Electronic Building Blocks
By Julian Edgar
Quick and easy construction
Great results on a low budget
Introduction to linear actuators – Part 1
Automate your gate or curtains, open and close doors and vents, rotate or extend
amateur radio antennas – and so much more using linear actuators.
R
ecently, there’s been an explosion in the
availability and use of linear actuators – powered devices
that extend or retract a rod on demand. These actuators
move relatively slowly but with a lot of force. They’re used in
hospital beds, solar panel trackers, reclining lounge chairs and
even to automate the movement of normally hidden TV screens!
From a technical hobby perspective, the list of potential uses
is limited only by your imagination. I intend using them to
automatically open and shut vents in a solar house. A quick
web search will show some amazing uses – from people who
have coffee tables disappearing into the floor of their house,
to automating the movement of farm gates, to opening attic
hatches in the ceiling.
So, what is their design – and how do you control them?
(like a bench vice) has an internally threaded nylon saddle (a
nut) riding on it. The saddle is prevented from turning by a
tab sliding within an enclosing tube and is connected to the
extension rod. Therefore, when the motor is turning, the saddle
How they work
Linear actuators are typically powered by a brushed, permanent
magnet DC motor. (You can find more compact versions with
brushless in-line motors and epicyclic gear boxes, but they are
much pricier.) Referring to Fig.1, The motor drives a reduction
gear-train that is connected to a threaded rod, sometimes
called a ‘leadscrew’. The rod normally uses a square-cut thread
64
A TV that disappears down into a cabinet! This is a professional
system, but you can make something similar using a linear actuator.
(Courtesy: Cabinet Tronix)
Practical Electronics | June | 2022
hits the end-stops and
stalls the motor.
Gearbox
The limit switches
are wired in series,
DC Motor
Nylon saddle with
and each switch has a
internal thread
diode wired in parallel
with it, with the two
diodes wired in
Actuator extension rod
reverse polarity to one
another. (See Fig.1.)
Leadscrew
This means that, once
a limit switch has been
Limit switch
Limit switch
tripped, reversing
the direction of the
actuator requires only
reversing the polarity
12-24V DC input
of the motor’s electrical
connections.
In this way, an
Fig.1. Most linear actuators have in-built limit switches. The limit switches are wired in series, and each switch
has a diode wired in parallel with it, with the two diodes wired in reverse polarity to one another. This approach actuator can be opened
turns off the motor when the actuator reaches the end of the travel, with movement in the opposite direction and closed with just a
achieved by reversing the polarity of the power feed. (Courtesy Firgelli Automations)
double-pole, doublethrow (DPDT) switch.
This can be a normal two-position switch (to fully retract or
extend the actuator) or a sprung centre-off switch (to allow
the actuator to be stopped at interim positions). With either
switch approach, the actuator will be automatically turned off
once it’s reached full extension or full retraction.
Note that when the actuator is fully extended, it loses some
of its lateral (bending) stiffness. This is because the nylon
saddle is located right at one end of the enclosing tube, and
the clearance between the saddle and the tube results in
Linear actuators are now widely available at prices much lower
more lateral movement than occurs when the rod is retracted.
than previously. The actuators use a brushed DC motor, a gear
Therefore, especially if dealing with high loads, it’s best from
train and a long, threaded leadscrew to power a rod that extends
a mechanical point of view to not completely extend the rod.
and retracts. (Courtesy: Firgelli Automations)
slowly moves along the threaded rod, extending (or retracting)
the rod. The large reduction ratio achieved by the gear-train,
and the relatively fine thread pitch of the threaded rod, allows
the actuator to develop a lot of force, despite the electric motor
being relatively small and its current consumption quite low.
With the current switched off, the rod stays locked in position.
