Preset
I was always fascinated by using the light of the Sun as a source for electrical energy. About ten years ago, after my course in electronic at The university, I bought some small photovoltaic cells to build a solar lamp. I never got around to finish it and after the recent surge in popularity of solar lamps, I thought it would not be worth the effort if you can buy them cheaply.
A few months ago, I found a big photo-voltaic (PV) panel (open circuit voltage ~ 21 V, voltage under load ~ 17 V, short circuit current ~ 200 mA) at my parents' home, intended to power the pump of a fountain for the garden pond. As I thought this was a rather wasteful use for the PV panel and the whole solar-powered pump system was never used anyway, I decided to build a solar lamp with it.
Design
After a lot of circuit simulation, searching for suitable parts and soldering of test circuits, I came up with this design:
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| schematics |
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| V1 (PV panel voltage) dependent DC simulation |
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You can download the
QUCS circuit simulation file
here. For the simulation to work you need to put the files in
this archive in the sub-directory named ".qucs" in your user directory.
V1 is the PV panel (with Rsolar representing the inner resistor) and V2 is the battery.
You can see that the switching of the light is done with a voltage divider (R1 and R2) and two transistors (T1 and T2). This eliminates the need for a light dependent resistor (LDR). If the voltage at the base of T1 drops below 0.7 V, it blocks and the voltage at the base of T2 increases towards the battery voltage and it conducts. This switches on the LED lights.
Charging of the battery pack is current-regulated by a LM317 IC with a 10 Ohm resistor (R6) as reference. This limits the charging current to about 125 mA. The diode D1 prevents discharge of the battery over the PV panel.
Unfortunately, I did not find a way to model LEDs in QUCS other representing them by five diodes in series.
Implementation
I bought ten warm white SMD LEDs (I
F = 60-80 mA, max 14 lm) and decided to use them all to have several lamps connected to a single central regulation unit. I was not sure how many of these LEDs I should connect in series. First, I bought a NiMH battery pack with 7.2 V (2000 mAh) and I found out that with this pack I could only connect two LEDs in series. This would have resulted in several lights with two LEDs in parallel with their own current regulation. So, I bought another 7.2 V battery pack to get 14.4 V. After destroying four of these nimble SMD LEDs due to my poor soldering skills, I settled on having two lights with three LEDs in series for each.
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| the circuit (battery pack in background) |
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| one of the lights (three LEDs in series) | |
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I know that my soldering skills are really poor, if non-existent. The 30 Ohm resistors in series with the LEDs are done using three 10 Ohm resistors.
Result
I placed the PV panel together with the battery pack and the circuit on the garage roof and placed one of the lights on the ground (inside a decorative candle housing connected via a 5 m cable) and the other under the garage roof.
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| up under the garage roof |
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| ground light |
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The lights illuminate the area in front of the garage which normally is a bit dark, especially because the motion sensor responsible for switching the illumination of the house behind the garage (visible in the photo on the left) is not really working properly.
The solar lights are switched on by the circuit at early dawn when it is not completely dark.
Issues
- Leaving efficiency issues aside (having a 14.4 V battery pack with three LEDs in series with a maximum voltage drop of about 11 V wastes some electrical energy if no efficient step-down converter is used), the main problem of the circuit is that the battery pack is not sufficiently charged during the course of the day - at least with middle European sunlight conditions. The diode and LM317 take away a few volts from the PV panel voltage and so the charge voltage is high enough only with really good lighting conditions. This leaves the battery pack not fully charged after an average day.
- Another point is the current regulation for the LED lights. The simple resistor in series results in a very bright light at dusk with brightness slowly decreasing as the battery pack discharges and its voltage drops. At midnight, the brightness has dropped drastically. A real constant-current regulation has to be used.
- With my poor soldering skills I should not have bought the tiny SMD LEDs, but should have used readily available 12 V LED lights (e.g. G4 modules).
I plan to address these issues and improve the circuit soon. All in all, this project has cost a lot compared to readily available solar lamps (more than €50 without the already available PV panel). But it was worth it just to see that my design ideas can be put into practice.