The unit I implemented can be operated in either of two modes:
1. Select from among eight different tone sequences
from a single doorbell switch.
2. Up to four doorbell switches, each with its own tone
sequence, for different doors/locations.
A combination of the two can be implemented if you
want to modify the code provided in the article downloads.
The circuit for the doorbell can be seen in Figure 1,
and is relatively simple. If diodes D0, D1, and D2 ARE
The three inputs of the PIC have their weak pullups
enabled; the software will invert the state of switches 0
through 2 after reading them. So, in this case, grounding a
switch creates a logical one.
The output circuit is a simple common emitter driving
the speaker. The PIC can drive up to 20 mA, but that’s not
enough to get a reasonable volume from a speaker.
The speaker I used is rated at 2W and its 92 dBA is
loud enough for our rather quiet home. Since it’s 8Ω, the
Build a Customizable Musical
When my wife and I moved into our current home a few years back, it didn’t
have a doorbell. We live on a quiet cul-de-sac, so it really wasn’t a problem
as we had few callers. After a while, I installed a wireless doorbell that lasted
about three years. Recently, after writing the series of articles on “A Digital
Analog — When a PIC Can Replace a 555,” I decided to make one using a PIC.
BUILD IT YOURSELF
By Larry Cicchinelli
22 September/October 2018