There’s two parts to the PumLantern. The “dark turn-on” circuit, and the
actual Pum circuit. Let’s start with the Pum circuit!
The Pummer is based around a very simple BEAM circuit called a “Bicore”,
which is simply an oscillator that has two outputs that change voltage.
When one is “high” (3V), the other is “low” (0V). Every so often, they trade
voltages.
We’re using this “trade voltage” behavior to create a charge-pump, which
is a fancy way of saying we’re building something that makes more voltage
than what it normally
access to. Here’s how it works (showing only one
pair of LEDs in operation):
has
C
IRCUIT
T
HEORY
The stored 1.7V is
added to the normal
3V now flowing back, like
a second 1.7V battery!
When the output swap polarity, the
stored 1.7V is added to the 3V, and
the LED goes super bright from 4.7V!
0V
+3V
Power
Flow
LED3
Super-bright PUM
from 4.7V!
LED1 (blocks
power flow)
+1.7V
With the bicore output + / -, it charges
the capacitor to ~ 1.7V, making LED1
light up while power passes through it.
+3V
0V
1000µF capacitor
charges up to 1.7V
(3V-1.3V=1.7V)
w
o l
F
r
e
w
o
P
LED3
(blocks power
flow)
+1.7V
LED1 “eats”
1.3V as light
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Page 14
The “dark turn-on” circuit is pretty simple. The chip has an “enable-low”
line, which means when it sees zero volts, it turns on. We attach this line to
the solar cell, and when it stops generating power (sun goes down), the
100kOhm resistor forces the chip on. That simple!
