MAKE 08: Pummer, Dude!

85Make:

N MAKE VOLUME 06, I WROTE ABOUT BEAM,

a branch of robotics built on low-end, mainly

analog electronics that is inspired by biology.

I described how to build two types of bots in the

BEAM taxonomy: Solarrollers and Symets. One of

the more obscure members of the BEAM family

tree is the Sitter, an immobile robot with few or no

moving parts.

One of my favorite types of Sitters is the Pummer,

a nocturnal, robotic plant that soaks up the sun

during the day; stores that energy in batteries or

capacitors; and then, when it senses darkness,

feeds power to a light which pulses, or “pumms,”

away in the dark. Since the electronics are simple

and minimal, you can have fun with the design of

your Pummer, creating a swanky piece of high-tech

art that will intrigue everyone who sees it adorning

your geekosphere.

How a Pummer Works

In “A Beginner’s Guide to BEAM” (MAKE, Volume

06, page 54), we talked about different types of

Solarengines (SE), which are simple power circuits

for actuating miniature robots. We mentioned the

nocturnal type of Solarengine. This is the variety

of SE used in many Pummers. All SE circuits work

in much the same way: the solar cell captures light

energy, converts it to electrical energy, and sends

it to storage, either in capacitors or rechargeable

batteries. When a trigger value is reached, the

stored energy gets sent off to do some sort of work.

In a voltage-triggered SE, the trigger is a set voltage

ceiling. In a nocturnal SE, the trigger is a threshold

value of light.

Looking at the circuit diagram on the following

page, you might be asking yourself: where is the

sensor that tells the Pummer that it’s lights out and

time to get with the pummin’? Ingeniously, the solar

cell and the circuit itself serve this purpose.

During the day, when light hits the cell and the

cell is sending juice to storage, the diode in the

circuit keeps the enable line set to high. When the

level of light/current reaching the cell/circuit falls

below a certain value (as set by the value of the

parallel resistor), the enable goes low, triggering the

discharge cycle and the pumming of the LED(s).

The diode, being a sort of one-way valve in a circuit,

prevents the current from flowing back into the

charging part of the circuit; it has no place else to

go but along the discharge path.

Pummer Circuits

There are a number of different Pummer circuits

you can use, from simple ones that power a single

LED, to more sophisticated ones designed to maxi-

mize power collecting and discharging, and ones

that can power multiple LEDs. The one shown here,

used in the Solarbotics Bicore Experimenters BCP

Applications Project (see makezine.com/08/

pummer), balances simplicity with circuit efficiency

and bang-for-buck; i.e., it makes a pretty damn cool

Pummer without too many building headaches.

This nocturnal SE circuit makes use of another

hallmark BEAM circuit, the bicore, which is the

basic “neuron” of BEAM “intelligence” (see MAKE,

Volume 06, page 54 and page 58). Here, the two-

state oscillator is used to create the flashing/

pumming behavior. The C1 and C2 caps are used

to set the blink/pause rates, and C3 handles the

“decay” rate of the pumms. You can play around

with these rates by trying different capacitor

values on a breadboard.

Other Pummer circuits, including those that

can handle multiple LEDs, can be found on

Solarbotics.net, in /library/circuits. Costa Rica

BEAM (costaricabeam.solarbotics.net) has a fairly

thorough library of schematics for Pummers,

including a circuit for making a Type 1 Solarengine

(which uses a 1381 voltage trigger) into a darkness-

activated power circuit.

Pummer Designs

One of the cooler aspects of a Pummer is that,

because it’s a Sitter and has no moving parts and

no concerns over weight, etc., the design and aes-

thetics of the robot can take center stage. You can

build Pummers to look any way you want. A lot of

builders, inspired by the idea of Pummers being

a sort of robotic plant life, put the LED(s) on a long

stalk or on multiple stalks. But Pummers have

also been built in the shape of modern sculptures,

hexagons, triangles, cubes, even a dragon with solar

MATERIALS

To build this Pummer circuit, you’ll need:

Solar cell that can deliver 3V at 20mA

(I recommend the SCC2433a from Solarbotics)

74AC240 Octal Inverting Buffer IC

AAA NiCad batteries (2) or you can use 10F “gold”

capacitors (2)

0.22μF capacitors (2) often marked with “224” on the cap

1000μF capacitor or 3300μF for a longer fade-away

1kΩ resistor

4.7MΩ resistor

LED any color, high-intensity LED recommended

Diode A low-voltage type, such as the 1N5818

Schottky or a germanium diode, is best, but a silicon

one works too.

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86 Make: Volume 08

Gareth Branwyn writes about the intersection of technology

and culture for Wired and other publications, and is a

member of MAKE’s Advisory Board. He is also “Cyborg-in-

Chief” of

Streettech.com.

cells on the wings and glowing LED eyes. Really,

your imagination and building skills are the only

limitations.

A large majority of Pummers are built using

paperclips as the main building component. Zach

DeBord, a BEAM builder from Chicago (whose

Pummers are pictured here) writes: “Buy a pack of

jumbo and regular sized paper clips. For the $2 you

spend, you’ll be able to build a whole fleet of robots.

I almost exclusively use paper clips and guitar

strings for my creations.”

Other common structural components are rubber

bands and heat-shrink tubing. “An assortment pack

of heat shrink (available at RadioShack and other

places) goes a long way,” says DeBord. “Not only are

your bots more interesting looking, but you can use

tubing in key places to reinforce weak joints.”

For more Pummer resources visit

makezine.com/08/pummer.

TOYS

GAMES

Zach DeBord’s collection of Pummers made from paper

clips, guitar strings, rubber bands, heat-shrink tubing,

and a dash of imagination.

Single-LED

High-Efficiency

Pummer

R2 4.7M 3V 20mA

74AC240

224

R1 100K

R3 1K

+C3

LED

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