Wiring up Wilf Rigter's SIMD1 V3

tl;dr: A FQP30N06L will work as the MOSFET in a SIMD1 V3 circuit, although it’s not a very bright solar engine with the parts I’ve used.

In the 1990’s, Mark Tilden cooked up the idea of BEAM robotics. Instead of using complicated digital electronics, he’d wire up simple biologically-inspired analog computers that often out-performed their digital equivalents.

Eventually, Tilden lost interest and a Yahoo Group full of people cooked up their own ideas, then those people lost interest in the whole subject… and I guess it kinda lives on with some pages that managed to be archived properly. When you were talking about a robot that could zoom around with a little bit of funky circuitry versus a BASIC Stamp, it was a huge thing, but now that you can throw a low-powered ARM microcontroller at your problem, there’s less of an advantage.

I still find some aspects of BEAM robotics cute. Especially the idea of little photovoric robots that skitter about the ground searching for light to eat. Or a plant that collects energy during the “day” via photosynthesis and then emits light during the “night” just like plants respirate.

One circuit that always looked interesting was the SIMD v3.

The thing that bugs me is that there was never an appropriate MOSFET specified. And I’d asked questions on the forum a long time ago and took notes and never actually bothered to get any of the recommended MOSFETs. So I had everything but the MOSFET for an embarassingly long time. Eventually, I decided that since the MOSFET that both SparkFun and AdaFruit carried as well as the one on The Bald Engineer’s list of transistors was the FQP30N06L.

So I wired it up with a 3v solar cell and a 0.22 farad supercapacitor I had and it worked. Here’s my schematic, with the parts labeled and the important blocks called out:


The main transistor, Q1, ends up doing double-duty. First, it prevents the solar-cell from discharging the storage capacitor in the dark, basically using a transistor as if it’s a diode. Second, it provides a signal to trigger Q2, the mosfet, to switch the load on, which is more obvious if you look at the SIMD1 v2 design and compare.

Also, if you look at the design, Q1 is providing a crisp switching voltage level for turn-on, which you wouldn’t necessarily get if you just used a MOSFET as the sensor.

In my case, I had a 3v solar cell and a 0.22 F supercapacitor to play with. You can put a voltmeter across the terminals of C1 and watch the capacitor charge, then check that the enable line (e) is about zero voltage when the lights on but then carries a voltage when there is power.

At least in my case, with a 3v solar cell, the whole thing is kinda marginal because the capacitor is never really powered up enough to get the LED overloaded even without a resistor and the MOSFET’s floating around VGS(th).

So I probably want to either use a higher-voltage solar cell or a lower VGS(th) MOSFET.