FRED Photopopper

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[edit] Description

Ben's use of the Fred SE for a photopopper takes advantage of an interesting feature of FLEDs -- they are photo-sensitive. In particular, light shining on a FLED causes that FLED's SE to be inhibited (to perform more poorly). So if you're building a phototropic FLED-based BEAM bot, you may have problems in bright light (both SEs in your 'bot are being inhibited, so neither side of your 'bot wants to fire) -- you can address this by partially shielding your FLEDs with heat-shrink tubing, or some paint (be careful here, you're trying to give your 'bot sunglasses, not blinders). Meanwhile, if you're using a FLED-based SE on a non-phototrope, you'll want to completely cover the FLED (with heat-shrink tubing, or black electrical tape, or dark paint...).

Once you understand how the independent solar engines work, you are ready to consider the dynamics of putting two FRED SE's together. These are coupled in two ways:

Electrically (through the supply rail)
Mechanically (the two SE's are physically connected together)

The electrical coupling means that both SE's will be constantly trying to trigger, but the first SE to trigger will make the supply voltage drop, and therefore stop the other SE from getting that tiny bit of extra voltage it needed to fire. In other words, the dark side fires first.

The mechanical coupling is quite loose, and means that if side 1 is in brightness then the side 2 will fire more often. After a while side 2 will turn the body of the FRED around, so that side 1 is 'darker' than it was before, and therefore more likely to fire first.

[edit] Schematics

Component Values

C1	3000 uF
C2	0.22 uF
R1	3.3 K
R2	33 K
R3	1 K

[edit] Shadow Protection

Component Values

C1	> 1 F
C2	0.22 uF
C3	470 uF
R1	3.3 K
R2	33 K
R3	2.2 K

[edit] Circuit Analysis

C3 provides enough power for the PNP transistors to latch the circuit. When C3 discharges, the Fred SE resets. C3 then charges from C1. This allows C1 to be a higher value, storing more power and keeping the voltage closer to the solar cell's peak power point.

The standard Fred SE continues to fire until the voltage in C1 falls below the forward voltage drop of the transistors. When this occurs, the two transistors cannot source the current needed to keep latched. Both of the transistors turn off and the SE resets.

This limits the total amount of capacitance on C1. Ideally, for the photopopper to be phototropic, C1 provides enough power for the SE that triggers to move the robot about 45-degrees. Any more than that and the popper ceases to effectively track the light.

Because the robot cannot store more energy than is needed for a single firing, the robot cannot “survive” the light source being removed. A shadow, for instance, can deprive the robot of power. This modification corrects that by allowing the robot to store up enough energy for several pops after the light source is removed.