3

FORGE

mark off the intermediate positions. Use this

information to calculate how much cable is required

between each sensor. Also work out where you will

mount the Raspberry Pi (somewhere easily visible

and near a power supply), and how much cable is

needed between the Pi and the first sensor.

PREPARING THE HARDWARE

The next step is to solder the header pins to the

Raspberry Pi. The Blinkt! LED module is ideal as

it’s nice and easy to connect to the Pi, and the

existing Python library makes setting the colour and

intensity of each individual LED really simple. The

downside is that it covers all 40 GPIO pins, making

it difficult to connect anything else.

Even though it covers all the pins, it actually only

uses four of them – numbers 2 and 6 for 5 V power

and ground, and 16 and 18 for LED control.

The DS18B20 digital temperature sensors

run off the 1-Wire protocol. This means all data

communication is transmitted on one wire, no matter

how many sensors are connected. The Raspberry Pi

can read these signals on pin number 7 (GPIO 4). The

sensors also need a 3.3 V and ground connection,

which are taken from pins 1 and 9 respectively.

By pushing pins 1, 7, and 9 of the header through

the plastic strip before it’s soldered, the three

pins required for the temperature sensors are left

sticking out of the back of the Pi, while the other

37 stick forwards in the normal way. The Blinkt!

module is connected to the front, leaving the

three connections for temperature sensors easily

accessible behind.

WIRING THE SENSORS

The sensors used here are pre-wired, with

red (+3.3 V), black (ground), and yellow (data)

cables attached.

A 4.7 kΩ pull-up resistor is required between the

data wire and +3.3 V. After this, it’s a simple matter of

daisy-chaining the sensors together, connecting red

to red, black to black and yellow to yellow, each time

extending the chain by the length required to place

the next sensor further down the tank.

Above

Eight sensors are fitted at equal intervals from the top to the

bottom of the hot water tank

YOU’LL NEED

Raspberry Pi

Zero W (but

not the latest

version with pre-

soldered header)

Case

Power supply

40-pin 2 × 20

male header

Blinkt!

8 LED array

8 × DS18B20

temperature

sensors (it’s

easiest to buy these

pre-wired in a

waterproof casing)

4.7 kΩ resistor

30 g tube

of thermal

compound paste

As the hot water is used up, the level of cold
water rises up the tank. When the cold water
reaches the top, that’s it: cold showers all round.
This project works by having a series of
temperature sensors fitted to the side of the tank
at regular intervals. By connecting these to a
Raspberry Pi, the rising level of cold water can be
measured – so you can keep an eye on your hot
water consumption and put an end to unnecessary
cold showers.

MEASURING UP
The Blinkt! LED module has eight LEDs, so this
project uses eight sensors – each sensor controls
the colour of one LED, changing it from red when
the water is hot, to blue when it is cold.
The first step is to determine how far apart the
sensors need to be. The top sensor should be
close to the outlet in the middle of the cylinder.
The bottom sensor should be about 5 cm above the
point where cold water enters. Mark these points
on the outside of the insulation with a marker pen.
Measure the vertical height from the top sensor
location to the bottom one, divide by eight, and

Using a Raspberry Pi Zero and a series of

digital temperature sensors, this project

will show you exactly how much hot water

remains in the tank

”

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