Hi Alex,

Series light sensors, eh?

Well let me think...

Ok here is the uCore Photo Bridge: a brand new way to influence a microcore
walker with light sensors.

I modified Fred's drawing, using cut and paste in my paint program, to
incorporate the new circuit and also to point out a slight correction to the
Miller Solar Engine. Note the small size of the attached schematic file
after reducing the colors to 16 and converting to gif format.

The uCore Photo Bridge:

The two Light Dependent Resistors in series form a ratiometric photobridge
connected between Nv 1 and Nv3 outputs which also control the rotation of
the rear motor of a two motor walker.

Depending on which Nv is active, the polarity across the bridge is reversed.
When the two LDR sensors are the same resistance, the bridge is balanced and
the midpoint voltage of the bridge is nominally Vcc/2. If the two LDRs are
not balanced, the midpoint will be offset from Vcc/2.

That midpoint voltage is the return point for the two new parallel timing
resistors isolated with diodes and connected to the inputs of Nv1 and Nv3.

Each parallel resistor and diode will affect the time constant of the
corresponding active Nv depending on the voltage of the photo bridge
midpoint.

When the midpoint of the LDR bridge is lower than Vcc/2, the active Nv will
time out faster. If the midpoint is higher, the active Nv will time out
slower.

With an unbalanced photobridge, the midpoint voltage will be higher for one
active and for the other the midpoint will be lower as the voltage across
the bridge is reversed.
The parallel resistors remain active until the voltage of the timing
capacitors on each Nv input drops below the photobridge midpoint voltage and
the isolation diode becomes reverse biased. After that the original 9M
resistor ensures that the Nv times out.

As a result the discharge voltage waveform of the active Nv will have a
double slope, first steep with two resistors in parallel to discharge the
cap and then slower with the single resistor to finish the capacitor
discharge.

So why is this photobridge better than the "traditional" LDR in parallel
with the microcore timing resistors arrangement?

Most importantly it is relatively insensitive to the absolute light level,
which in the tranditional circuit would cause very short steps in bright
light and long steps in dim light.

That problem of sensitivity to absolute light level also occurs in light
seeking master slave bicore walkers.

The uCore Photo Bridge only reacts to the ratio of the LDR resistance values
and insensitive to the absolute light level.

This makes the period of leg rotation relatively constant over a wide range
of light levels.

This circuit comes piping hot from the mind of yours truly, so it is as yet
untested but I think it will work spectacularly well. This may be an
opportunity to test out this circuit and help develop into a tried and
proven design.

enjoy

wilf

----- Original Message -----
From: "submachinegunn" <rockbitea@...>
To: <beam@yahoogroups.com>
Sent: Saturday, June 14, 2003 8:23 PM
Subject: [beam] Re: I need help with my micro core sensors.........


> Cool. Thanks for your help but I still have some questions about
> somethings you said.
>
> > Light sensors like Cadmium Sulfide cells can be placed in parallel
> with the
> > Nv timing resistors to decrease the Nv duration and the rotation
> of the
> > motor so that one leg on the ground has a shorter stride than the
> other leg
> > on the ground.
>
> Are you saying that I need to hook up all of these timing resistors
> to the photo cells or are 2 photo cells (made into a voltage
> divider, right?) for one timing resistor is enough? Sorry if I am
> not understanding this.
>
> Sincerly:
> Alex
>



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