Okay, so we've got three motors, three H-bridges, and a Step-up converter. So we've got the motor side of things covered. We'll discuss measuring the motor rotations, measuring light levels, and choosing a CPU dev board later.

I'm just going to steam on as much as possible, because I'll have to wait for up to a month now anyway for parts to arrive from China (I hope they arrive).

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Calculated Current Usage

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We need a battery to power everything. A solar powered self recharging one of course.

Motor control:

Let's say, 6 hours, twice per hour, 10 seconds, at maximum power of 100 mA @ 12 VDC: 200 second at 12 VDC, at 100 mA. so we need 200/3600 * 100 mA = 5.56 mAh.

The step up converter wastes 10% of it's power, and let's give the H bridge also 5% wasted power.

So we need to multiply the required motor run energy by 115%: 6.38 mAh.

Basically, we can just ignore this number. If it's not sunny, we're not going to bother changing the motors. If it's sunny, the built-in solar panel could easily recharge this feeble amount of energy usage.

Now, hopefully we can get a CPU dev board that can run on 1 mA standby power. That shouldn't be too hard, really, they should easily outperform that.

Let's demand that it will keep running for one week without ANY light. I know, it's not realistic, but hey, we're doing worst case.

That's 1 mA * 24 hours * 7 days = 168 mAh.

So we're looking for a solar recharing battery with 168 mAh, that puts out 5 VDC.

Ebay to the rescue!!

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Finding the right solar charging battery

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This is the cheapest I could find. £5.98

Solar panel:5.5V/80mA
Voltage Output:5.5V
Li-polymer Battery:1350mAH
Size:9.3*4.3*1.1cm

I've actually used this thing before and it worked quite well. It is massively overspec'ed for what we need, which is good.

However, we can also make it more difficult (= interesting) for ourselves, and save some money (hopefully). Why not build our charger ourselves?

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Building Our Own Solar Charging Battery

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We can just buy a separate solar panel ( 5V, 40 mA, £1.09) and 4x1.2 Vdc NiMH batteries? (1000 mAh, £1.09)

But, mime, the battery voltage adds up to 4.8 VDC, and the solar panel adds up to 5V, aren't you worried about exploding batteries?

From a 5V,40 mA = 0.2 W solar panel? No. That amounts to 40/10 = 0.04 C of charge, which is equal to the advised trickle charge to keep them topped up.

How about self discharge? See above argument.

How about discharging too much?? Hmm perhaps. Let's see what happens.

Apparently, you have to stop discharging a NiMH at 800 mV. At 4x 800 mV, this would be 3.2 VDC for the entire pack. This is something that the step-up converter could still work with.

So to prevent that, we need a discharge protection circuit, with a cutoff voltage of 3.2 VDC. Let's see if Ebay has them.

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Over Discharge Protection Circuit

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There are plenty of cheap Li-ion protection circuits, for instance, this one, at £1.51.

• Charging voltage DC3.7V, Low voltage, over charge, over load protection.
• Charging for single battery or few battery in parallel, Suitable for Multi-cells form DC3.7V, such as Oppo A100 A113 A115 A121 A127 A520 cell phone.
• Charging limit voltage is 4.2V, Battery capacity is 900mAh.

Our battery is 1000 mAh at 4.8 V, so that fits. We don't have to be fully charged. This protects the batteries, but limits the use to 3.7-4.2 VDC.

However, they don't mention the minimum discharge voltage limit, and that is what this is all about. So we can't really use this after all.

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The Showdown

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So. Have we made it cheaper by buying our own solar panel and batteries?

The all-in solar charger was £5.98, and the custom hacked solution

£1.09 for the solar panel

£1.09 for the NIMH batteries

£0.99 for battery holder 4x AAA batteries

That amounts to £3.17, so roughly half price, but without under voltage protection.

How much danger is there from undervoltage?

1000 mAh, 1mA current use (worst case), then discharge time = 1000 hours, or 41.67 days.

And we've put in a solar panel, which works even during the day. If we have full sun for only half an hour per day, then the current use of that day is accounted for.

So let's live dangerously, and go for the cheap Mac-Gyver option! This is hackaday.io after all.

We can always buy the more expensive option later.

What do we need still? a motor revolution sensor, a Real-Time-Clock (RTC), and a CPU dev board (low power).