The nylon saddle’s tab, that prevents the saddle from turning,
also has another purpose. It closes a microswitch at each
end of the rod’s travel, thus turning off power. That is, these
limit switches prevent the motor driving the rod so far that it
Specifications
The main specifications of typical linear actuators are:
n Operating voltage:
12V and 24V are common
n Rod extension:
25-300mm
n Maximum force:
150-800N (×0.1 for approx ‘force’ in kg)
n Limit switches:
Most actuators have one at each end
nF
eedback position pot: Far fewer actuators have these
n Weatherproofing:
Versions available for outside use.
Industrial linear actuators are very expensive (from £250 and
up); however, ‘hobbyist-grade’ linear actuators from sellers on
eBay and companies like Banggood and Aliexpress are much
cheaper – starting at about £30. Note also that some companies
selling ‘branded’ actuators are simply reselling the cheap
Chinese ones with their sticker slapped on top! In that case,
you are paying a premium just for the warranty.
If you intend to use a linear actuator in a demanding
industrial use, or it needs to be weatherproof, I suggest that you
buy one of the higher price units, but for general household
and hobbyist use, the lower price actuators should be fine.
Power supply
Neater, but (much) pricier industrial linear actuators are available
with in-line brushless DC motors and epicyclic gearboxes. Avoid
these unless you really need the extra quality.
Practical Electronics | June | 2022
The current draw of a linear actuator depends on its size and
the load being moved – the current draw increases with both
factors. However, the size and mechanical power of linear
actuators typically used in hobby and home applications means
that a current draw of around 2A is typical for a loaded actuator.
Power can be provided by a suitably rated mains switching
power supply (eg, a plugpack) or via a lead acid or lithiumion battery. Because the duty cycle of an actuator in normal
use is very low (how many times a day do you open and close
the door of a chicken coop?), renewable energy sources like
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Linear actuator
DC motor
M
DPDT
centre-off
switch
12-24V DC input
Fig.2. A DPDT switch can be used to
manually control a linear actuator. Push
the toggle in one direction and the actuator
extends, move the toggle in the other
direction and the actuator retracts. The
presence of in-built limit switches in the
actuator prevent damage, even if the switch
is held too long in one direction.
solar panels or wind turbines can be used
to maintain battery energy levels. This
allows the actuator to be located away
from mains power.
Switched manual control
The simplest way to control a linear
actuator is to use a DPDT, centre-off switch
– see Fig.2. When the toggle is moved in
one direction, the actuator extends; and
when the toggle is moved in the other
direction, the actuator retracts. As stated,
the limit switches in the actuator prevent
damage, even if the toggle switch is held
too long in one direction. Because of the
relatively low current draw, a normalduty switch is fine.
If you want to use pushbuttons to
control the up and down movements,
you can add a diode and a DPDT relay.
See Fig.3. Momentary pushbutton 1
feeds power to the linear actuator, with
the relay in its normally closed position.
SW1
6A10
SW2
Linear actuators come with and without position feedback. This unit does not have position
feedback – it has only the two power connections. Those actuators with position feedback
have an additional three wires for the potentiometer.
Momentary pushbutton 2 feeds power to
the actuator but additionally causes the
DPDT relay to pull-in, so swapping the
polarity of power feed to the actuator
and giving reverse direction movement.
The diode prevents pushbutton 1 from
triggering the relay.
If the actuator is to be fully extended
or retracted on each use, the normally
open relay outputs of two timer modules
can replace the two pushbuttons. To use
this approach, measure the time taken for
the actuator to extend under load (eg, 30
seconds) and then add (say) 25% for the
timed period. When the timer is started,
the relay will pull in and the actuator
extend. The actuator will stop when it
reaches its internal limit switch, and then
a short while later, the relay will click
off as the timed period elapses. Follow
the same approach for the retraction
control. Using the timers (rather than say
simple latching relays) stops the relays
from having to be pointlessly engaged for
long periods. The timer modules that you
select will need to be of the type triggered
by external pushbuttons. Using timers
in this manner allows the actuator to be
operated by single press ‘up’
and ‘down’ buttons. Want
your TV to rise out of the
+
top of a cupboard? Press the
‘up’ button and walk away. It
M
Remote control
If you want progressive remote control
of an actuator, an off-the-shelf system is
available. It costs under £10 and is available
on eBay – search under ‘DC HOUSE Linear
Actuator Controller DC8-30V Wireless
Remote Motor Control Switch’. (For
example, at the time of writing, one of
many on eBay is item 313431937657,
which is just £7.99, including delivery.)
