Mix up some PVA glue, three parts glue to one part water. The water will help the glue soak into the paper and improve drying times. Rip up white paper and dip into the glue mix before applying all over the skull. Ensure it is fully covered, and apply two or three layers for strength. It’s a good idea to leave the skull 30 minutes to one hour between each layer to dry. Then leave overnight for it to harden.

Once it’s dry, we need to decorate the skull. This can be done with a final skin of paper using a printed image of a skull, a technique called découpage. An example skull image is available on Wikimedia Commons: hsmag.cc/eBBMCv. Apply this in the same way as the papier-mâché layers, by ripping the picture into smaller pieces and sticking them onto the skull. Use plain black paper for the eye sockets and red paper for the bowl. An alternative would be to paint or colour these with a permanent marker.

To give the skull a protective skin, it is coated with shellac. The ‘button’ or ‘garnet’ colours work best. It dries quickly and gives an antique appearance. One coat should be sufficient. If you use a dedicated brush for shellac, you can give it a quick rinse in thinners before storage, and then a soak before the next use.

Glowing Gemstones

Select some acrylic gemstones of a suitable size to fit into the eye sockets. The stick-on or stitch-on type, that have a flat side, should be easiest to use. The flat side is painted silver and helps reflect the light. So that the light from the LED will shine through, lightly sand the silver paint.

A reflector cone is formed from kitchen foil. To ensure there is some overlap to join the cone, we needed to draw our template slightly oversized. Draw a 15 mm diameter circle, and a second circle of 40 mm around that. Cut out the large circle and slit the disk to the centre. Now cut out the inner circle.

Form the foil into a cone and wrap around a 10 mm LED. Tape this into place. If your gemstones are not round, you may need to make some adjustments to the shape of the cone. You can do this by folding over the edges, pinching and folding, or by cutting off the excess. Fill the cone carefully with hot glue and stick the gemstone to the top and leave to cool. Repeat with the other LED and gemstone.

Wires and crimps

You will need two lengths of different coloured wire for each LED, which are long enough to pass through the skull and out of the back. This will mean you can connect the Gemma before sliding it back into the box. Crimp one wire to each leg of the LED. Make a note of which wire connects to the anode of the LED. Use a bradawl or skewer to make a hole in the back of the eye sockets and into the cardboard box. Pass the wires through the holes and out of the box. Crimp a resistor to one of the LED wires.

You’ve got the touch

The sensor for the touch is formed from the screening from the screened wire. Remove the outer plastic cover of the screened wire and slide the braided screen off the wires. Flatten the braid and form into a loop to fit around the rim of the bowl. Twist the ends together and crimp to a wire. Make a hole from the bowl through to the electronics box and pass the wire though. The braid can then be attached to the bowl with hot glue.

Strip the ends of the wire and wrap around an M2.5 bolt, and twist the end to form a loop. Form a loop on the end of each resistor. Bolt one LED anode wire to the D0 connection on the Gemma, and the other anode wire to D2. The cathode wires should be bolted to the 0 V connector. Bolt the sensor wire to the A0 connection.

Brain Power

The Gemma M0 uses an ATSAMD21E18 processor, and this is powerful enough to run a subset of the Python language called CircuitPython. Combined with the provided libraries, this means we can build this kind of project with just a few lines of code.

import time
import board
import touchio
from digitalio import DigitalInOut, Direction

We start with instructions to use the libraries for time, board, and touchio. For the digitalio import instruction, we’ve just picked the items we need. This can be a useful technique to simplify the code you write later.

touchpad = board.A0
touch = touchio.TouchIn(touchpad)
led1 = DigitalInOut(board.D0)
led1.direction = Direction.OUTPUT
led2 = DigitalInOut(board.D2)
led2.direction = Direction.OUTPUT

The pins need to be configured before we can use them. The A0 pin is configured for touch, D0 and D2 are configured as digital outputs. Note how Direction and DigitalInOut don’t need a prefix as we imported those specifically above.

def detected():
for i in range(3):
led1.value = True
led2.value = True
time.sleep(0.5)
led1.value = False
led2.value = False
time.sleep(0.2)

This code defines a function we can call when the skull is triggered. It flashes the LEDs three times, with a short delay between each flash.

detected()
while true:
if touch.value:
detected()
time.sleep(0.05)

To check that the lights are working, the code calls the detected function when the Gemma first powers. The while statement creates a loop that will keep running until the power is turned off. It checks the touch sensor to see if it has been touched. If it has, then the code calls the function defined earlier to flash the lights.
Upload the code to the Gemma and remove the USB connector. Fit the batteries to the battery pack, and plug into the Gemma. Test everything is working, then fit the electronics into the back of the skull. Add some treasure to the bowl, e.g. sweets. Your monkey skull bowl is now ready for use.

Capacitive sensing

The capacitive sensors on the Gemma work by charging and discharging the output pins and measuring the voltage. The pins have a small amount of capacitance and the microcontroller is sensitive enough to measure this. When a person is very near to the sensor, they affect the circuit by acting as a second capacitor. This means that when the microcontroller charges and discharges the pin, it will see a change in behaviour and know that it has been touched.

Other ideas

• Paint your skull in candy skull style for a ‘Day of the Dead’ look.

• Use PWM to make the eyes fade up and down, rather than flash.

• Use servos to make your skull move.

• Swap out the Gemma with a Circuit Playground Express board, and add some audio effects to your project.