The DigiX > DigiX Projects

4xMOSFET for DigiX (and others)

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cboden:
Due to the fact that the DigiX provides only 3.3V on the GPIO pins it's not so easy to manage MOSFETs with it. Most logic level MOSFETs are designed to work with 5V instead of 3.3V. So you need a little level shifter for driving them. There are several possible ways to do that - but I decided to use a simpel 74HCT circuit for that. All chips of that familiy recognize 3.3 as high and they provide 5V as output signal. If you try to do the same, please be aware that this is only a feature of the 74HCT familiy AND NOT of the 74HC familiy. So be carefull that you choose the right chip :-)

I decided to do it with the 74HCT08. It compines four logical AND gates. So to use it as a level shifter simply route the input signal to both input pins of a gate. To have a defined low signal when not connected, I use a 100k ohm pulldown resistor. To reduce EMC between the level shifter and MOSFET I use a 100 ohm resistor. If you try to use that schematic for PWM you may need to reduce that value or even replace the whole resistor by a bridge because the size of it effects the time the MOSFET needs for changing the gate capacity. Another 100k ohm resistor between gate and ground is responsible to unload that gate capacity when switching off the MOSFET.

To have a visual response of the state for the channels (and the power) I added  LEDs with suitable resistors. If you re-use the schematic for your own project you may decide to left that away. You can do this without effecting the rest of the project. But if you decide to left them in you may have to adjust the value of the resistors so that they fit the needs for the LEDs you choose. The 100 ohm work for my green LEDs.

By using the 74HCT chip the schematic works with both 3.3V or 5V input signals. So it can be used with the DigiX as well as with the Digispark. But please be aware that it has to be powered with 5V to work proper even if you use it for the DigiX. You can use GND and +5V from the DigiX for that.


cboden:
I did a tiny 2x2 inch PCB based on that schematic in KiCad. Ordered also some of them by my favorit chinese manufacturer and will let you know how good they work once I have them in my hands.

You will find the KiCad project attached. Here a drawing of the PCB:



Everything is through the hole so that you can re-build it without greater problems.

gogol:
What is the reason, that you have not used your eagle files for a CORE DigiX shield?

cboden:
good question :-)

To be honest: I did that project first in Eagle (and also have some PCB's based on that layout in front of me) but then I thought it would be a good example for me to introduce myself to KiCad as I think to migrate to it. Another reason is that I think we should use open source for open hardware project in general.

I plan also to re-do the pinhead layout again in KiCad and publish the result again here in the forum if somebody is interessted in.

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