The system operates on 8-30V (the actuator
must also use a voltage within this range)
and can handle up to 5A.
The controller comprises a receiver/
relay module and a remote control with
an extendable antenna and two press
buttons. The receiver module is fed 12V
and in turn feeds the linear actuator.
Pressing one button causes the actuator
to extend for as long as the button is
pressed, or until the actuator reaches
full extension. As you’d expect, the other
button causes retraction in the same way.
The system is very neat and simple to
fit as the polarity changeover and on/
off relays and receiver are integrated
into one compact module. In testing,
the system had a line-of-sight range of
over 50 metres – ideal for gate opening
and closing.
Safety!
–
DPDT
relay
Fig.3. Operation of the linear actuator by two momentary
pushbuttons: button 1 feeds power to the linear actuator
via the relay in its normally closed position. Pushbutton 2
causes the relay to pull-in, so swapping the polarity of the
power feed to the actuator. The diode prevents the relay
from operating when pushbutton 1 is pressed.
66
will smoothly rise, stopping when full
height is reached.
Any mechanical system that uses a linear actuator will need to be
stiff and have the required strength to support the loads. Carefully
consider what could happen if something breaks – we don’t want
people being dumped out of powered reclining armchairs because
the forces destroyed the framework!
Don’t use linear actuators to lift heavy weights that are
positioned above people, and remember that the actuator becomes
less stiff in bending when fully extended.
Remotely opening or closing a door or gate should be done
only when it is in view of the operator, otherwise there is the
possibility of trapping or squashing people or animals.
Practical Electronics | June | 2022
This cutaway view shows a typical linear actuator. The electric
motor is to the right, and the reduction gear train closest to the
camera. The lead screw and the white nylon saddle that rides it
can also be seen. (Courtesy Firgelli Automations)
In the next issue we will use a sophisticated programmable
controller that gives excellent position accuracy of the
linear actuator, even with varying voltages and loads. It also
incorporates over-current control and allows the acceleration
rate of the actuator to be specified.
(Top) This low-cost remote control is specifically designed for use
with linear actuators. The receiver module simply inserts in the
power connections to the actuator. Using the remote’s pushbuttons
allows the actuator to be either fully or partly extended, or retracted.
(Bottom) The inside view of the receiver module showing the two
relays and receiver. It’s a very neat unit.
Using a timer to give a specific extension?
If you want to extend and retract the actuator repeatedly to less
than full extension, you may think that a timer can be used.
For example, using a four-second timed period to extend to the
required length, and another four seconds of reverse polarity to
retract. But unfortunately, it’s no use just operating the motor
for a pre-set period and assuming that the rod will end up at
exactly position ‘X’. Why not? Because these motors are very
responsive to voltage changes, even a 0.5V difference in drive
voltage will result in the actuator ending up in a different
position. The actuator speed for a given voltage also depends
on load – another reason you cannot just used a timed period
to set the actuator position.
Instead, precise placement of the actuator requires the use
of a feedback signal. This is achieved in many linear actuators
by using an inbuilt feedback pot – eg, 10kΩ. The voltage signal
from the pot then indicates the position of the rod. (Note that
actuators with in-built pots usually still have limit switches.) We
will cover using this feedback signal in Part 2 in the next issue.
Conclusion
Whether you choose to control a linear actuator with a simple
DPDT switch, pushbutton-operated relay or via a remote
control, the ready availability and relatively low price of
linear actuators means that lots of exciting projects can now
become an easy reality.
Practical Electronics | June | 2022
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