[TriEmbed] Pinewood derby help

[Shane Trent]

Craig,

That circuit looks a little wonky. It seems they want to use the 2N3904 for
voltage translation (the FET is not logic level sensitive, it is famous
because you could buy it at Radio Shack) but they do not provide a ground
reference for the emitter of the 2N3904, the emitter only connects to the
FET's gate.

The BJT would turn on when the gate voltage (through the 4.7k resistor)
reaches around 0.6V above the emitter voltage but the emitter voltage is
floating with no ground reference. So if the circuit did turn on, it would
pull the FET gate voltage up and turn on the FET. Turning off the FET has
it's own issues though since there is no path to drain current from the FET
gate, the FET will turn off in the semi random time it takes for the gate
charge to leak away. You could make it work by putting a large resistor
between the FET gate and GND but the FET would turn on faster than it would
turn off.

It would be much more effective if you could use a more sensitive FET that
could be driven directly from the logic pin (via a 1k or so resistor). This
would let you drive current to turn on the FET as well as sink current to
turn off the FET.

Note they are also missing a kickback diode across the load so you would be
dumping nasty high frequency, high voltage spikes back into the power rail
and eventually your chips will start acting oddly before dying. Do not skip
the kickback diode, the body diode of the MOSFET does nothing to help tame
the inductive kick when the FET is switched off.

Shane

On Sun, Jan 8, 2017 at 12:20 PM Craig Cook via TriEmbed <
triembed at triembed.org> wrote:

> Ah, learning more.  This is the project I am looking at:
>
>
>
>
> https://chip.hackster.io/user024507/motor-on-9c6bd1?ref=channel&ref_id=5276_trending___&offset=22
>
> Is the 2N3904 working as an isolator?
>
> The donor truck contains these:
> BR8550D 064 (x 4)
> C1815Y 5L
> Super Chip STRX3A, 60100395
>
> Could I reuse the C1815Y for my isolator?
>
> I also have these new PN2222ABU, BC547B, P2N2222AG
>
> I could attach a picture of donor truck board, but it would have to be
> less than 100k.
>
> Thanks
>
> Craig
>
> ------------------------------
> *From:* Carl Nobile <carl.nobile at gmail.com>
> *To:* Craig Cook <cncook001 at yahoo.com>
> *Cc:* TriEmbed <triembed at triembed.org>
> *Sent:* Sunday, January 8, 2017 11:10 AM
>
> *Subject:* Re: [TriEmbed] Pinewood derby help
>
> Yes, you will need something like a MOSFET to handle the higher current
> that the motor will draw and a different power souce that what is being
> used for the CHIP, but that won't isolate the motor from the electronics.
> You will still need an opto isolator.
>
> ~Carl
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