Little Alice

This is a servo. It goes bzzzzzt. And moves. It’s a tiny motor that can be turned to a specific angle by giving it a specific signal. It’s both technically crude and impressively sophisticated:

It’s basically a little more and a load of gears, but with a very important addition – the potentiometer inside is turned at the same time as the final cog in the gear chain. This potentiometer sends a feedback signal back to the control board, so that it knows when the motor has reached the desired position (or not).

Even though the motor turns (relatively) quickly, and has very low torque (relatively) because of the gearbox, the “horn” on the top rotates relatively slowly (and has much higher torque).

It’s the movement of all these gears against each other than makes the familiar bzzzzt sound, when a servo moves.

All servos work in pretty much the same way – they have a guaranteed range of about 90 degrees (some servos can move in a wider arc than this).

To make a servo move, we send it a “pulse” of a specific width, every 20 milliseconds or so. A pulse that is 1.5ms long tells the servo to move to the “centre” point (many call this “90 degrees”).

A pulse every 20ms or so that is only 1ms long would cause the servo to move to about 45 degrees. A pulse every 20ms or so that is 2ms long would cause the servo to move to about 135 degrees. Some servos accept pulses of less than 1ms (and can therefore move to an angle of less than 45) and pulses of more than 2ms (pushing them beyong the 135 degree angle) but this is never guaranteed.

Some servos “chatter” if you try to make them move beyond a 90 degree range.

I mounted the tiny 9g servo inside the base of my diorama and hot-glued the end of the rod (on which the dolls head had been glued) to the centre of the servo “horn”. I ensured that the servo was at it’s maximum rotation (135 degrees) and glued the doll’s head at it’s extreme rotation (facing the Alice character on which the doll sits).

Now, when I set the servo position to 45 degrees, the doll appeared to be face outwards, towards the viewer.

By now, I had an assembled diorama with a seated Alice, a spooky doll (with light-up LED eyes) and the ability to make its head turn to either face Alice, or towards the viewer.

It was really creepy.

I wanted more. It needed to make a sound…..