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Are there problems in my 3D printer design?
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FlakeyCrust:
I'm trying to build a 3D printer. Unfortunately, I only have a basic understanding of electronics. I have encountered some problems and it would be nice if I could verify that my design is sound before doing any more troubleshooting. I'm mainly worried about signals or power going to the wrong connections, so I have omitted the resistance and capacitance values from my diagram. I haven't had any problems getting the components to work individually (although I haven't been thorough in testing them).

I have included the schematic as an attachment. The wires are colour-coded. Red lines are connected directly to +5V, orange lines to +12V and black lines to a common ground. The rest of the wires are blue. It should be noted that the actual wiring is not represented by this schematic. For example: I have wired the components as if the 5 and 12 volt rails have separate grounds, but the power supply I'm currently using only has one set of ground wires, so they are connected.

Edit: I just noticed that for the 2 channel relay, ground is connected to GND, COM1 and NC2. It should be GND, COM1 and COM2. Please disregard this; it's only a mistake in the schematic.
hagster:
Are you sure you want to design your own main board? There are loads of cheap boards availiable.

Seems like you are using a DC motor for the z axis. How will you accuratly position this?

The mode and enable pins on your steppers are floating.
brucehoult:
Yes, not using a stepper for the Z axis seems wacky. That's the thing that needs the MOST precise control in the whole printer!

Also a relay to control the hot end heater?  You want to keep the temperature steady to certainly 1 degree or so, and under a variety of load conditions (e.g. feed rates). It needs to be controlled by a PID algorithm. So you *really* want a PWM output there, going through a MosFET, not a relay.
FlakeyCrust:

--- Quote ---Are you sure you want to design your own main board? There are loads of cheap boards availiable.

Seems like you are using a DC motor for the z axis. How will you accuratly position this?

The mode and enable pins on your steppers are floating.
--- End quote ---

It's because I'm using a DC motor for the Z axis that I'm not using an existing main board. I actually got the motors from an old printer that broke down (a cheap plastic gear broke) and a really old printer I bought used for next to nothing. The stepper motors were rather small, and seemed inadequate for the Z axis, considering the weight, but there was also a rather beefy DC motor. So I decided to use that with a reduction gear made from the remaining gears.

Near the top left corner of the schematic is a box containing an LED and photo-transistor: that's a photo-interrupter. The motor spins a disk with uniformly spaced holes and the photo-interrupter will count how many holes go by, to measure the exact amount of rotation.

As for the floating pins: I just looked at the A4988 documentation and it says the MSx pins already have pull-down resistors, but it doesn't say anything about ENABLE having one, so I will add resistors for the ENABLE pins.

Thanks for taking a look.
FlakeyCrust:

--- Quote from: brucehoult on August 10, 2018, 05:57:16 am ---Also a relay to control the hot end heater?  You want to keep the temperature steady to certainly 1 degree or so, and under a variety of load conditions (e.g. feed rates). It needs to be controlled by a PID algorithm. So you *really* want a PWM output there, going through a MosFET, not a relay.

--- End quote ---

I'll change that. I might just use another heat bed module (which is basically just a MOSFET) for the hot end, since I have a spare.
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