1 Topic by holoped

  • holoped
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Topic: Questions about the electronics

Hey all,
I'm trying to build a scanner myself and I don't understand the electronics.

https://s3.amazonaws.com/freelss/bb.png

Can someone confirm that this does not expose the PI pins to 5v? Because that could fry the PI.

What are the pots for? They are not voltage dividers, since the third leg is not connected. What do they affect?

What kind of lasers should I get? 5v lasers?

Thanks.

2 Reply by hairu526

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Re: Questions about the electronics

holoped,

5V is sent to the power pins to power the Raspberry Pi, not the I/O pins.  You should not use the normal USB power plug of the Pi.  The pots were originally intended to allow for control over low currents but they are not needed.  You should not include them in your circuit.  The outputted voltage is 4.5V, so you need a laser that can operate at this voltage level.  This includes most 5V lasers.

Uriah

3 Reply by holoped

  • holoped
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Re: Questions about the electronics

Hi Uriah.

The polulu A4988 operates at 5v logic, and the pi has 3.3v logic.
Usually the two should not mix, so I was concerned.

Thanks for your reply.

4 Reply by Viseng

  • Viseng
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Re: Questions about the electronics

Holoped,

You are quite correct, there is an interface issue with any electronics scheme which connects the Pi directly to the A4988.

The schematic indicates that the driver is connected to +5V, yet the Raspberry pi GPIO are 3.3V logic level outputs.  The datasheet for the driver confirms that the logic HIGH must be >0.7*5V  that is >3.5V.  The driver logic supply must either be connected to 3.3V or level translators used to make the interface compatible.  The former is acceptable since the A4988 is specified to work at 3.3V.  Failure to adopt either approach may result in erratic or non-operation, depending upon device and supply tolerances, temperature or amount of noise on supply.  Quite a few people have reported issues with erratic stepper operation; this is probably the reason.  My own experience confirms this.

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5 Reply by n2ri

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Re: Questions about the electronics

so why not use an adjustable POT for stepper motot like most 3D printers do?

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6 Reply by elmoret

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Re: Questions about the electronics

The ATLAS 3D Scanner uses the DRV8834, which supports 3.3v logic levels.

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