Can U use 2 transistors in place of a relay to switch 2 different voltages

Discussion in 'The Projects Forum' started by icadj, Feb 9, 2010.

  1. icadj

    Thread Starter New Member

    Jan 2, 2010
    Hello everyone

    The original of this circuit has O/P from pin 3 triggering a transistor, firing the relay. I have bought 3 replacment relays and they make as much noise as the old one does ,,,,,,, and I am getting very fed up with the noise it makes

    Can any plz advise wether or not replacing the relay with 2 transistors as shown will work. If not is there a way of not using a relay?

    The time scale of off and on periods are ,,
    24v ON 35-38secs (aprox) 12v OFF 7-10secs (aprox)
    24v OFF 35-38 secs (aprox) 12v ON 7-10 secs (aprox)

    I can NOT replace or reduce the unit that needs 24V as it is sealed. Both units are lighting displays and the 12V system is built around the 24v system

    Any help would be brilliant
    thanks in advance
  2. mik3

    Senior Member

    Feb 4, 2008
    You can use solid state relays which are noiseless.
  3. russpatterson

    Senior Member

    Feb 1, 2010
    I recently switched from using relays to MOSFET's in my project. They're a little trickier to learn but once you get them figured out they're pretty handy. Cheaper and smaller than relays. You can get them rated for all kinds of voltages and current.

    I'm using the irf530 Check out this schematic that MikeML posted for me (an improvement on my own).

    The MOSFET is M1. MOSFETS have a Gate, Drain, and Source. In this case the Source is connected to ground and the Drain is connected to the negative side of the device your switching. You put your control voltage on the Gate. So your pin 3 would go to the Gate.

    The trickiest thing for me was the fact that MOSFETs are driven by voltage and BJT transistors by current flow. The VGS(th) or voltage difference between the Gate and the Source is what causes them to conduct. If you switch the negative side of your device then this gets easy as your gate voltage will always be higher than the voltage across the source. Here's a link to my project if you want to look at my schematic.
  4. joemmech


    Jan 22, 2010
    You can try using a MOSFET for a much faster switching time.