Solar Energy Project

Megan MacMurray and I set out to make a solar-powered night light for her nephews for this project. To accomplish this, we planned to use the SIMD1/Solar Regulator solar engine that could supposedly light a flashing led for up to 12 hours on a full charge. In our ideal vision for the project, we were going to build a couple of these engines in the first week or two of the project, then build cute housing for them during the second phase. However, as with many ITP projects, things weren't as easy as we planned. We found the Solar Regulator too confusing to build without previous experience, so we started with more basic circuits. Plus, we had to wait for the special 1N34A germanium diodes to come in to build the solar regulator.

The 1381-based solar engine

To build this 1381 engine, we first had to spend a ridiculous amount of time figuring out what was going on with the 3904 NPN and 3906 PNP transistors. Just so I don't forget, here it is. When you're looking at the flat side of either transistor, the emitter is the left pin, the base is the middle pin, and the collector is the right pin. In schematic diagrams, the emitter is the pin with the arrow on it.

We tried two different capacitors - with the 5V 1uF capacitor, our led stayed dimly lit constantly. With the larger 6.3V 4700uF capacitor, the led blinked brightly. The speed of the blinking increased as the solar cell was exposed to more light. Hint: click on the image to view it in Flickr with notes posted about various components.

Later, we also created a free form version of this engine, which took our soldering skills to new heights. Thank you to Rob Faludi for giving me a soldering tutorial and lending his forceps. The free form engine shown here is the same as the breadboard version, except for the substitution of a small motor for the led used previously. Soldering the free form version proved to be incredibly frustrating, as the legs of the transistor were fragile and tended to break off at the base, precluding any efforts to re-solder them together. This version has more wires than necessary because I was down to my last transistor and couldn't afford to break it, so I soldered and then hot glued wires to each pin to avoid having to bend them (or even touch them) again.

Simple Nocturnal Engine

Next we found a schematic for a very simple noctural engine, pictured here, on http://grant.solarbotics.net/Circuits.htm that worked very nicely. We were able to light an led for about 15 seconds after charging for about 30 seconds. Charging for longer didn't have an effect on the amount of time the led stayed lit for -- perhaps we need to experiment with a larger capacitor in the future. Note that the schematic doesn't specify which capacitor or diode to use. We tried both Zener and 1N400 silicon diodes and got similar results. The "large caps" we used were 4700uFs and 1000uFs. Below are both the breadboard and free-form versions of this nocturnal circuit. Megan has a video of the nocturnal circuit at work available on her site.

SIM D1 / Solar Regulator & PowerSaver Flasher

Finally, we got to the SIMD1/Solar Regulator and PowerSaver Flasher circuits. The schematic shown to the right (credit: BEAM wiki) was difficult for us to understand, especially because it appeared to have 2 powers and 2 grounds. Jeff helped us understand that while the solar cell provided power and ground, those lines were different from the 2V and 0V lines that provided power and ground to the PowerSaver Flasher circuit. It's also important to note the the red led shown in the SIM D1/ Solar Regulator circuit should NOT light up -- if it is lighting up, that means it's connected to power from the solar cell instead of the 2V line. Another problem we encountered was that we didn't have a 74HC14 Schmitt inverter, so we used the similar 74HC04 hex inverter, which had the same pin layout. However, we discovered that the Schmitt inverter is essential to the functioning of the circuit, as it has the unique property of hysteresis. I don't understand what hysteresis is, but it's important --take my word for it. When we borrowed the right inverter, everything suddenly worked. Ta da!

Our success with the SIM D1/Solar Regulator was mixed - we were able to make an led blink rapidly for an extended amount of time - we never timed it, but it exceeded 30 minutes. However, the led was extremely dim and wouldn't have worked for a night light. We're interested in pursuing the improvement of this engine for the final project, so we'll see what happens.

Megan compiled the following list of parts necessary for these two circuits:

Solar Regulator Parts

• Two 2n3904 NPN Transistors
• 2n3906 PNP Transistor
• Two 100k resistors
• 1000uF Capacitor (should be 1 F for best performance)
• Germanium Diode (1N34A)
  
• Solar Cell SCC3733
  
• 0.1 uF Capacitor
• red led (this is important as different color leds have different resistance)

Flasher Parts

• Schmidt Inverter 74HC14
• 1.0 uF Capacitor (not recommended in schematic, but works)
  
• 100k resistor
• 10 mega Ohm resistor
  
• .33 uF Capacitor
  
• led (pick a color, make it pretty)

Resources

BEAM Reference Library hosted by solarbotics.net
BEAM wiki hosted by wikipedia (these two sites have a lot of the same information, but the wikipedia site is cleaner and easier to navigate)