making a delay off timer

Discussion in 'General Electronics Chat' started by sdowney717, Oct 16, 2012.

  1. sdowney717

    Thread Starter Member

    Jul 18, 2012
    Running into wall on these things but found one willing to make.
    The description about the TC is a little chalenging to me

    What I want to do is take the circuit on the right and make the resistor variable. What value ohms variability to get the delay off from 1 to 20 seconds?

    I also have some NTE123 trans and lots of 330uf caps.

    Can I use the NTE123 in place of theses suggested 2N2222? Circuit modify to use them?
    Is the diode across the relay coil needed?
    I plan to put 3 of the caps in parallel to make 1000uf.

    his description below which having some trouble with

    "The time delay for the common emitter will be approximately 3 time constants or 3*R*C.
    The capacitor value for a 15 second delay will be 15/3R = 1327 uF."

    Can someone explain this to me, how he comes up with the values?

  2. sdowney717

    Thread Starter Member

    Jul 18, 2012
    I put this together and it works!
    Powered with an old 12v car battery charger

    I Used these parts

    1 - 1N4007 single diode
    1 - 2.2K resistor
    3 - 330uf cap = 990uf
    1 - relay with 156 ohm 12vdc coil

    1 - NTE123

    with 2.2k resistor power off delay is ~10 seconds
    with 1.0k resistor power off delay is ~6 seconds

  3. sdowney717

    Thread Starter Member

    Jul 18, 2012
    further testing
    only 2 caps is 4 seconds
    only 1 cap is 2 seconds.

    This will be perfect for my fuel pump issue on the boat.

    For the head, I will go with about 10 seconds.
    I am thinking different relay coil resistances will affect the power off delay time?
  4. BSomer


    Dec 28, 2011
    Good to see that you got it worked out. You should use a diode across the relay though. This will help in protecting the circuit from spikes when the relay shuts off. When the magnetic field around the coil of the relay collapses there could be some large voltage spikes that can go back into the circuit and ruin the transistor or other components.
  5. sdowney717

    Thread Starter Member

    Jul 18, 2012
    does adding the diode across the relay affect the time delay?
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
    No, it prevents back emf from the relay damaging the transistor.
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Which means STOP using this circuit until you add that diode as you are killing the transistor.

    Don't use that transistor in your boat: get a fresh one that never saw the relay open without that diode.

    Aside: I happen to have designed the delay timer inside some brands of MIL-PRF-83726-28 thru -31 relays used on commercial and military aircraft, so I know all about driving relays.
  8. sdowney717

    Thread Starter Member

    Jul 18, 2012
    I took a poor picture of my little project board but it gives a general idea.
    black wire is ground
    purple is the trigger
    red is the relay and 12v positive
    blue is the other side of relay.

    I added the diode across the relay on the board. And used a new transistor.

    I am using a standard automotive 40 amp relay which has an 83 ohm coil. So this will wire into the fuel pump relay control circuit already on the boat.
    The case is from a broken Omron timer relay. For the little board I used the Omron board and removed all components and copper tracing. There are 2 little slots on this board which I passed the wires through which serve well as a strain relief.

    The device is in the wheel house well protected from the weather.
    Last edited: Oct 20, 2012
  9. sdowney717

    Thread Starter Member

    Jul 18, 2012
    I made the timer for the electric head flush pump. With the var resistor mini pot I can vary from 8 to 12 seconds which should be enough flush time. Raritan Lectrasan has a flush timer of 1 to 20 seconds with it factory set at 10 seconds. Minipot taken off old ATX PS, not enough resistance range to increase the delay time further. Would be nice to have one with range from 1 to 10,000 ohms. Would old VCR have those minipots?
    Fixed resistor is 1K. Cap is 1000uf. All these parts salvaged from various old useless pieces of electric stuff.

    It is important to have a resistor in series with minipot as if you turn minipot resistance to low, the NTE123 will be destroyed too much current flowing from base to emitter.


    showing the cutup salvaged part of a board from an old broken PC power supply. Easy to drill new holes where needed.
    I removed some copper trace by grinding, others by peeling off the board. Cut the piece off board using a grit cut off wheel.

    fits well inside an old pill bottle. Will be mounted inside a dry cabinet in the head.
    Will use a 40 amp auto relay.
    Red wire to 12vdc pos and one side of relay coil
    Black to ground negative
    Orange to other side of relay coil
    Green to trigger pump switch 12vdc pos.

    Last edited: Oct 21, 2012
  10. The Question

    New Member

    Oct 16, 2009
    I have a question. what is the purpose of the relay coil? is it just there to draw amperage?
  11. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    The relay coil is there to produce the magnetic field, when a current is passed through it, that pulls the relay's contacts together.

  12. cajunsniper

    New Member

    Jul 29, 2013
    how can i make it delay 5 minutes
    and how can i make it delay 45 minues

    de 5 minutes is for a bathroom fan
    and the 45 minutes its for the AC of my room
  13. doug08


    Jan 30, 2011