Are you ready for Halloween?

In this Instructable we will show you a fun alternative to a candle to light up your jack-o-lantern. It’s a light source that can create and combine two patterns for a creepy effect: a breathing pattern and a pseudo-random pattern. These patterns, as well as the LED power driver, are all achieved together in a single Silego GreenPAK SLG46116 chip.

Step 1: How It Works?

The breathing pattern causes the illumination intensity to wax and wane. It’s implemented by using a pair of counters with periods that offset from each other to generate a time-varying PWM. The reset pulses of the two counters are XOR’d into the clock input of a toggling flip-flop. The resulting output thus has a pulse width equal to the offset of the counter outputs. Since the periods of each counter is offset, the flip-flop pulse width slowly changes up and down, completing a cycle at the beat frequency. The PWM is relatively slow, giving the observed effect a flickering characteristic as it “breathes.”

The other pattern is made using a pseudo-random generator that produces a relatively unpredictable pattern of pulses. The pattern is implemented using a Linear Feedback Shift Register (LFSR) approach. A 10-bit shift register is created from a “pipe delay” block, combined with a couple of DFFs. Certain bits along the shift register are tapped (one on the 10th bit and the other on the 7th bit), then XOR’d and fed back into the 1st bit. This creates a maximal length pseudo-random sequence from those 10 bits. The 10th bit served as the LFSR’s output. The first bit is initialized high, otherwise the LFSR will have no high value to clock through and remain low.

Step 2: GreenPAK Design

Two different combination patterns are implemented: one using XOR and the other using AND, for different pattern results. The Ghoulish Illuminator has total four pattern modes selected by SEL0 and SEL1:

SEL0SEL1OUT

0 0 Ghastly Gasp (Breathing)

0 1 Sinister Strobe (Pseudo-random)

1 0 Ghoulish Gleam (XOR)

1 1 Lurid Luminescence (AND)

The particular GreenPAK device chosen for this design includes a P-FET power switch that has a maximum current rating of 1A. Two jumper wires on the breadboard implementation allow the user to select one of the four pattern modes shown above. A four-bit Look Up Table (LUT) is used to merge the two select signals, and the two internally generated signals (LFSR and Breathing Pattern) into the signal that directly controls the power switch, and therefore the pattern seen on the LED.

The GreenPAK is powered by 3 AA batteries (about 4.5V). A 10Ω resistor limits the output current for the LED to about 100mA, which is well below the current limit for the power switch. The P-FET output drives a 0.5W LED which illuminates the jack-o-lantern. The circuitry with minimal external circuitry was implemented on a breadboard, and attached to cardboard and fixed to the jack-o-lantern lid with a screw through the stem.