Difference between revisions of "Doorbell"
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LEDs on an AC supply
Revision as of 12:06, 12 January 2009
This article covers the choice and fitting of wired doorbells.
- Wired doorbell
- Wireless doorbell
- Mechanical doorbell
- Door knocker
Doorbell kits are easy, as you don't need to worry about voltage, ac or dc, regulation or anything else. You even get all the bits in one pack. If you're not fussy, pre-packed kits make life simple. Kit systems tend to be basic, and a lot of the sections about extra options in this article won't apply.
Self assembled systems
Assembling your own system from diverse parts is more complex, but not difficult. And of course its much more flexible, with more choices and various extras available.
You get a choice of
- Chime (mechanical)
- Electronic sounder
- Buzzer (mechanical or electronic)
Mechanical sounders can produce voltage pulses on the cable, so mixing electronic and mechanical sounders on one circuit is not best practice, and may kill the electronic sounder.
As a general rule of thumb:
- Bells are the loudest
- Chimes are medium volume, but much more pleasant to the ear
- electronic sounders can produce various sounds, but generally aren't so loud.
More sounder options
Fire bells and very high output electronic sounders (eg) can be used outdoors to cover a large area, or indoors for the hard of hearing. These need to run off a bigger power supply than the usual door bell.
Ding-dong chimes can be confused with similar sounds on TV, especially if a person's hearing isn't good. A simple resolution to this is to swap the 2 bars, changing the sound to dong-ding.
Piezo electronic sounders produce high frequencies, and are less easy for people with poor hearing to hear. Sounders with a wide range of frequency output are more likely to be heard.
Use of more than one sounder enables each to be best positioned for audibility in different house areas. This is another good option for the hard of hearing.
When more than one bell circuit is used, a different sound for each one differentiates which bellpush is being pressed. Historic bell systems used to use indicator panels, but with different sounds available and multiple sounders affordable this is no longer necessary.
For very large systems its possible to use electronic sounders that produce different sounds depending on which input is activated.
Adding another sounder
Its possible to mix mechanical and electronic sounders, but this can create issues that can result in the electronic one dying or bahaving not so well. Adding mechanical to existing mechanical or electronic to existing electronic is a simpler option.
Bells and dingdongs can be synchronised by running the two in series rather than parallel. This sounds rather better, but raises minor electrical issues.
The mechanical interrupt switch on one of the sounders is wired over (shorted), so that the action of both sounders is dictated by the same interrupting switch. The supply voltage needs to be increased (usually doubled) to run the 2 in series.
In most cases this will work off the bat, but if the mechanical speed of the 2 sounders is significantly different it can fail to work, due to the slower one failing to hit the bell as it tries to move at the speed of the faster bell. If this should happen, wire across the switch mechanism on the faster bell and use the switching mechanism of the slower one dictate the switching speed.
AC only bells with no switching also exist. These use a hammer vibrating at mains frequency. If used in series, these high speed bells and a low speed interrupting bell should not be mixed, else they will perform badly together.
The bell push is just a momentary switch. It doesn't matter whether the switch goes in the -ve or +ve feed to the bell.
Since it switches low voltage at not much current, the switch can be pretty much any type of momentary push-to-make switch. Custom switches are perfectly DIYable where a character bellpush is wanted.
Bell push switches are routinely low quality. Various better quality commercial momentary press switches are available in the larger electrical accessory ranges, eg MK, including:
- standard plateswitch style
- architrave switch
- weatherproof/external mains
- momentary pull-cord switches
Rubber tubes connected to a pneumatic switch are used on some petrol station forecourts, to ring when a vehicle drives over it. These can be useful on the main property entrance(s) for larger properties, though they're not vandal proof.
Over-door switches are available for shop doors.
On circuits with multiple bellpushes or sounders, on/off switches can be inserted into the system to optionally mute one or more buttons or bells. In the circuit digram here:
- Sw1 silences one bell on one circuit
- Sw2 silences both bells on one circuit
- Sw3 silences one button on one circuit
Heavy steel cased switches prevent physical breakage.
Connecting a large capacitor (4700uF) in series with the bellpush (and a leakage resistor across the capacitor) prevents the bell being stuck on. When the switch is pressed the bell will then only ding once; if the switch is wedged in the pressed position it will sit silently after the first ding. This is unsuitable for noisy environments or hearing impairment, where more than a single ding is required.
