Part Number: BEAGLEBK
Hi
I'm trying to build a thyristor softstarter for asynchronous motors with the BeableBone Black.
To this end I made a Schmitt-trigger stage, that can be connected (accidentally) to AC,
before the BBB has power.
In that stage, the trigger output might be erratic, but is clamped by a
TVS-diode and/or a Zehner-diode.
The trigger is isolated from AC by a transformer and it's ground is always connected
to ground of the BBB.
See www.elektronik-kompendium.de/.../st555.htm "Bild 5" (figure 5)
on roughly how I did this (the rectifier is left out, one leg of the transformer
is connected to ground).
The BBB reference manual states, that inputs *must not be driven*,
before the processor is fully awake.
My best guess so far is, to use a bus driver like the SN74AHC126, or a CD40109B
with enable pin pulled to ground and connected to the /sys reset-output (Pin P9-10),
in order to keep GPI of the BBB floating during boot up.
During operation the BBB shall be powered by the 5V-jack, also supplying the SN74AHC126
via DC_3.3V or VDD_5V (Pin P9-5) somehow reduced to 3.3V (resistor + zehner?).
A resistor shall limit the input of the SN74AHC126 <20mA in the unpowered stage.
Is this an advisable approach?
If not, why, and what else should I do?
Is it good/bad to pull the output of SN74AHC126 to ground?
Best Regards, Armin Faltl
P.S.
In addition I will need an analog input from a potentiometer, supplied from the BBB.
While I have more controll over this, I still fear issues before/during boot up with
things like noise.
