[Diy_efi] WB LED Display dimmer

[Daniel R. Nicoson]

Bruce,

I took a trip to Washington DC this week and was driving in Virginia after
dark with your LED display merrily lighting the way.  More than one Virginia
policeman looked very hard at me to see if that bunch of flashing lights was
a radar detector (they're illegal there).

Is there a simple modification that would allow me to add a dimmer to the
LED's?  That display is great but sometimes could use a dimmer.  I haven't
had a chance to pull the data sheets on the chips involved.

Any thoughts?

Thanks,

Dan Nicoson

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]On
Behalf Of bcroe at juno.com
Sent: Wednesday, March 31, 2004 7:40 PM
To: diy_efi at diy-efi.org; bcroe at juno.com
Subject: [Diy_efi] Need help designing a circuit


31 Mar 04  Adam Wade <espresso_doppio at yahoo.com> writes:

> I'm trying to develop a small, solid-state device that
> will light a warning LED when one leg of a motorcycle
> stator start to become weak.

> The stator generates current in place of an
> alternator.  Most motorcycles us a 3-leg stator,
> making 3-phase AC between 8-80 V, then putting it
> through 6 diodes to rectify it, and to then feed the
> voltage regulator.

> I am looking to take a tap off of each of the 3 legs,
> and then to compare the AC voltages.  A difference of
> more than 1 V between each leg would trigger the lamp.
>
> Is there a quick and easy way to build such a thing?
> It would have to handle the full range of stator
> voltage.  Having the LED light when there was no
> voltage would also be handy, so one could see that it
> was operational at key-on.

If you can rectify the voltage off each phase to a representative
DC voltage, this isn't hard.  A comparator (LM239) can detect
when 2 voltages cross.  With 10V to power the '39, take a 1:15
voltage divider into an input, then take a 15:(1 X .85) or 17.65:1
divider into the other input of a comparator.  Maximum voltage
the comparator sees is 80/15 = 5.33V (must be well below the
'39 power).  If the voltage on the 15:1 drops below 85% of the
voltage on the 17.65:1, the comparator will change state.  Build
3 of these in a ring ('39 has 4 comparators), and you can pick
up any weak phase.

If one divider goes to a voltage instead of ground, you can make
a voltage difference detector (instead of percentage) with all
equal dividers.  Or make a combination of voltage and percentage.

Make the comparator output go low on fault; tie 4 comparator
outputs together (open collector) so any one can signal fault.
Use the 4th to detect no output before starting.

Bruce Roe

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