Wednesday, April 20, 2011 (updated 3/18/2012)
Pimp My Doorbell Button
appliances draw a tiny bit of power all the time in exchange for a
convenient or useful purpose. For example, a cordless phone with base
station, a TV with remote control or the microwave oven with its glowing
clock and fancy Lieutenant Commander Data console touch buttons.
Now don’t get me wrong. Doorbell
lights are helpful, convenient and luxurious. Not only do they
pinpoint where the doorbell is located, but pushing the button turns
the light off, indicating to the visitor that the doorbell rang
inside. Oooo fancy!
"But why would I want to aid the
Kirby salesman at being able to reach me better"? This is a
discussion for a later blog topic, "How to electrify door-to
door Solicitors with 24,000 Volts while still keeping little Jimmy
and Mrs. Snow (from down the street) out of harm’s way".
When I find that an appliance in my
house draws power all the time for no good reason, its gets shut off.
For example, the sprinkler timer and A/C compressor during the winter
As I near the end of my list of phantom
power consuming appliances, I’m starting to look at the efficiency
of these devices. Why should a TV draw 20 watts in standby when it
could provide the same function drawing less than 1 watt?
One night while turning off the porch
light, I noticed that the doorbell light was providing quite a bit of
light all by itself. It seemed like overkill that this doorbell
indicator was acting more like a beacon of hope for tempest tossed
sailors than a doorbell light.
A doorbell is a simple circuit. It has
a low voltage transformer that steps 120 volts AC down to about 21
volts AC which then runs through low voltage wiring to the doorbell
switch. The doorbell switch has a small incandescent bulb wired
across it. The tiny light bulb provides light but has too high of
resistance to actuate the solenoid. When the suspecting visitor
pushes the button, the circuit is closed, shorting out the light and
energizing the solenoid.
The solenoid has a small hammer that
compresses a spring and smacks into a "ding" bar, (I just
made that up). When the switch is released, the solenoid de-energies
and the hammer flies back the other way into a "dong" bar,
(ha ha, I’m such a dork).
During the "ding-dong"
sequence the entire circuit draws over 2 amps from the 21 volts AC
provided by the transformer. The total power consumption is about 50
watts but only lasts the length of "ding dong", so who
Even with all my fanaticism, I don’t
lose any sleep over a dinky 50 watt ding dong. If someone were to
ring my doorbell for ½ a second, 20 times a day, every day for a
year, aside from me trying to kill them, the total time spent ringing
the doorbell would only add up to 0.5*20*365 = 3650 seconds or ~1
hour per year. At 50 watts, that would only cost ½ cent per year.
What concerns me is the power draw of
the incandescent doorbell light itself. The continual power draw of
my doorbell (with light) is about 4 watts. That might not seem like
much but if you consider that it runs 24-7 all year long, that adds
up to 35KWH annually and costs $3.50/year.
"That’s still not a big deal"
you say? Well, saving a little energy everywhere really adds up.
After all, "Find a penny, pick it up, all the day you’ll be a
scrounge. A scrounge who just made a penny that is".
The heady days of the incandescent
indicator peaked around the time of the original Star Trek series.
Today the indicator light of choice is the LED.
My original LED doorbell design started out as a simple
white LED with a current limiting resistor. That works okay but since
the LED is being powered off of alternating current, it flickers at 60
Hertz. I don’t know about you but that annoys the crap out of me. It
might even send poor aunt Edna into an epileptic seizure each time she
steps foot on my porch. I must rectify the situation.
I modified the circuit to include 4 diodes and a 3.3uF capacitor. 3.3uF was chosen for exactly 3 point 3 reasons:
- I already had one
- It's large enough that the LED won’t flicker and
- It's small enough that the LED will go out when the button is pushed.
To my dismay, my circuit caused the doorbell circuit to ring continually with an annoying solenoid buzz.
In Chinese Accent: "Confucius say: A ding with no dong is dung".
Adding a 1.8K resistor in series with the rectifier stopped the solenoid from triggering.
For comparison, I measured the current draw of both lights.
| Incandescent Light||21.8VAC ||75mA ||1635mW |
| LED Light|| 21.8VAC|| 3mA|| 65mW|
Less power draw on the inefficient low voltage transformer
and less current leaking through the solenoid resulted in even more
power saved. The 1.5 watt savings from swapping out the
incandescent light nearly tripled into 4 watts when the power
consumption was measured on TED.
You may say that 4 watts is a trivial
power reduction. That may be true, but the sailors sure are furious.
I swapped out the old transformer with
a switching supply that I had lying around. After some
experimenting, I learned that the solenoid that drives the
"ding-dong" bars doesn't care weather you give it DC or AC.
This is a good thing because switching supplies are DC and not only
that, they are way more efficient than a buzzy old transformer.
I didn't exactly have a switching
supply lying around that put out the right voltage. I did have a tiny little 9V 1Amp
supply. I opened it up and quickly found a 27KΩ voltage reference
resistor (surface mount) that controlled what the output voltage
should be. After some trial (and lots of error), I arrived at a 68KΩ
resistor value to give me approximately 20 volts DC out. I also added an additional capacitor to beef up the surge current rating of the supply, helping it to drive the solenoid.
It was a really easy mod to do to a $3.00 power supply from DealExtreme
. I can make it whatever voltage I want from 3V - 25V, simply by changing out the reference resistor. While it is possible to go higher in voltage, the electrolytic caps (and maybe a transistor) would have to be swapped out with high voltage ones.
Comparison of the old transformer (right) and the new switching supply (left).
My doorbell is actually louder now than it was before. I think DC is causing the clapper to hit the bars harder than it was before.
I should also note that I removed the bridge rectifier and filter cap circuit that I put in the doorbell switch previously. The 20VDC drives the resistor and LED directly without any adverse flickering. The LED still goes out like normal while the button is pushed.
All in all, I shaved off an additional 1.6 watts of standby power.
"But John, isn't going to all this effort for a lousy 1.6 watts a little fanatic?" Absolutely! But I am on a quest for the lowest phantom power possible.