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Wireless Doorbell Mod Help!

Discussion in 'General Electronics Discussion' started by CoDi, Dec 30, 2013.

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  1. CoDi


    Dec 30, 2013
    Hi everybody!

    I am new here so I apologize in advance for my little knowledge in electronics. I recently purchased this wireless doorbell system:

    But I want to modify it so I can use this vintage Mid Century button:

    These buttons were originally used on wired low voltage doorbell systems and the cool thing is that they were lighted. Here is the button taken apart:

    The idea would be to replace the original bulb with a led like this one that actually fits perfectly inside:

    Here the Thomas Betts button taken apart:

    And this is the circuit inside:

    The switch has 4 contacts. I was thinking of de-soldering and removing it and wire the old Nutone which has 2 contacts but I am a little lost on how to approach this because of the four contacts.

    Another simple and rudimentary solution would be to leave the switch always pressed and wire the nutone switch in between the battery contacts, but I am not sure if that would be the best solution. Any suggestions?

    Thank you so much!
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    These pressbuttons on the PCB are soldered to the PCB with 4 leads. Actually there is only one contact inside. See this datasheet for example. It is tehrefore perfectly possible to replace them with your vintage switch.

    What I think will not work as you expect is the lighting effect. It looks like the vintage switch has only two contacts. The lamp was inserted between the two contacts. While the switch is open (not pressed), the voltage across the contacts is enough to light the lamp. The resulting current, however, is too small to activate the classic doorbell.
    When you press the button, the lamp is momentarily shorted, a higher current flows to activate the doorbell.
    This will not work with the remote doorbell because:
    - the doorbell may recognize the lamp (or an equivalent LED) as a closed circuit and cannot distiguish it from a pressed button (due to the higher sensitivity of the electronics). The bell will be active all the time.
    - the current available is probably much too small to light a lamp or LED.

    You will have to modifiy the vintage button in such a way that the LED's connections are separate from the pushbutton connectors. Then you can light up the LED independently from the pushbutton function.
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I agree exactly with what Harald said. You will have to modify the pushbutton to separate the contacts from the bulb or LED.

    You can then connect the contacts in parallel with the existing pushbutton in the new transmitter and it should work fine.

    But there's another issue: finding power for the LED. An LED draws less power than a light bulb, but it's still a significant amount of power. Typically they're run at 10~20 mA (milliamps) and they require around 3.5V forward voltage (for a white one). This is 35~70 mW (milliwatts) and that's a lot more than those little button cells can supply - for more than a few hours, at least. Even the larger batteries in the receiver / chime unit won't power an LED for more than a few weeks. So how are you going to power the LED?

    Do you still have the original pair of wires that runs to the door? If so, you could use those to feed power from a DC adapter to the LED.

    Also, before you go modifying the pushbutton, you should check the appearance of the button cover with a white LED behind it. It may not have the genuine "Retro Sickly Yellow" look that you get with a genuine light bulb. Also if you take it outside at night, with a 9V battery and a collection of resistors from around 180 ohms up to 820 ohms, you should be able to estimate the correct current to give the desired brightness.

    Edit: Actually, you CAN avoid modifying the pushbutton, but some extra components would be needed, to provide a simulation of the original circuit and detect when the button is pressed. You should even be able to retain the light bulb - if you want to. This method will take a bit more work though.
    Last edited: Dec 30, 2013
  4. CoDi


    Dec 30, 2013
    Wow! thank you so much Harald and Kris! You guys are amazing!

    I thought it would be easier to run LED's with batteries. Worse case scenario I could just connect the contacts in parallel with the Nutone button and dissable the light on the switch :( but Kris, I am curious of how would you simulate the original circuit so it detects when the button is pressed and keeps the light effect.

    I have the old wires from the old button but unfortunately they go nowhere. We did some renovation and one of the guys removed by mistake the chime system and covered the wall with Drywall. That is why I am installing a wireless now.

    Really close to the button there is a cat5 cable from an old intercom that is not being used. This cable runs to the garage, could I use that for something?
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Detecting a button press when the indicator is connected across the button can be done by feeding a current into the button and monitoring the voltage across it.

    When the button is open circuit, the current will flow through the lamp or LED, and will cause a voltage to appear across it. When the button is pressed, the lamp or LED is shorted out, and the voltage across the pushbutton drops to zero. There is still current flowing through the pushbutton, but there is no voltage across it.

    This is easier to detect with an LED than with a light bulb, because the light bulb will probably need more current than the LED, so the current source would need to be more powerful. But the principle is the same.

    I can draw up a circuit, but I need to know whether you're going to use an LED or a light bulb, and I need to know the characteristics of whichever component you choose. For the light bulb, I need to know the rated voltage and either the current or the power rating. For the LED, I need to know how much current you will be running through it to get the correct brightness, and the approximate forward voltage at that current.

