February 4, 2009
I'm trying to build a Power-Smart Head from a design in the Solarbotics' community pages. It doesn't seem to be working, and the community seems to be dead so perhaps someone here with more electronics knowledge can help me.
The basic article is here. There's a circuit schematic here, and a circuit layout here. You may notice that the components in the schematic and the layout don't quite match. I've followed the schematic. The motor I'm using is supposed to be ideal for use with solar cells. The spec is here (although the server is currently off-line for some reason).
If I replace the solar cell with a 4.5v battery everything works as expected. With the solar cell nothing happens.
When disconnected from the breadboard, the solar cell produces about 3v - 3.5v (depending on how close I hold it to an incandescent bulb). Connected to the breadboard with both the super-caps disconnected I get about 2.3v. Connecting one capacitor in I get 0.7v or 0.8v; and with both in 1.5v. I think the solar cell is producing between 0.1mA and 0.2mA.
I noticed that the voltage seemed to be creeping up slowly, so I left it for an hour or so. There's now 2.4v coming from the solar cell. I assume that the solar cell is charging the super-caps, and when it reaches some value greater than the trigger voltage for the 1381 the rest of the circuit will come to life. Does this all sound reasonable - should I just be more patient?
I'm using a 1381C (spec sheet here), which should trigger at some point between 2.0v and 2.2v. Is this the voltage between Vdd and Vss (pins 2 and 3)? I have 2.08v and it's slowly rising.
Now the whole thing seems to have fired/discharged. There's 0.46v across the solar cell and 0.29v across the 1381; both rising. However, the LED didn't flash and the motor didn't turn.
Assuming the circuit is basically OK, how can I make it charge faster? Two solar cells in series or parallel? One super-cap instead of two? A smaller super-cap (eg 0.047F as shown in the layout)?
Would a higher trigger threshold on the 1381 increase the likelihood that the motor would turn?
Any and all pointers gratefully received.