Creepy Cave Crabs – Light detecting light & music synths

by Jimmie on October 27, 2008

creepy crab 01
Check out the Flickr set

I made these for the Willoughby and Baltic Halloween show at the Charles River Museum of Industry. They use a single Hex Schmitt trigger inverter IC to generate the sound, and two RGB color change LED to generate the lights. I then encased them in friendly plastic, and painted them with silver paint.

Here is a short video on what they look/sound like:

I’ve also drawn up a quick schematic with the circuit. The capacitors were all mixed from 0.1-0.35uf, and the photo resistors were also grabbed randomly from my collection. Same with the diodes. The only thing I made sure to have as a constant was the light detecting eye circuits. They were tuned to the specific photo resistors I had multiples of, so you may need to adjust the value a bit, but 100k should be a decent place to start to make them very sensitive to light.

creepy cave crab schematic


  • A small perfboard
  • A 9v Battery snap


  • 1 Hex Schmitt trigger Inverter IC 7414(Jameco)
  • 6 Various capacitors from .1-.35uf
  • 6 Various photo resistors
  • 6 Various diodes
  • A 1/8″ audio jack


  • 2 NPN 3904 Transistors
  • 2 100K Resistors
  • 2 RGB slow blink LEDs (ebay – “rgb led slow diffused”)
  • 2 photo resistors (use similar ones if you want the same level of light detection for both eyes)

I purposefully wanted to have them scream loudly when the light comes on, but if you want to use these in normal light conditions, I would suggest that you play around a bit. These can be tailored to any light level, so just play around with what you’ve got. The fun thing is that this circuit is extremely forgiving and works in many ways, so you just need to play around to get the sound/sensitivity that you want. I’m sure you could even put a potentiometer in series with each photo resistor to tune them.

Here you can see the circuit running on a breadboard. This is the one that is running on the right in the video:
creepy crab 06

Once I soldered everything together, I then covered it all in friendly plastic, and painted it with silver paint. I did this because the crabs are going to be setting in a dark room with an oscillating light, which will have them in alternating light and dark.

I’ll actually be running an open hack at the Willoughby and Baltic tonight, where we will be making a few more of these for the show. Feel free to stop by, even if you aren’t a member. Come check them out, or just drop by the space and see it. I’ll post more on these later, and I’ll throw up an Instructable if I manage to find a few free hours before the Halloween show.

{ 4 comments… read them below or add one }

1 Trevor Henry March 13, 2010 at 16:46

See, Jimmie, that’s what I meant earlier today – without knowing all this stuff, how does one begin ??

quote – “1 Hex Schmitt trigger Inverter IC 7414” unquote

Er, what is one of those ? I am tempted to ask – wtf ?

2 tHaH4x0r June 1, 2010 at 17:23

Hi, Trevor,

First let me explain what a hex schmitt trigger inverter IC is. The 7414 is the chip name and refers to a inverter, it turnes bit wise 1’s into 0’s and 0’s into 1’s, but here it is used as an oscillator. The Hex says that there is a total of 6 inverters in 1 chip, but there is something special about this inverter, its a schmitt trigger. I am not going to explain what a schmitt trigger is because it is a little bit to complicated, i can give you a website for a electronics tutorial:
If i remember, schmitt trigger explanaition is on page 49.

3 RICHIE October 9, 2012 at 02:48

Is your project interesting, congratulations. I WILL DO ONE, thanks for sharing.

4 Crow October 31, 2012 at 04:19

Very nice, I really liked it and will try to find the time to build one (or more!) soon.
I have most of the components laying around, only missing the light changing LED’s which I may really buy considering their low price on ebay.
I liked the “Creepy crab” idea, and that made me laugh to see how they scream when you turn on the light, heh.
I’ll play around with making the circuit into a creature as well, so regarding that: how about maybe integrating a small speaker into the creature’s body? I have a bunch of small speakers (maybe an inch or so in diameter), and will try to see how it works.
Thanks or posting this!

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