●●●●
BY DAVID WILLIAMS
BUILD A
JUMBO LED
DIGITAL
THERMOMETER
Anyone with a tropical fish tank knows that the water temperature must be
maintained within a relatively narrow temperature band – typically around
25°C (or 77°F), depending on the type of fish. In most home aquariums, this
is achieved using a thermostatic heater. When the temperature drops below
a preset level the heater switches on, and when it rises above a higher
preset, it switches off.
Many aquariums are also equipped with a thermometer, but it can often be difficult to see or
read accurately. This project was born from the need to
easily check the water temperature at a glance. It features
three 2. 2” large LED digits that are easy to read from
across the room and a precision DS1822 temperature
sensor. Temperatures can be selected to display in
Centigrade, as well as Fahrenheit.
The DS1822 is a direct-to-digital temperature sensor
with ± 2°C accuracy over a - 10°C to +85°C range. It is
presented in a compact TO-92 package and transmits
digital temperature readings using only three wires. It is
easy to encapsulate the DS1822 in silicon or epoxy for
a robust and waterproof sensor that makes it perfect for
this project.
Of course, a digital thermometer with a large LED
readout and a remote temperature probe is not just
limited to aquarium owners. This project will appeal to
anyone that wants accurate digital temperature
measurements.
Home beer brewers, hydroponic gardeners,
amateur weather watchers, or folks just interested in
energy management will all find this to be a very useful
device.
38 May 2010
Circuit Description
At the heart of the jumbo LED thermometer is an
ATMEL AT89C2051 microcontroller. It has been
programmed to handle a variety of functions, including
display multiplexing, temperature conversion, and seven-segment encoding. The AT89C2051 has 2K of Flash
program memory, 128 bytes of RAM, 15 I/O lines, one
16-bit timer, an analog comparator, and three interrupt
sources. It is fully compatible with the Intel MCS- 51
architecture and instruction set.
Even though the microcontroller is handling most of
the functions of the thermometer, some additional
circuitry is required. Refer to the schematic shown in
Figure 1. The circuit receives power from a 9-12V DC or
AC wall-mount transformer. The full-wave bridge DB1
ignores the incoming voltage polarity if it is DC or else
rectifies it to DC if it is AC. The voltage is filtered by
capacitors C7 and C4, and then regulated to five volts DC
by U1. Capacitors C6 and C3 provide additional filtering
for the five volts.
Capacitor C5 and resistor R10 provide a RESET signal
to U5 at power-on. The three LED displays (DISP1-3) do
not require a regulated DC voltage, so they are driven by
