Wiring a Bell switch + LED to 746

[hsbhachu3d]

The idea is to wire the standard bell ON/OFF switch with a little 3mm LED inside the switch to flicker when there's an incoming call with the switch set to off. Basically, bell off, light on and light off, bell on.

The function of the LED is to work like the Neon light on some handsets. The advantage of this is, you can have a Warm white LED for an Ivory phone, Red LED for a Red one and Green LED for a Green phone etc.

I've attached a schematic that will help.

*request for sticky*

[Dave Moll]

That sounds a good idea. At least you still have some indication of ringing when the bell is silenced. I think I have a 746 with bell switch somewhere - or at least a spare microswitch and button I can add to a 746. I'll have to dig one out and give it a try. I know I have some spare LEDs.

[hsbhachu3d]

https://www.youtube.com/watch?v=BvVmUxoB8gA

[JohnBHanson]

Easy way is two leds in parallel but with opposite polarity, Connect this in series with the bell and in series with a 1k resistor and a 3.3k resistor.

Wire the on/off switch common pin to the junction of the bell and the
1k resistor. The other switch wires go to the other side of the bell and the other side of the 1k resistor


eg
1k 3.3k |--led+--|
-+-bell--+--/\/\/\/\/--+--\/\/\/\/\/-----+ +------
| | | |-+led---|
+---switch-----------+


Hope that helps

[Oldcodger]

Something oft forgotten is that as the ringing current is AC, then a capacitor can be used in place of a resistor,again using a diode in reverse parallel across the LED. Incoming ring to common contact of switch(before and between bell and bell capacitor). Normal contact to bell,other contact to other side of bell, via impedance capacitor and LED.
Bell circuit then becomes- switch normal- ring via switch normal to bell capacitor to bell and return.
Switch operated - ring via switch operated to LED &LED capacitor ,to bell and return .
I'd suggest a small rectangular LED.

It should be possible to do this on a 706 , using a small ( SAY 3mm Led) in a hole drilled on the switch .

[hsbhachu3d]

JohnBHanson The LEDs don't have to be paralel and reverse polarised since the ringing current is AC. Should there be DC, the bell will obviously not ring Lol.

Oldcodger Not really. It is a pain to remove the cover the next time if you dill the LED in. The switch is hollow inside anyway. If you see how I did it, the little 3mm LEDs in my video sit in the bottom of the switch. The casing can be taken off without fear that the LED wires will break off.I did consider rectangular LEDs but they cost about 10 times as much as standard 3mm LEDs. Also the capacitor and diode is not that bad of an idea but I'd still opt for resistors than drying capacitors being replaced over time.

[Nickthedentist]

Like Dave, I like what you've done.

But note that it is normal practice when running LEDs from an AC source to wire two in parallel, one each way round, and feed the pair via a cap.

In fact, some manufacturers have actually developed two LEDs and a cap in one package, and marketed it as an AC LED: http://www.ledsmagazine.com/content/.../LynkFig1b.jpg

Hope that helps.

N.

[hsbhachu3d]

Should the capacitor be polarised and does capacitance value matter with with what voltage you feed in?

[julie_m]

You can get away without the reverse parallel diode here because an LED has quite a low PIV (5 - 6V) and so provides a discharge path by doing a convincing impression of a Zener diode. The big series resistor limits the dissipation to what it can handle.

It still looks a bit like subtracting components to meet a price point, though .....

[hsbhachu3d]

ajs derby I did post a video in post #3 of my phone working with the LEDs.

[Oldcodger]

Originally Posted by hsbhachu3d

Also the capacitor and diode is not that bad of an idea but I'd still opt for resistors than drying capacitors being replaced over time.

I used a capacitor on a LED replacement for a neon on a cooker. Still working well several years later when we parted with the cooker.

Originally Posted by hsbhachu3d

Should the capacitor be polarised and does capacitance value matter with with what voltage you feed in?

In my case, I used a non polarised capacitor rated at 250v. The value was calculated from the impedance using the same formula as would be used to calculate a series resistor and then converted to capacitor value using the formula for calculating the impedance at mains frequency.