555 timer to delay motor start

Discussion in 'The Projects Forum' started by jimzinvt, Nov 1, 2014.

  1. jimzinvt

    Thread Starter New Member

    Aug 14, 2013
    Hi - first time poster here. I'm a 55 year old tinkerer....very minimal understanding of electronics, but can usually figure something out if I bang my head against it long enough.

    Here is what I'm trying to solve: I've built a vacuum pump for vacuum bagging fiberglass/carbon/epoxy RC airplane wings. It is controlled by a n.o. vacuum switch operating a relay that turns the pump on if the vac. drops below the set pressure and off when that pressure is reached. The pump cycles on/off too frequently, tripping the over-temp. switch on the motor. I want to add a timer to delay the restarting of the motor for a minute or two every time it shuts off.

    I've been poking around on this site and found this circuit in the "Worksheets" section. If I'm understanding things right, this may be a good starting point for what I'm trying to do? As configured it supposedly has a delay of ~4 seconds. And changing some resistor & capacitor values will produce a different delay? Maybe a variable resistor to make the delay adjustable?

    Thanks for any guidance, corrections, or slaps upside the head you'd like to offer!

    - Jim

  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    can't see what the cmos gate is for on pin 3?
  3. jimzinvt

    Thread Starter New Member

    Aug 14, 2013
    Honestly I'm not sure why you would or wouldn't have the gate there (like I said, I'm pretty ignorant about this stuff). Happy to make any corrections though. (and be schooled on why :) )

    So connect the output direct to the transistor, along with the 0.1uF cap? What about the valueless resistor there? And what transistor to use?

    I tried to run this on the DoCircuits on-line simulator but it says I have "hanging wires". I don't see any, except the control pin on the 555. Should that be grounded or tied to something?
  4. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    The CMOS gate is a negative-logic input (low input is true) AND gate. When the power to the input relay is off, the gate's lower input is pulled high through the 10K and 470 resistors. The output of the 555 monostable is low, so the gate's output is low. (H+L=L) The transistor and output relay are off. When the input relay is turned on, it grounds the capacitor which triggers the 555 and puts the lower gated input low. The 555'd output is then high for the duration of the delay. And the gate's output is still low. (H+L=L) The transistor and output relay are still off. After the delay the 555's output goes low and the input's relay is holding the lower input of the gate low, the gate's output now goes high (L+L=H). That turns on the transistor and output relay.

  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Here's how you could do it more simply without the need for that input relay. Delay is adjustable ~1.5-3 min with component values shown.
    CMOS gate and FET require anti-static precautions.
  6. jimzinvt

    Thread Starter New Member

    Aug 14, 2013
    Thanks for your help guys. But I'm bowing out of this project.

    I'm kind of embarrassed to admit this, but I've found a commercially available adjustable time delay relay circuit for just $8 that should do what I need. I would have loved to figure out and build this on my own, but this saves me a lot of time and probably money, and I can get on to building sailplane wings, which is where this project started.

    Thanks again!