Let's Make Robots!

First time building MSE, having problems.

I bought a solar cell from Solarbotics.com that has a standard Miller engine etched onto it ( http://solarbotics.com/products/scc2433b-mse/ ), then I got my hands on some transistors and capacitors to use. I soldered everything up onto the board, hooked up my power capacitor and a motor. I have my multimeter connected to the leads of the power capacitor and I can watch the voltage climb up to 2.6V and then stop. Its my understanding that when it gets to this point, the circuit is then supposed to dump all that voltage into my motor, but nothing happens!

I'm wondering if the transistor I substituted for the 1381 voltage trigger is to blame. I am using a TC54VC2102EZB, which according to its datasheet is supposed to trigger at 2.1V. But like I said, I can see my power cap go up to 2.6V and nothing happens.

Here are the values I'm using for the rest of the circuit, and a picture showing how I connected things to the solar cell:

C1 = 4700uF

C2 = .47uF

TC54VC2102EZB instead of 1381 for voltage trigger

2n3904 transistor

Solar cell rated at 4.5V / 18mA.

Here is what my soldered circuit looks like: http://imgur.com/gVtD4.jpg

Any help is appreciated!

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It looks like you're missing  a diode in there.  Without it the trigger doesn't see ground.

Ah! I thought I read that the diode was optional, but when I put it in I was able to get an LED to flash :) Thanks!

I've noticed that when I measure the voltage across my power cap in standard fluorescent room lighting it tends to hover around 1.5V without climbing or falling, but starts to climb when I shine a flashlight on it. I'd like to have this thing work indoors, is there a way I can modify the MSE circuit to trigger at a lower voltage? I don't know if I can get a 1381 with a low enough trigger voltage for that.

Or maybe I should just tell whoever I give it to to always keep it by the window?

I'm glad it works for you. I remember my first SE success. Ahhh.. :)

Yes, the diode is optional, but you'd need to jumper those two pads to complete the circuit. I do believe it would lower the trigger voltage (by whatever the forward voltage is of the diode) as well.

Fluorescent lighting sucks. Very little light to harvest from those things from distances greater than very very close (the whole square of the distance thing). If you want to have it work in such conditions you'll need a panel rated at a higher voltage, sorry.

There is no way to have a trigger to go off at a lower-than-rated voltage. You can make it fire at higher voltages by wiring a pot into the circuit, but not lower.

You are correct, MN1381s only go as low as 2.1v I believe. You could use a MAX8212 which has a programmable trigger point. 1.5V is almost at its lowest threshold. It uses an open-drain instead of a CMOS output, however. You would just need to substitute a PNP or a P-FET instead of an NPN transistor to get it to work. Solarbotics has a pdf with an example of such a circuit here.

When I tried to get my first BEAM bot to trigger a pair of 1381J MSE's indoors it took 8.0V panels to make it work! Barely. That was with what I thought was decent incandescent lighting too.

Just figures for you to consider.