ground between the two could result in a "ground loop", and lots of noise on the audio, which would be a big problem.

    You could use the mic on signal to drive the emitter side of an optoisolator/optocoupler like a 4N25; 8mA current is all they really need. The output of the optocoupler could be used to drive the base of a Darlington transistor or MOSFET gate to sink current from the relay's coil.

    See the attached.
    Ignore Rdefault; it's required in SPICE simulations that all nodes have a path to ground.

    R2 ensures that the optocoupler shuts off quickly when the mic signal goes to 0v.
    R3 keeps the gate of the MOSFET turned off when the optocoupler turns off.
    D1 is required; without it the relay could "zap" the MOSFET when the MOSFET turns off.
    V1 and V2 are your +/-12v supplies.
    Last edited: Oct 26, 2010
    DeminJanu likes this.
  4. DeminJanu

    Thread Starter New Member

    Oct 26, 2010
    wow thanks for the quick replies!
    The 5/12 V sources & board DO have separate grounds, and I believe your concern over ground loops is well placed - I know these can be big problems for audio noise.

    Thanks for the circuit schematic, and especially for the explanations! I like the optocoupler idea (I'm in photonics, so that is particularly neat)

    That looks pretty simple! I'll see if I can find the parts and get cracking!
    If anyone else has pre-packaged ideas I'd love to see those too (just so that, 5 years down the road, someone doesn't find my circuit hobbled together and get all confused...)
    Last edited: Oct 26, 2010
  5. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    If you don't fancy extra soldering, you can just use a solid state relay as the interface. Only an extra 220Ω resistor is required.


  6. bkr1969

    New Member

    Dec 11, 2010
    I'm trying to do something similar, though not in the audio arena. I'm builing a PIR sensor to activate a drinking fountain for my cat and need to switch on a 12V (AC) pump with the output from the module I'm planning to use (http://www.glolab.com/dp-003A/dp-003A.html). It will be powered by the 12V adapter that came with the fountain, but I'm not sure how to activate the pump with the relay output from the module. Any help is appreciated.
  7. wayneh


    Sep 9, 2010
    We used a Clapper for a while, but quit because we're not there enough for the cat. We thought we were saving wear on the pump, which is now in its tenth? year of continuous operation. My advice: Let it run.