Lost with 120VAC Project Wiring

Discussion in 'The Projects Forum' started by mia631, Apr 12, 2011.

  1. mia631

    Thread Starter New Member

    Apr 12, 2011
    Hey all,

    I have undertaken a project that i have done a great deal of reading for and learning about to supplement my newbie knowledge, but my efforts have come short in a few areas. I have finally given up googling for now as i have not been able to find the answers I need to move forward. Hopefully someone can point me in the right direction and I will be on my way back to Google, to return with answers for others upon completion of my project.

    So, this project requires 2 lamps, driven by a single ballast from a common US (120V) home, with a power button/switch that will automatically power off after, say 30 seconds. It seemed simple enough to me, get the correct ballast, endcaps, and lamps, then wire everything and get a timed power switch to take care of the rest. However, I have been unable to find such a timed power button/switch, and have been reading more and more about circuits, and at this point I am unsure whether this will need a circuit board and timer (555 timer maybe?) to accomplish this. I have found a wealth of info on DC circuits, but not much on AC circuits and honestly don't know how much DC relates to AC in regards to circuit boards and circuitry.

    Are timed power switches commonly sold? Do I need a circuit board? Where should I look to find info on AC circuitry? Hopefully I am on the right track even, but at this point, i am just not sure. Finally, I am not one that looks to be fed the answers, I am willing to read and learn as much as I can on my own if I can be pointed to the right info, but Ive come to a point where im just lost and dont even know what to search for. Thanks ahead of time for all guidance.
  2. K7GUH


    Jan 28, 2011
    In the bad old days, one might use a d.p.d.t d.c. relay, with rectifier supply, and a capacitor. The current would be enough to pull in the relay, but as the capacitor discharged through the relay coil, the relay would drop. There are timed switches for bathroom fans and darkroom timers (expensive, but they work precisely). Of course, nowadays you can use an NE555 timer to drive the relay. Your solution does not have to be elaborate.
  3. wayneh


    Sep 9, 2010
    Are you saying you want a fluorescent light to stay on for about 30 seconds after it's switched on?

    I don't know how to do this in the AC world, but it would be "easy" to build a DC timer circuit that controls a relay to switch an AC load.

    First you need a low voltage (up to 16v, say) DC power supply, which could be any old wall wart you have. If you can find one that provides DC, all the better. Otherwise you need a full wave bridge (4 diodes, or a single device meant for this) to rectify the AC coming off the transformer. The wall wart transformer can power up along with the lamp when switched on.

    Timing can be accomplished by using a 555 timer IC, or an RC tank and a comparator. People often choose between these strategies based on their experience with one or the other and what parts they have laying around. Starting from zero, I'd go with learning the 555. It's probably a more precise tool.

    The final step is the input and output. The input is simply being turned on. You want the circuit to hold the relay closed upon turning on. This is "automatic" using the comparator approach, and probably with a 555 timer also.

    The output is the signal from your timer that it's time to open the relay and shut off the light. I think the 555 can handle the current for the relay coil directly. If not, or you choose to use a comparator-based strategy, you can use a small MOSFET to switch the current through the relay coil. You'll want a diode wired "backwards" across the coil to absorb the spike the coil produces when current is removed. Of course you'll need a relay that is specified for handling the voltage and current demand of your lamp. And I'd go double or more on the current spec.
  4. someonesdad

    Senior Member

    Jul 7, 2009
    If I was faced with this task, my first choice would be to look for a suitable mechanical timer switch. I have one that lets me turn the air conditioner on in my room for up to 12 hours. I installed it 20 years ago when I built this addition onto the house and it was $12 (how can I remember that and not remember what I had for lunch yesterday?). Of course, the challenge will be finding one with a 30 second delay. However, any hardware store should have these timers, so see what they have and you may be able to special order one. It will be more reliable than any equivalent electrical device. And they're dirt-simple to hook up. Here's a link -- they make a 5 minute model that might be suitable for you unless your 30 second requirement is fixed in concrete. Amazon sells the 5 minute one for $20.

    Next, look at time delay relays. They come in all kinds of types and typically range from $10 to $100 or more. Someone somewhere makes what you want, so it's just a matter of searching.

    If I had to make a one-off device, I'd go the route similar to what wayneh described. But I have a prototyping board set up to work with PIC processors, so I'd just write a simple program and use an 8 pin 50 cent PIC processor to provide the delay. It could drive e.g. a 2N2222 transistor or MOSFET to turn a relay on or off. Or, it could drive a solid state relay directly. Then a small push-button switch would be the only input needed. Thus, the total parts would be a 3 to 5 volt wall wart, the PIC, a push-button switch, and a solid state relay. It wouldn't be hard to add a little pot to let you adjust the delay time. Actually, the real reason I'd pick this route is that I already have all the parts and a whole bunch of solid state relays that were given to me. :p
  5. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    sounds like googling 'darkroom timer schematics' could come up with a bunch of suggestions?

    Senior Member

    Jul 1, 2008
    I've used many high wattage infrared heat lamps in motel bathrooms. They were all controlled by a single knob wall control. Admittedly, I think 20 minutes was there max setting but you can bet they make them for longer periods.

    As already stated, you can home brew one with 555s but not if you have limited electronics background.
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    Such timers are manufactured, typically used to control lighting in the entry hallways and staircases of multiple dwellings - with the result that someone can be left halfway up the stairs in the dark. There may be safety and security aspects to consider when installing such a system! It is also notable that the life of fluorescent lamps can be considerably shortened by frequent starting.

    It may be better to obtain a commercial version, of a type compliant with any legal and insurance requirements which may apply to your situation. Additionally, you should consider whether regulations may require the switch to be installed by a professional electrician.

    There are two basic types. The first is an older pneumatic type, which may now be considered obsolescent, and which has a poor reputation for reliability, with a tendency to shorten its time delay as it ages. Some suspect that this suits the requirements of landlords who install them, as the lighting cost is further reduced as the timer ages! See link:

    More recent designs are electronic; they may be more satisfactory, but note that some are unsuitable for fluorescent lamps. See link: