I have modified the two motor walker posted
earlier to include this feature [stops the walker
when it gets to the light and moves back toward light
when in shadow]. In addition I made a small
correction to the auto PNC
which were not connected correctly in the earlier design.
senses bright conditions by measuring the current
that passes through two series photodiodes
(PD1 and PD2). These photodiodes
form a voltage
divider that generates a control voltage
at the midpoint of the two photodiodes.
controls the photobridge circuit
so that it's output oscillates when the light is
balanced. The photodiodes
should be relatively low sensitivity or should be shadow
masked to decrease their sensitivity.
To sense bright conditions both photodiodes
must be strongly illuminated so that the total current
through both increases above 0.3mA. That causes a 0.3V
across each 1K resistor
which is sufficient to turn on the NPN
of the current
connected to the transistor
when turned on, generate about an additional 1% extra
through the 1K resistors.
This is positive feedback that causes the bright sensor
to snap on. When the bright sensor 2N3904 transistor
turns on, the collector
changes from +V to GND and this generates the active low
The Halt signal is used to tristate / disable the
HC125 buffer in series with the Nv3 output. As a result
outputs continue to pulse until Nv3 goes low and high
again but the rising Nv3 output signal is blocked with
the disabled HC125 buffer. A 10K resistor
to GND at the 125 output holds the Nv4 input capacitor
low. All microcore
Nv outputs are now high (inactive) and both motors are
stopped. In this state the circuit
is in the standby mode and does not draw much power.
If the circuit
is operated from a battery
with on board PV solar panels, these must be lined up
with the photo sensors to be pointed in the optimum
direction for charging the battery
while the light is bright.
When the ambient light level drops or a shadow is cast
on the photodiodes,
through the photodiodes
drops and the bright sensor circuit
rapidly turns off. The HALT line goes back to +V and the
125 buffer is enabled. The high level that was present on
the Nv3 output is now gated through the HC125 to the Nv 4
and starts the Nv4 pulse and the sequence of multiples of
pulses before the next bright spot is encountered.