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CIRCUIT NOTEBOOK
Interesting circuit ideas which we have checked but not built and tested. Contributions will be
paid for at standard rates. All submissions should include full name, address & phone number.
Automated tyre inflator/deflator
ing winter, then as the ambient temperature increases going into summer,
they can become overinflated, which
could lead to the tyre popping in extreme cases or suffering excess wear.
This circuit takes the guesswork out
of the equation; rather than letting the
air out, then re-inflating them to the
correct pressure, it does it all for you.
The main components are a microcontroller, 0-100PSI pressure sensor, solenoid driver, motor driver and
LCD screen. Pushbuttons S1 & S2 are
used to set the desired pressure, which
is shown on the LCD screen. It then
checks the tyre pressure and if it is
higher than the set pressure, actuates
the pressure release valve solenoid
until the tyre pressure drops to the
set pressure.
Alternatively, if the tyre pressure
is too low, the motor driver is activated to power up a 12V compressor
which then raises the pressure until
it reaches the set point. The motor is
then switched off.
The prototype was built on a Curiosity
development board.
►
The 0-100PSI pressure sensor is
connected directly to the pipes near
the hose connection point, although
an onboard sensor can be used
instead.
►
siliconchip.com.au
Australia’s electronics magazine
►
I decided to build my own digital
tyre inflator, mainly because pretty
much all the affordable ones I found
online could only increase the tyre
pressure, not decrease it. Sometimes
you just want to dial in a pressure
value (in PSI, kilopascals or bar) and
let the device do all the work for you.
It runs off a 12V battery, so it could
even be kept in your car and used on
the road.
Keep in mind that if you keep your
tyres inflated to the right pressure dur-
The final build uses a custom PCB
design and a significant heatsink. This
all mounts closely to the motor, pipes
and valves.
December 2020 85
In either case, when the tyre pressure reaches the target, the piezo buzzer sounds to let you know.
In terms of the air plumbing (not
shown), the following are all connected together via threaded pipes and
joiners: the air hose, 12V compressor,
pressure sensor and pressure release
valve. Make sure all the pipes and connectors are gas-tight; use yellow Teflon
tape on each set of screw threads. The
air release valve is not shown on the
circuit; it is a readily available type
and connects to the terminal marked
“VALVE” near the bottom.
86
Silicon Chip
The output from the absolute pressure sensor at upper left is 0.5V at 0
PSI, 2.5V at 50PSI and 4.5V at 100PSI.
This signal is reduced via a resistive
divider to be within the 0-3.3V range
that the PIC24 micro can handle, then
filtered and buffered by IC3a, part of
an MCP6004 quad rail-to-rail op amp.
It is then fed into one of the PIC24’s
analog input pins, AN6 (pin 25).
The output from an optional second pressure sensor (eg, ambient)
goes to AN7 (pin 24), while the output of an analog temperature sensor
(IC4) is similarly fed to AN8 (pin 23).
Australia’s electronics magazine
This could be used to compensate the
pressure setting for variations in ambient temperature, but you would need
to change the software. Analog input
AN9 (pin 26) is used to sense the battery voltage, for the under-voltage cutout to protect the battery.
The air release valve is powered by
Mosfet Q4, which is controlled via an
opto-isolator for simplicity. It is driven
from one output of MCP23S08 I/O expander IC2, as many of IC1’s pins are
occupied driving the display.
Similarly, the motor is driven by a
parallel pair of high-current Mosfets
siliconchip.com.au
(Q2 & Q3), which are in turn driven by
Mosfet Q1. It is controlled by another
opto-isolator, this time driven from the
RA0 output of IC1 (pin 2). There is no
need to reverse the motor, so this part
of the circuit acts as a switch.
The I/O expander (IC2) also handles
sensing when pushbuttons S1 and S2
are pressed, along with controlling the
air release valve, piezo buzzer, some
of the low-speed LCD signal lines plus
indicator LED2.
Circuit power comes from a 12V battery which must be able to supply a
siliconchip.com.au
substantial amount of current to drive
the compressor motor. Power for the
rest of the circuit goes via protective
fuse F1 and reverse polarity protection
diode D1, with the piezo and pressure
relief valve running off that 12V rail
(it’s also fed to IC3d for battery voltage sensing).
The rest of the circuitry runs off 5V
or 3.3V, derived from the 12V supply
by low-dropout linear regulators REG1
& REG2. An in-circuit serial programming (ICSP) header is provided to allow microcontroller IC1 to be re-proAustralia’s electronics magazine
grammed in-circuit, while the COM1
serial port header is provided for debugging purposes.
The original prototype was built
by hanging the various modules off
a PIC24FJ256GA7 Curiosity development board from Microchip, while my
final version uses a custom-designed
PCB. The PIC24 firmware files (source
code and HEX file) are available for
download from siliconchip.com.au/
Shop/6/5637
Tom Croft,
Sunnybank Hills, Qld. ($150)
December 2020 87
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