The assembly consists of
- a stepper motor with 200 steps/turn
- a single hand mounted on the motor's axis
- a Reed relay to indicate position 0
Two versions exist, and they are pin compatible regarding the parallel port connector.
On the power board, a 5 V regulator generates the proper voltage for supplying the FPGA board.
The stepper motor can be regarded as having a stator with coils repeating the same 4-phase sequence and a rotor acting as a magnet. Driving current into one of the coils forces the rotor to turn facing to the selected coil. The actual motor has 200 steps per trurn.
Tests have shown that the motor can be driven to rotate at up to 1 or 2 revolutions per second.
A Reed relay is used to indicate the 12 o'clock position. A Reed relay is a switch which is controlled by a magnet: When the magnet is close to the sensor, the contact closes.
In the chronometer assembly, a magnet is mounted on the tip of the hand. When the hand comes close to the 12 o'clock position, the Reed relay closes, but this does not imply that the hand is already at the proper position. Some extra steps might be necessary to effectively reach the 12 o'clock position