8 February 2017
Q & A n WITH KRISTEN A. McINTYRE
A Visual Doorbell
QI have a relative who is hard of hearing, and I would like to create a circuit that would enable his doorbell button to
light or maybe even flash a table lamp. I
have measured the voltage at the doorbell
and it reads 16V AC on my meter if that
helps with the design.
AThis is a great question for me since I’m also hard of hearing. I wear hearing aids, after a somewhat unpleasant series of operations on one of my
ears. By the way, hearing aids are a great thing and have
made my life better in many ways. If it’s viable, please
encourage your relative to try them. Mine even connect
to my iPhone and beam phone calls, music, and podcasts
directly into my brain!
Hearing is a very important sense, but one that can be
supplemented with visual cues. It should be easy to control
a light with the doorbell, but we may need to think a little
bit about how to make it effective. I’m going to go through
a few designs of increasing complexity to demonstrate how
I think about problems like this.
The naive approach is to just take the doorbell signal
and rectify it so we can use it as a control signal. The
rectification doesn’t have to be efficient, but just enough to
be able to sense the state of the button. Controlling some
other thing in the house (like a lamp) can be done with
either a traditional mechanical relay or with a more reliable
solid-state relay. Let’s assume it’s going to be something
connected to the AC mains.
An example of this first approach is shown in Figure
1. In this case, we just rectify the bell signal, and hope
we have enough current / power available to switch a
relay. That probably works just fine. We choose a 24V
coil because the peak voltage for 16V RMS AC is 16 x √ 2
= 22.6V. There’s a slightly more sophisticated approach
shown in Figure 2.
We take the AC signal, pump charge into a capacitor
with a bleed down resistor, and then switch a transistor.
That buffered signal is the control
for some kind of relay (a mechanical
relay is shown). Note the snubber
diode used to keep the relay coil’s
inductance from destroying the
control transistor when the relay is
turned off. A solid-state relay won’t
You will need a power supply
for this second approach, of course,
and it needs to be isolated from the
doorbell. Most wall warts are isolated.
The doorbell signal could be further
In this column, Kristen answers questions about
all aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest to
the hobbyist. Feel free to participate with your
questions, comments, or suggestions. Send all
questions and comments to: Q&A@nutsvolts.
• A Visual Doorbell
• On Driving LEDs
•Text Messaging When in Need
n FIGURE 1. Simplest
n FIGURE 2. More
sophisticated active control.