Making your own switch housing is an easy way to replace a piece of plastic with a quality feature. Numerous designs, materials and construction methods are possible. Wood is an easy and minimal cost material to use, which can be worked to a wide range of styles. It may be sawn & planed or sanded, carved with basic Hand Tools, or turned with a lathe, or a even a drill & screw & grinder
Overuse of the bellpush often occurs if the caller can't hear the sounder, and this can be quite annoying. Ensure the caller can hear a sounder, whether the main one or an additional one.
Many bellpushes have a low power filament bulb inside, which lights when the switch isn't being pressed. This connects across the switch contacts. Such lamps are generally intended for use with mechanical sounders, if used with electronic ones they may cause continuous sounding.
Filament lamps are generally short lived, and for a bellpush they should be run well below rated voltage to achieve long life. This inevitably gives them a yellowy light colour. If desired this can be countered by using pastel tinted paper for the printed name instead of white card (but not all colour papers are light fast).
LEDs aren't used in most bell systems, but Ultrabright LEDs are a good long life replacement for bellpush filament lamps.
- LEDs are available in several colours as well as white
- LEDs last in the region of 50,000 hours if run well below max rated current
- they cost 10s of pence
- they consume miniscule power.
- mechanically robust, unlike filament lamps
- To illuminate a small paper sign or a backlit plastic ring, choose an ultrabright LED
- when the LED is viewed directly rather than used to light something, either choose high brightness or use ultrabrights with a higher resistor value
- Common indicator LEDs are not suitable.
- don't treat these industry standard LED terms too literally
A dc supply is used with LEDs, with a series resistor to limit current. When using LEDs with a mechanical sounder, a reverse connected ultrafast diode across the switch contacts is also required to avoid premature LED failure. LEDs with at least 1,000 mcd output are recommended.
+-----|>|-----+ Either 2 LEDs or a LED -----| |-----^^^^----- and a diode are used +-----|<|-----+ LED Resistor 1 or 2 LEDs on an AC supply
There are many more LED circuit options, including simple circuits with colour control. See LED Lighting#Ballasts
For maximum brightness, the following resistor values give around 20mA:
- for an 8v dc supply:
- blue & white: 220 ohms
- green: 270 ohms
- red, orange, yellow: 330 ohms
- for an 8v ac supply:
- blue & white: 330 ohms
- green: 470 ohms
- red, orange, yellow: 470 ohms
- for a 6v dc supply:
- blue & white: 100 ohms
- green: 150 ohms
- red, orange, yellow: 220 ohms
- for a 6v ac supply:
- blue & white: 150 ohms
- green: 220 ohms
- red, orange, yellow: 330 ohms
LEDs are generally best run at reduced current to achieve long life, use a higher resistor value than above to achieve this.
At 20mA a LED will consume 40-80mW, or 0.35-0.7kWh per annum, at a cost of 3.5-10p per annum. At 10mA a LED only costs 2-5p per annum to run.
LED brightness can be reduced by using higher resistances. Twice the resistance above gives around half the output and longer LED life.
Pink LEDs deteriorate rapidly, and aren't recommended. Pink can be obtained by mixing red & white LEDs, or red & blue.
A snubber is not usually fitted, but if you put time into making a fancy switch it makes sense to take another minute or 2 to make it reliable & long lived.
Improvement in switch reliability and longevity is gained by fitting a snubber across the switch contacts. This also eliminates one of the causes of interference to audio equipment & digital TV.
Snubbers make more difference with mechanical sounders, which are invariably inductive loads, and thus hard on switch contacts.
Mains voltage snubbers aren't effective for low voltage use. More suitable component values would be:
- for dc systems: a 15-22 ohm resistors plus a 10uF capacitor.
- for ac systems: 15-22 ohms & a 1uF non-polar capacitor.
A filament bulb in the bellpush acts as a snubber, but these are often not fitted, and when they are they're often not replaced when they fail. Snubbers last for life. LED lights in bellpushes don't act as snubbers.
Mains | | ____|__|_____ | | ______ | Transformer |------------------------| | | | | Bell | | |------+ +------|______| |_____________| | | | | | | | | | | | | +--Switch--+ Basic system diagram
The basic circuit can be optionally expanded as follows:
Mains | | ___ ____|__|___ | | Bell Bell Lamp Bell Bell | |-|~ +|--+----------+-------+-------+-------+--------+ | | | | | __|__ __|__ | __|__ __|__ | | | | | | | | | | | | | | | | | | === C | | | | (_) | | | | | | | | | |_____| |_____| | |_____| |_____| | | | | | | | | | | |___________|-|~ -|--+ Sw1 | +-------+--------+ |___| | | | | Transformer | +-------+ | BR | | | | Sw2 | | Light | Light | | / \ | / \ | +--Switch--+ +--Switch--+ | \ /| | \ /| | Snubber | | Snubber | | | | | | Sw3 | | | Light | | Light | | / \ | | / \ | +--Switch--+ +--Switch--+ | \ / | \ / | Snubber | Snubber |_______________________| With optional extras
Batteries are only workable with unlit bellpushes. Use of an electronic sounder makes a battery last much longer.
