Will the proposed "the 12V" relay act about the same with a 3.3 V closed circuit?

Discussion in 'The Projects Forum' started by TheManKinahan, Mar 5, 2016.

  1. TheManKinahan

    Thread Starter New Member

    Mar 18, 2008
    Mods Edit:
    Please don't hijack other member's thread, now you have your own.
    This thread was split from -- Need help with a Constant to Momentary Relay

    Since this thread is a couple years old maybe I can briefly hijack. I haven't done any circuit stuff in quite a few years. I have my keurig on an automatic outlet with a manual switch as well. I was trying to make sure it was heated up and ready to go in the morning, so I cut the power switch and soldered it together. I think it's ~3.3 V circuit. Now I discover its a momentary switch, and the display goes off, and only stays on if I hold a button (such as brew) down.

    Will the proposed "the 12V" relay act about the same with a 3.3 V closed circuit? Can I increase the time (to close to 4 seconds, by increasing the capacitor value by 4s/1.5s or 8x (8000 uF)? Lastly, would the relay close with a 5V line from something like a iphone/usb cable? I assume similar to a ssr, the close current is very very minimal?

  2. #12


    Nov 30, 2010
    Do you mean this circuit?
    There are some 555 timers that will work at 3.3 volts.
  3. TheManKinahan

    Thread Starter New Member

    Mar 18, 2008
    It seems more complicated than I would need. First, I've read the voltage for the LCD, etc is 3.3 V, but I don't know for sure. Is there not a simple analog momentary relay? I looked over the 555 documentation you provided and the wikipedia, which mode would I utilize?
  4. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    This may be a bit more tricky that you want it to be unfortunately.

    An interface that usually works is an OPTO-FET. One of the coolest is the vactrol VTL5C1. http://www.excelitas.com/downloads/dts_vtl5c1c2.pdf The cool part is leads. So, if the "switch" will operate with a 200 to 600 ohm resistor, your good to go.

    OPTOMOS relays can be used if you need a lower on resistance.

    The nice part os you don't have to use a real relay and you don;t have to figure out what your working with. e.g. A scanned matrix of switches.

    One think I did one was to make the input handle something like 6 to 30 V.

    The one thing your going to have to do is find out what power supplies are available. e.g. 3.3, 5 or 12 V

    Unfortunately, delays on power up or what might be called a "delay on make timer" always seems to be problematic. The 555 is disastrous and long time periods > seconds also cause issues.

    You will have to test and see if the machine can stomach a closed button held before power up and then released or does it need a certain amount of time to power up and then it has to be pressed for a minimum time and it also has to be released.

    I don;t know how this: Timerblox http://www.linear.com/product/LTC6993-1 (multiple versions) behaves on power up.

    It might be possible to combine a reset generator (e.g. http://www.st.com/web/en/catalog/sense_power/FM1946/SC665?s_searchtype=keyword) and a timerblox to get the pulse you need.

    This http://www.galco.com/buy/SSAC/TS1423 has other issues, but it is a delay on make. 120 VAC does dissapear instantly. You would have to use a real 120 VAC relay. It might not work unless you can hold the button during/before power up and then release it.
  5. TheManKinahan

    Thread Starter New Member

    Mar 18, 2008
    Now that I've looked into it more, it does seem like the 555 IC is perfect. I think it will be very forgiving with closed times, and it looks like there are versions with a cap, an adjustable timer (pot) and screw terminals already made for 2 bucks. I'll try that and report back.

    I looked for a hot 12/5/etc connection, but unfortunately those connections are all at the top of the machine. It was a PITA to take apart last time, so I think I'll just pickup a 120->5 V adapter. and hide it on the bottom.