    But your first problem is providing a constant DC supply for the illuminator. If you use batteries, you'll be replacing them every few weeks, or more often, depending on their size. It's not really a workable option. You need a AC-DC adapter or "wall wart" that can provide a suitable voltage for the illuminator. For an LED, 6V DC would be appropriate. For a light bulb, the bulb's rated voltage, or a little bit higher, would be right.

    The Cat 5 cable could be used to feed this DC supply to the pushbutton. In fact you could probably just connect that cable straight to the pushbutton and have the power supply and transmitter at the other end. If the other end is near the chime/receiver part, you wouldn't even need radio communication! But it might be simplest to stick with what you have.

    If you use the Cat 5 cable, you should use all of the wires in it, for minimum resistance. How long is this cable?

    And as Harald and I explained, the simplest option would be to separate the LED from the contacts inside the pushbutton. Is that feasible?
  6. CoDi


    Dec 30, 2013
    Thanks Kris!

    I think the idea of using that cat5 cable to connect to the transmitter and to light the bulb/LED is great.

    The cat5 cable is about 500 to 550 inches long, not sure of the exact length because it goes through some walls.

    The light bulb is rated 16v max. Here is a picture of it:

    I am not sure what current I should use in case of using the LED, I need to run some tests.

    To separate the LED from the contacts inside the push button I think the best would be to drill a small hole, run the wires through it and insulate them so it doesn't interfere with the button. Might be a little tricky because its small, but its doable.

    Here some more pictures of the inside part of the push button:

    Also, the Thomas&Betts transmitter is currently using two 3V Lithium cells CR2032 connected in series.
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    OK. I think your simplest option would be to drill holes and mount an LED inside the button. The LED can be powered via the Cat 5 cable. This means the pushbutton can be connected directly to the transmitter board, in parallel with the existing pushbutton, with no special circuitry needed.

    With this arrangement, the LED will not go out when the button is pressed. Is that a problem?

    Is there any other reason why you might not want to use an LED?

    I wasn't suggesting that you should power the transmitter from the Cat 5 cable as well. It's probably best to leave that as-is. The two CR2032 batteries will last a long time because the transmitter is a very low-power design.

    500~550 inches is 12.7~14 metres. Standards-compliant Cat 5 cable has a maximum resistance of about 0.1 ohms per metre per conductor. Cat 5 cable has four pairs of wires. If you connect two groups of four wires together at both ends (doesn't matter which, as long as you do the same at both ends), total circuit resistance will be 0.05 ohms per metre, or a maximum of 0.7 ohms for a 14 metre run. That's small potatoes. It won't affect anything significantly.

    So you need a mains-powered DC power source at the other end of the Cat 5 cable, rated for around 5~6V at 0.1A or more, and you connect the door end of the Cat 5 cable through a resistor (to limit the current) to the LED.

    You can calculate your resistor value using a rearrangement of Ohm's Law: R = V / I, where
    R is the resistance of the resistor, in ohms;
    V is the voltage ACROSS the resistor, in volts, which is equal to the power supply voltage minus the LED's forward voltage. For a 6V power supply and a typical white LED with a 3.5V forward voltage, V will be 2.5V.
    I is the desired LED current, in amps. For example, 20 mA is 0.02 amps.

    So if your power supply is 6V, you could start by trying a resistor value of:
    R = V / I
    = 2.5 / 0.02
    = 125 ohms
    120 ohms is a close preferred (standard) value.

    If that's too bright, increase the resistor value.

    As I suggested in an earlier post, you might want to test the LED with the button cover, to see whether the colour is right, and what current gives the right amount of illumination.
  8. CoDi


    Dec 30, 2013
    Thanks again Kris! I am really impressed by your knowledge and quick response!

    I agree going with the LED. I do not have a problem at all in having the button lighted at all times.

    Oh no, me neither. I was just wondering if I could use some of the wires from the cat5 to connect the Nutone button to the transmitter. Would it be ok if I use 2 pairs for the LED and 2 pairs for the button?

    I am going to be testing soon and will report the results. Thanks again for everything!

    Happy New Year!
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Yes, you should be able to use two cores from the Cat 5 cable to connect the pushbutton to the transmitter. I thought you would be mounting the transmitter right beside the button...?

    Happy New Year to you too :)
  10. CoDi


    Dec 30, 2013
    I thought so too, but it might be too visible on the other side of the wall, so I was wondering if maybe I could do that in case I can not hide the wire.

    I will let you know how everything goes as soon as I have time for testing. Thanks again!
  11. CoDi


    Dec 30, 2013
    Sorry it took me a while to post but I managed to make everything work. It actually was very easy and probably the hardest part has been to hide the cables :) Also, I ended up using a 180 ohms resistor, the LED was too bright.

    Thanks again for all your help!
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