The modern fashion is to cut manufacturing costs by using AA or PP3 batteries. These work, but have much shorter lives, and you end up with repeated battery replacement and lots of missed calls. Over the long term, small batteries are a false economy.
Kit Power supplies
The power supply in a kit will match tbe sounder.
Other power supplies
If assembling your own system you'll need to do a bit of thinking re the power supply.
Standard bell transformers can be used with a lot of sounders, but not all. These are ac output transformers, typically 5-8v. Mechanical sounders usually use these. They're the simple choice if your sounder will accept the ac voltage the transformer produces. If it won't, the ac may kill the sounder. The next section explains supplies for sounders requiring dc, and how to run any sounder on a wallwart. Ac sounders also run happily on dc (often requiring a bit less voltage on dc).
For dc wallwart supplies:
- for electronic sounders, use a wallwart of the sounder's rated voltage
- for mechanical sounders, use a 1A wallwart of 3v above the sounder's rated voltage. Adding a 2200uF-4700uF 16v (or more) capacitor across the transformer's outputs (connecting + to +, - to -) can improve reliability & volume.
The above rule of thumb will work fine in nearly all cases.
DC power supplies explained
If you want to understand power supply choice properly, and pick the optimum supply, here's the deal:
Electronic sounders use low power, and will run fine off the rated voltage of dc supply. No extra capacitor is required.
Mechanical bells and ding-dong chimes use a lot of current when they're sounding, well above the rated current of your average wallwart. This isn't a problem for the wart, since it only delivers this current for a tiny percentage of the time. However it does have implications for the sounder. The current draw results in the wart delivering well below rated voltage during sounding, due to transformer copper losses. This drop is partly made up for by picking a higher voltage wart.
An added issue is that mechanical sounders suffer from stiction, and some require an initial current & voltage kick to get them moving freely, so they work well & give good volume. The capacitor delivers this, by charging to above on-load voltage and delivering this to the bell for a very brief moment when the bellpush is operated. This is entirely harmless to mechanical sounders.
Most wallwarts deliver well above rated voltage when off load. This doesn't matter to mechanical sounders since they're not connected, and a momentary overvoltage when connected is beneficial rather than harmful. Light bulbs however do care very much about voltage, and the bellpush bulb should be rated at or above the voltage the wart delivers when offload. A multimeter will show the off load voltage. Hence the bulb and bell should have different voltage ratings, yet both be powered by the one supply.
Bell transformers that mount inside a CU (consumer unit, aka fusebox) are available. They cost more to buy & more to fit.
Options to differentiate which bellpush was activated:
- use 2 separate bell circuits with different sounds
- add a capacitor to one bellpush so one of them only gives a single ding
- more complex options are also available involving electronics that store and indicate whch switch was last activated.
- Victorian mechanical indicator boards can be used, or new ones made. These electromagnetic mechanical indicators use 2 switch circuits feeding one bell circuit. A simple electrical version can be made with a bistable relay and 2 lamps or an electromagnetic pointer.
Bell wire is the cheapest wire that will do the job, and is the usual cable used. In large properties, long runs of bell wire can affect operation with mechanical sounders, the solution to this is simply to increase the power supply output voltage to compensate.
Standard 4mm bell wire staples are used to fix the cable to the wall. Since its a low voltage system, there's no requirement for insulated fixings, and any type of staples may also be used.
Polarity doesn't matter for ac systems. With dc sounders it does. Bell wire has a tiny ridge moulded along one edge, and this is used to maintain correct polarity throughout the wiring.
Although door bells are low voltage circuits, mechanical sounders produce higher voltage pulses on the cable, so use of bare or enamelled wire is not recommended. For the same reason, mixing electronic and mechanical sounders on one circuit is not recommended, and can sometimes kill the electronic sounder.
If you live alone, keeping the receiver in your pocket ensures you don't miss calls. Some wireless sets are specifically designed for this.
Bell circuits are very simple, and should present no difficulty if you have a multimeter to see what's going on where. The one possible surprise is that water in the bellpush can sometimes pass enough current to operate an electronic sounder.