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did you know that understanding edge
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triggers can drastically improve your
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PLC understanding how oneshots work
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within the PLC scan is vital in
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programming PLC's in this post we'll
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dive into the world of programmable
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logic controllers and explore the
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techniques and strategy secrets you need
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to know to trigger your edges precisely
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and accurately We will demonstrate the
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secrets with the free doour designer
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software and PLC simulator Whether
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you're a seasoned PLC programmer or just
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starting this post has valuable insights
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and practical tips to take your edge
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triggering skills to the next level So
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what are you waiting for let's unlock
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the secret to oneshot edge triggers in
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scanning Introduction to PLC inputs and
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triggers The fundamental operation of
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programmable logic controllers PLC's
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Scan refers to the cyclical process
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where the controller reads inputs
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executes the control logic and updates
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outputs This scan is the heartbeat of
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automated systems ensuring seamless
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operation and precise control Inputs to
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the PLC are like the eyes of the control
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system They let us see what the system
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or machine is doing All PLC's will have
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input filters This is the minimum time
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required to register that an input is on
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Edge triggers a key concept in PLC
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programming are essential for capturing
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specific events or conditions in your
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PLC program Positive and negative edge
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triggers detect changes and input
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signals triggering actions based on
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these transitions Harnessing the power
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of edge triggers within PLC programming
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you can optimize control systems
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streamline operations and elevate the
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performance of your industrial
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processes PLC scan read input filters
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Start the Do More Designer programming
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software This entirely free fully
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functional package includes a PLC
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simulator It will program the entire Do
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More series of controllers like the BRX
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H2 and T1H Start a new project by
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selecting the new offline icon on the
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main page This can also be done using
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the main menu ashar file new project
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harsh offline Alternatively you can also
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use the shortcut combination of cotrol
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plusn We can select the controller we
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wish to program when the new offline
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project window appears In our case we
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will select the BXD1ME 18813 controller
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Leave the title of our project as the
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default as we are just demonstrating
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Select okay Call up the system
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configuration window from the main menu
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gates PLC This can also be called up by
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selecting system configuration under
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tools in the project browser window Each
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PLC software differs regarding the
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filters location or whether they can be
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set See the documentation for your
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controller Select the BRX local IO on
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the left hand side of the system
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configuration window Then select filters
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under the input filter
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heading The setup discrete input
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response time window will be displayed
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Here we can adjust the filter time of
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the inputs I like to view this screen in
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time nanoseconds because I do not have
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to calculate frequency based on the 75
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MHz clock We can have input responses
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from 1 microcond to 100 milliseconds or
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0.001 seconds to 0.1 seconds If the
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actual input signal is shorter than the
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filter time the PLC will disregard the
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signal When the input is on longer than
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the filter time then the PLC will send
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this to the CPU This input will be
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recognized as being on When the input
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turns off the filter time is added to
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the signal to the CPU As mentioned
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previously all PLC's will have input
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filter times You may have to review the
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specifications for your controller to
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determine what minimum frequency or time
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is required to send the input to the CPU
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so it can be used in your
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program PLC can execute
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program ladder logic is the most common
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of the five recognized PLC programming
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languages Our do more designer software
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will only program in latter logic The
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cyclic nature of the PLC scan allows
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your program to be scanned rapidly We
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will demonstrate this with the do more
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designer PLC simulator Start a new
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offline program using the
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simulator A normally open contact X0
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will input an increment D0 instruction
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Connect to the simulator Download and
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When we turn on X0 the increment
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instruction will add one to D0 Since we
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have a cyclic scan this will happen
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every time X0 is on If we need to do
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this only once then this is where our
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leading or trailing edge one shots come
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in Note that our simulator works
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independently of the do more designer
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programming software It must communicate
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with the simulator So there is a slight
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lag in the numbers within
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D0 ladder logic and oneshot
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implementation Leading edge detection is
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known by several different names within
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PLC programming one shot positive edge
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negative edge differentiate up
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differentiate down etc Please refer to
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your PLC manual for the terminology used
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in your PLC controller We will insert a
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new rung and add a leading edge oneshot
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contact for input X1 This will increment
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the D1 register The third rung will have
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a trailing edge oneshot contact for
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input X2 This will increment the D2
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Accept the changes to the program and
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write them to the PLC simulator
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Turn on X1 and X2 D1 register has now
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been incremented just once This is
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because we turned on X1 which changed
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the input from off to on The instruction
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in the scan cycle detected this leading
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edge and activated the output
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incrementing D1 Note that D2 did not
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increment This is because X2 is looking
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for the trailing edge or transition from
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on to off Turn off X1 and X2 The D2
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register has now been incremented just
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once This is because we turned off X2
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which changed the input from on to off
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The instruction in the scan cycle
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detected this trailing edge and
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activated the output incrementing D2 D1
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did not increment again because X1 only
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looks for a leading edge or transition
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from off to on The power of one shot
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triggers capturing specific events or
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conditions with precision and
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accuracy Multiple inputs one shot
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PLC's will often have oneshot
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instructions built into specific
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instructions One of the most common
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instructions is the counter In the do
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more designer programming software the
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leading or trailing edge is represented
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by a triangle on the input leg of the
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instruction This example will increment
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the counter only on the leading edge of
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the input conditions Please refer to
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your controller manual for specific
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instructions on the input signal
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conditions So far we have just looked at
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individual contacts for our leading and
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trailing edge oneshots PLC's also have
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output instructions that can set one
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shots within your program These are
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usually used with internal memory bits
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because they are only on for one scan
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cycle at a time Add a positive
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differential PD leading edge oneshot and
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a negative differential ND trailing edge
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oneshot output instructions We will set
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the exact parallel and series contacts
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We can now utilize our program's
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internal contact bits C 0 and C1 This
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integration enhances the responsiveness
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of control systems and opens up a world
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of possibilities for fine-tuning
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automation processes to meet your
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diverse industrial needs By mastering
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these triggers you not only enhance your
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technical skills but also contribute to
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the advancement of industrial control
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systems Embrace the power of oneshot
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edge triggers to revolutionize your
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approach to PLC programming For those
8:54
looking to expand their knowledge in
8:56
industrial automation and control
8:57
systems exploring additional resources
8:59
and further reading can be
9:01
invaluable Whether you are a beginner
9:04
seeking to grasp PLC basics or an
9:06
experienced professional aiming to
9:08
enhance your skills the right
9:09
educational materials can make a
9:11
significant difference in your
9:12
understanding and proficiency To review
9:15
our PLC guide and approaches to creating
9:17
PLC programs click here Click here to
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learn more about the Do More Designer
