Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!


  1. Dec 26, 2008 #1
    I have a simple circuit switching activating a 110V timer but would like to show an LED ON indicator at the switch SW when the switch is closed, I can do this easely enough if I run the wire from the LED to ground but this means I need a ground nearby, so my questions is it possible to place the LED in PARALLEL with a resistance R so that when the switch is closed the LED lights up !!!!.

    I only want to run 2 wires to/from my switch hence the problem. if I put my LED in series with the circuit it doesnt work if I put my LED across the switch it works in reverse IE the LED is on when the switch is open.

    This must be a simple problem can anyone help ?

    LED is 110V AC

    Attached Files:

  2. jcsd
  3. Dec 26, 2008 #2
    That would only light when the timer has also closed the circuit except that there might be some leakage through the timer when it is not closed causing a feint glow You want another R (R2) in series with the LED. Select the main R to give you about 2.6 volts across it and R2 to put a few milliamps through the LED. The LED will have a about 1.6 V across so R2 needs to drop 1 volt. Putting an (another) LED and series R after the switch and then to N will light when SW is closed whether the timer has operated or not.
  4. Dec 26, 2008 #3
    Thanks for your quick reply , I'm still confused The LED I have is 110V AC (it does however have a Resistor in series built into it) are you not giving me an answer as if I was using 3V LED or does it not matter can you give me a rough diagram or is that too complicated.

    The layout I drew before was purely how I thought it might go but if you a have a better idea please feel free to explain, do you mean that what I have is nearly right but I need to add another R2 in series with the LED and keep R1 (R on my diag) where it is ? what would the values be for R1 and R2 approx so that the 110V LED lights (i think it draws about 0.8mA).

    The Timer operates an ondelay to activate a NC contactor to switch off an A/C unit, it all works great its just that sometimes I dont have an earth to locate onto so that my panel light is on when the contactor is operating.

    thanks for you help so far.....
  5. Dec 26, 2008 #4
    Maybe it is neon you have? Sounds like it is not suitable. You need an LED which develops 1.6 - 2 volts with a few mA running through it.

    R1 is in series the LED and the two are in parallel with R.
  6. Dec 26, 2008 #5
    As pumblechook implies, this is often done with a neon bulb. You might be better off using a switch with integral neon lamp.
  7. Dec 26, 2008 #6
    yes your right its a neon but I would like to continue using them if I can although I checked and they only last 5000 hours !!!!


    I'm sure it can still be done though, I'm using a reed switch and a magnet but the light must go on (since its a locating light) when the switch is closed ie when the magnet is removed away from the reedswitch
  8. Dec 26, 2008 #7
    Thank you for your suggestion but I must stick to my magnetic reed switch and the integral light must work when the switch is closed, in fact its easy to make it work the reverse you just put the light across the switch in Parallel.

    the hard bit is the opposite when there is no obvious ground otherwise I would run another cable but I'm trying to stick to 2 cables, when there is an obvious ground like metal perlin then you connect the other side of the neon to it and bingo it works since when the switch is closed the live goes through the light onto the ground.....

    pumblechook please see pics too.

    Regards to both for help, now what do I do ?

    Attached Files:

    Last edited: Dec 26, 2008
  9. Dec 26, 2008 #8
    In that case you would want to place the neon bulb electrically across the load. No resistor is required. If you want it physically located at the switch, the bulb is connected to neutral and the switch wipper (per your sketch).
  10. Dec 26, 2008 #9
    I do want it located physically at the switch as that is the indicator that the system is in GREEN mode hence saving electricity when the timer is started . I understand what you mean when you say that I must connect it as per my diagram but what must be R so that it still sends 110V to the bulb and acts as a contact between the wiper and the load side of the timer, in other words if I just put the neon bulb in place of R it would not work right ? so then what resistance would be sufficient to give me a voltage drop of 110V and yet offer a circuit from wiper to timer on the live side.

    I thank you again for your help and hope that you understand what i'm trying to say.


    Light on when Timer is on
    Light Must be at switch
    only 2 wires to switch (Neutral and Live)


    When there is a earth nearby, yes its true that I can dump the neon neutral side to it but its not always the case when working with clients who dont have nearby earth source or in Concrete walls ect.....
  11. Dec 26, 2008 #10
    Hold on. I've confused you. Sorry. The switch has two ends. I'll call them "hot" and "load."

