Nv-based, and other poppers --
all built on a common BEAM chip
To date, I've collected three 74*14-based
circuits for photopoppers
-- one (Dozer) by Darren Busson, based
on one head circuit design (the
Schmitt Comparitor Head, or SC
Head), another circuit by Cliff
Boerema based on yet another head
circuit design, and a third
circuit by John-Isaac Mumford.
In November of 2001, Darren Busson
that he'd played around with the circuit for the SC
Head, and come up with a variant suitable for photopoppers.
His result is simple, and as such, suitable for very small
Droidmakr (Cliff Boerema) came up with
an interesting idea for a light-tracking head with a form of
peripheral vision. As often happens, the circuit turned into
something different -- a photopopper:
Droidmakr's peripheral vision "roller"
(click to enlarge)
Bruce Robinson posted
this on the circuit:
1) Normal behaviour is both motors running
2) When the robot is pointing in the general direction
of a light, increased light on one of the "normal"
causes the motor on that side of the robot to stop. This
causes the robot to pivot about the stopped wheel and
face toward the light.
3) When "peripherial" photodiodes detect a difference
between one side and the other, they over-ride the
that detects the brighter light reverses the motor on
that side so the robot pivots sharply about its center
(one wheel forward, the other reverse) to face more
toward the light.
4) When the robot bumps into something on one side, it
over-rides all the photodiode
circuits and reverses the motor on the OPPOSITE side,
causing the robot to pivot sharply away from the
5) When both touch sensors contact an object, the
robot backs up.
All done with a single 74HC14
being a motor driver).
Meanwhile, Wilf Rigter posted
this comment on its operation:
The behaviour of this circuit depends on the
location of the photodiodes.
From the title it would appear that all 4 photodiodes
face forward but the 2 inner PDs
face directly forward and the outer 2 are angled to the
left and right. Then the effect is that the robot turns
towards a light that is in directly in front by turning
off one motor and pivots around the stopped wheel. But if
there is a brighter light on the periphery (front left or
front right) then the bot reverses one motor and the
robot rotates around its center towards the light. So
light in front only = small adjustment and brighter light
to the side = rapid large adjustment. This effect of a
strong peripheral light response mimics natural behavior.
The tactiles switches behave even more strongly: if a
switch is closed then the bot turns away unconditionally.
If both switches are closed the robots reverse straight
back regardless of light level.
In fact the behavior is even more complex and
interesting as the light conditions themselves change
with turning, etc. All in all, IMHO this circuit is
destined to become a classic.
The multiple Si diodes
provide a "dead band with memory" between one or two
inverters switching state. They are used as a
constant voltage drop and can be replaced with LEDs,
each of which has a higher forward voltage drop providing
the same function as 2 or 3 series diodes.
John-Isaac Mumford started off by
simplifying the Maxibug design, and wound up with an
entirely new circuit -- Mazibug (first posted
in March, 2003):
I'vee used a zener
and a 4148 diode
in place of six 4148s. Then i removed the reverse
function and cut down the inverter
drivers for the motors and changed the biasing(?)
to 1M rather than the posted 1K. What you get is a robot
which starts out photophobic, then when the voltage
drops (to around 2.5v) it turns around and heads towards
the light. All on one chip!
The size of the zener and cap
should be modified to suit. I think solarizing might be a
little difficult, but possible.
I tried the same setup with the 74*240 (with an extra
inverter per motor) and 7404, but the 74HC14 seems to
Wilf Rigter did a bit of tinkering, and turned this
circuit into a more-efficient solarized 74*240-based critter
For more information...
The home page for Darren's circuit and robot is
The discussion of the peripheral vision design
Dave's redrawing came in this
posting. Cliff now has a page up on this 'bot on
his own site here.