Clear as a bell

Our house has a doorbell consisting of a set of the traditional (some might say “old fashioned”) “Ding Dong” chimes, triggered by a pair of hardwired bellpushes at the front and back doors. Rather than eating through lots of sets of batteries there is a small 12v mains transformer that powers the whole setup. Since the system is mains-powered, it’s possible to support bellpushes that contain a very small incandescent bulb, wired across the contacts which “glow” all the time, making the bellpush easy to find at night.

Except they don’t. Although the life of incandescent bulbs is inversely proportional to their voltage, most tiny 12v bulbs only last a few thousand hours; if we assume about 5000 hours, that’s less than seven months in this application. Worse, whenever someone presses one of the bellpushes the current through the bulb is snapped off, and then back EMF from the coils in the chimes unit will drive an even bigger transient voltage through the bulb when when the bellpush is released – which shortens the bulb life even further. My last replacement lasted less than 6 months.

To add insult to injury, you don’t seem to be able to easily buy replacement bulbs – only much more expensive complete replacement bellpushes. And changing just the little bulb is a fiddly job which further encourages you to just buy a complete replacement bellpush. No wonder most people quickly give up and leave them “unlit”.

So I decided to go solid-state, and upgrade my existing bellpushes to LED lighting. The basic idea is to change the little incandescent bulb with an LED, which will last at least 50,000 hours. However, there are two problems with this approach that we need to overcome:

  1. Since the actual chimes operate by way of electromagnets, the voltage in the system is alternating current
  2. The back EMF from the coils is going to generate a much larger voltage than the LED can cope with

LEDs normally operate on a maximum of a couple of volts. However, it is possible to buy LEDs with a suitable ballast resistor integrated into the LED package to allow them to work directly from a 12v DC supply. Two of these, connected back-to-back, can be used to replace the incandescent bulb. One of the pair will glow on each phase of the AC. Actually, they’ll each flash 50 times a second, but we just see a glow, which is good enough. Unfortunately the transient voltage spikes (back EMF from the chimes) in the system would completely destroy the LEDs when the bellpush is first pressed.

To avoid this, it is possible to take advantage of a Voltage Dependant Resistor (VDR). This is a simple passive component whose resistance varies according to the voltage placed accross it. When the voltage is below a rated value, it presents a very high resistance – essentially it is an insulator. When the voltage rises above that rated value its resistance falls rapidly, becoming a conductor, and allowing current to flow through it, preventing excess voltage from forming across it.

By wiring a VDR rated at 12v across each coil, the voltages across the coils can be clamped to a maximum of 12v, preventing any back EMF voltages from forming in the circuit. The result, in my case, is a pair of bellpushes that glow a gentle blue colour, and should continue to do so for years to come.

This circuit diagram should make things clearer; the bits in the “dotted T” outline are in the chime unit:

Doorbell circuit diagram

Also refer to this photo from the HowStuffWorks site to see how a chime unit works mechanically:

Inside of chime showing solenoids

The pistons of the solenoids rest just above the “dong” tone bar (at the bottom). When energised the pistons push upward against springs. One (the right hand one in the above picture) is able to contact the “ding” tone bar (at the top), the other is not – there is a plastic buffer to prevent that. When de-energised, springs force the pistons to rebound back against the “dong” tone bar, before they settle into their resting positions. So one solenoid causes a “ding-dong” sound, the other only a “dong” sound.

Finally, refer to Wikipedia to understand how the solenoids create back EMF when they are de-energised and the springs push the pistons back through the coils to their original location.

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