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Screaming Creep head on: an angry ball of red, orange and yellow fur with a yellow mohawk, heavy black vinyl brows over wide white eyes, an orange sneering lip and two huge cream buck teeth. In each pupil sits a photocell - a small silver disc with a zigzag track, ringed in orange - staring straight at you.

Build Log · 555 Theremin, Monster Edition

Screaming Creep

555 Optical Theremin Fresh off the bench

A 555 theremin wearing a gutted Grumblies. The toy's own electronics are gone; in their place is the two-photocell 555 oscillator from my 555 Optical Theremin guide, with the two LDRs in series sitting right in its pupils. The light on its face plays the scream - wave a hand over its eyes, lean in close, cup both and it goes quiet. An on/off toggle and a bare-shaft volume pot hide in the fur on the back of its head, and it runs on a 9V jammed where the AAs used to live.

  • Type555 optical theremin in a toy shell (one-off build)
  • ShellGrumblies rage-monster, the fire-colored one - gutted
  • EyesTwo photocells in series - the theremin's timing resistor
  • ControlsOn/off toggle · volume pot, both through the back of the shell
  • OutputIts own speaker - no amp needed
  • Power9V battery in the gutted AA bay
  • StatusFinished Aug 2026 - one of a kind

What it is

Grumblies are little interactive rage-monsters - provoke one and it rumbles, growls and finally blows its top. This one no longer needs provoking. I gutted it - the stock electronics came all the way out - and built a 555 optical theremin into the empty shell: the same circuit as my 555 Optical Theremin guide, with the two photocells in series as the timing resistor. Those cells sit in its pupils. Shadow falling across its face, a hand cupped over one eye, the sweep of a flashlight - the creep sees all of it, and the scream follows the light.

The rest of the surgery hides in the fur: an on/off toggle and a volume pot come straight through the back of the shell, bare shafts only, no knobs - part the fur to find them, flip it awake, and set how loud the screaming gets to be. Underneath, the stock battery bay is gutted and a 9V on a snap lives where the AAs went, so it runs on the same battery as everything else on this bench.

Hear it scream

Demo filmed by Onion Madder on the actual unit.

What's inside

A 555 oscillator where the toy's brain used to be, with photocell eyes - two LDRs in series as its timing resistor, one in each pupil, so light is the instrument. A ball-top toggle through the back of the shell switches the creature on and off - no more shaking it awake - and a bare-shaft volume pot beside it, buried in the fur. And the 9V conversion - the AA compartment stripped out, a snap connector soldered in, and the battery door still closes over all of it.