    Connect one lead of the bulb to the neutral wire and other lead to the switch's load end.
  12. Dec 27, 2008 #11
    Yes that would work but then remember that the only part of 2 wires that run into the switch are the live and the switched live result and I dont really want to run 3 wires since the distance between the entrance switch and the fusebox may as much as 20 meters in some cases and i'm doing more than just one home installation.

    If there is no solution then I will have to run 3 wires and in your case you are right any neutral or an Earth would work in fact the advantage of running 3 wires with an earth is that if you cut the wire by accident you have a safety.

    My switch wiper should be drawn the other way too !!! I was just doing a quick illustrative sketch....

    Is there really no way in which I can put the Neon in parallel with a Resistor so that the voltage drops to the Neon is 110V and yet there is enough juice to power the 1W Timer.
  13. Dec 27, 2008 #12
    no, it's fine either way
    I don't understand why you need a third wire, no following where various curcuit elements are, but in any case...

    The load is one Watt, you say, so it draws 10 milliamps. I think we can assume that it draws 10 milliamps whether it has timed-out or not. That's enough to light an LED rather dimely.

    If I absolutely couldn't obtain neutral and switched-power where I wanted it, I would try two small LEDs in parallel with each other. Put them antiparallel to each other--anodes tied to cathodes. Put the pair between the switch and the load.
  14. Dec 27, 2008 #13
    OK will try that remember they are not LED as discoverred they are NEON's....will that still allow the current to flow to the load ? furthermore the LED needs to light up as much as poss to show the keyholder where he must place them in order to restore power upon returning to his condo in sometime dark entranceways.

    Thank you so much for your help and will keep ya posted on the parallel LED issue, as i might have to swap to LEDS as NEON green only have predicted lifespan of 5000 hours !!!!

    Also as an option could I use a standard 3V LED and a resistor and drop 3V across it thus giving me what I want ?
  15. Dec 27, 2008 #14
    At 10 mA a modern LED will be pretty bright. I have used some at less that 1 mA. So you could just put one in series with the timer (will only light when the timer is closed unless there is a small current when it is open). You will need a second LED (or a diode) in reverse parallel with the first if you are using AC.
  16. Dec 27, 2008 #15
    Ok got some results using a range of R in Parallel with an LED if I use the LED on its own it lights but doesnt operate the timer I tried several LED together and they helped to start the timer but then failed when the small current the CONTACTOR draws.

    So then I decided to use just one LED in Parallel with a range of R from 75k to 1k measuring the current in the circuit and observing the LED brightness and seeing if it operates the timer (30 sec ONDELAY) and contactor in turn

    R mA LED TimerOK ConOK
    75K 2.2 High Y N
    22K 5.5 Med Y Y
    10K 8.3 Med Y Y
    5.6K 11.5 Low Y Y
    1K 16.5 Off Y Y

    from the above results it looks like I could get away with using a Resistance in the range 10 to 50K but anything outside that and the light either doesnt come on with low resistance or with high resistance the timer doesnt get enough Juice.
    Last edited: Dec 27, 2008
  17. Jan 3, 2009 #16
    OK I have measured the Current drawn By the timer at 16.5mA and the GREEN NEON light at 1mA, So what safe value Resistor shall I use to have the light on when the timer is on assuming the resistor is placed as per my orginal diagram. (in parallel with the NEON).

    I have in practice made a 22K R work and the current draw in the circuit has dropped to 5.5mA when the green light is on and the timer is operating but why has the current dropped ? problem is the higher R the Brighter the LED but the less current is being passed to the Timer to the point where it wont operate at all.

    Shall I just stick to 22K is that a safe value that wont overheat ? I want to understand the theory so if one of you guys could explain it to me what is happening ?

    Also should this circuit be fused with a ????mA fuse just to be safe as I'm running on 110V on a 2x 20AWG sometimes distances of up to 20M, I know the current draw is small but what worries me is shorts in the even of a wire tear. Can I just put a little fuse near the timer on the Live side ?

    Also I feed 220V to my contactor from the timer on similar cable 2x20AWG again its not the current draw I'm worried about (2.5W) its the fact that I havent stepped down to a safe voltage and I would rather be safe a put a protection in there too. most of the cable is in trunking 7mm x12mm but still you never know !

    Any coments ideas ? thanks guys....
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook