94 Silicon chip Australia’s electronics magazine siliconchip.com.au
Commercial clock radios are aw-
ful. The cheap ones can’t even keep
the time accurately. If you spend a
bit more, you get accurate time, and
may even get battery backup, but the
sound quality is still horrible and they
often don’t have basic features like
dimming.
They also use 1980s style 7-seg-
ment displays. You need to spend a
lot of money to get one with decent
sound quality.
This clock radio has very accurate
timekeeping, battery backup, tempera-
ture measurement, an FM stereo tuner
with a powerful stereo amplifier and
stereo speakers.
It has automatic dimming, eight
station presets and is very power-effi-
cient. It features a colour touchscreen
with nice fonts for the clock, making
it easy to set the clock and program
the presets.
It is based around the Micromite
LCD BackPack V2, although I used the
original BackPack board and added the
new backlight circuitry.
The circuit is very simple because
it makes use of four modules that can
be cheaply purchased from eBay or
other similar sites.
The first one is an LM2596-based
DC-DC converter (MOD4). Initially, I
used a linear regulator, but the voltage
drop from 12V to 5V and the amount of
current required to drive the backlight
meant that it got very hot and wasted
a lot of energy.
These modules are available from the
Silicon chip Online Shop (siliconchip.
com.au/Shop/7/4916). The module is
adjusted using the onboard trimpot to
give a 5V output.
The DS3231-based real-time clock
module (MOD2) is extremely accu-
rate and has battery backup and an
internal temperature sensor that can
be read through the I^2 C bus.
This is the same module that was
used in the Silicon chip Super Clock
project (July 2016 and updated in
July 2018) and described in the Octo-
ber 2016 “El Cheapo Modules” article
(siliconchip.com.au/Article/10296).
The modules are designed to use a
rechargeable button cell and have a
built-in charger circuit. This is bad for
standard lithium batteries (CR2032),
so the resistor just above the SCL la-
bel should be removed. This mod-
ule is available from the Silicon chip
Online Shop (siliconchip.com.au/
Shop/7/3519).
The FM radio module (MOD3) is
based on a TEA5767 IC. It is a PLL-
tuned FM stereo receiver. It is very
small and difficult to solder, but this
can be made easy if a PCB is created
with 10 pads to suit the module. It is
also controlled using an I^2 C bus.
The stereo amplifier (MOD5) is quite
small but contains a stereo Class-D am-
plifier which can deliver about 10W
per channel. This seems like a lot of
power, but small speakers suited to
clock radios are really inefficient.
These two modules are available
from the Silicon chip Online Shop
(siliconchip.com.au/Shop/7/5024 &
siliconchip.com.au/Shop/7/5025)
The whole circuit is powered by
a 12V switchmode plugpack. Only
the amplifier module needs such a
high voltage, so the DC-DC converter
drops the voltage to 5V for the rest of
the circuit.
The 3.3V regulator on the BackPack
provides the 3.3V rail. To improve ef-
ficiency, the amplifier module is only
powered when required, so its 12V
power is switched by relay RLY1.
The FM radio module’s analog out-
puts are fed to the amplifier module
through a 4052 analog switch, IC1.
This allows the micro to feed multi-
ple different sound sources to the am-
plifier: either from FM radio, a stereo
auxiliary input, the alarm sound from
the micro or no source.
Sounds are fed to the amplifier mod-
ule via 100nF AC-coupling capacitors.
This relatively low value was deliber-
ately chosen to limit the bass through
the amplifier. Small speakers can go a
lot louder if you don’t try to get them
to reproduce bass, and you don’t needheavy bass when you wake up in the
morning.
The amplifier output is fed into a
crossover network before going to the
speakers, to allow separate woofers and
tweeters to be used. I used Jaycar Cat
AS3034 3-inch woofers and some old
ribbon tweeters that I had, although
any small tweeter would be suitable.
The ambient light sensor is a light
dependent resistor (LDR1). I used Jay-
car Cat RD3485, although others could
also be used. This forms a resistive di-
vider with a 27kW fixed resistor and
the resulting voltage is fed to an analog
input on the BackPack module.
The software calculates an appropri-
ate level of backlight brightness and
updates the PWM duty cycle on pin- On the V2 BackPack, this controls
the backlight brightness.
The Micromite has native support
for the real-time clock module but ad-
ditional code has been added to read
the temperature. You can download
the MMBasic source code for this pro-
ject from the Silicon chip website.
I originally tried adding a Bluetooth
receiver to the circuit, but eventually
gave up. There are many cheap Blue-
tooth receivers available both locally
and from China that produce stereo
signals that can be connected to the
external input.
This circuit could be installed in
an old stereo radio cassette or clock
radio. I made a custom case from sty-
rene, resulting in a very compact unit
that only uses a small amount of space
on a bedside table.
Dan Amos,
Macquarie Fields, NSW ($90).
CIRCUIT NOTEBOOK
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paid for at standard rates. All submissions should include full name, address & phone number.Touchscreen clock radio using a Micromite BackPack