The read/write heads of a hard disk drive are moved back and forth over the magnetic platters by a linear motor. This motor consists of a coil that moves in a strong magnetic field, combined with some sophisticated electronics that drives it such that the read/write heads are quickly positioned to the desired location. By now enough hard disk drives have crashed that every enthusiast should have no trouble getting his or her hands on one and using it for other purposes. As the head motor has a fairly long stroke and can supply considerable force, we used it in this project to build a special sort of clock. If you simply apply a DC voltage to the coil, the arm jumps from one end to the other one with a bang. If you reverse the polarity of the voltage, the arm moves in the opposite direction. The voltage applied to the coil can be controlled by a PC with the aid of a Darlington circuit (Figure 1).
We used one of the pins of the Centronics port on the computer (K1 in the schematic drawing) to drive the circuit. Here the control signal is provided by pin 2 of the Centronics connector, which corto bit 0 of port H378. Pin 19 (ground) is tied to the ground line of the control circuit. Use a hefty AC power adapter for the power supply; it must be able to deliver at least 2 A. The mechanical design of the clock is rather unusual. It consists of a length of curtain rail arranged at an oblique angle, along which a steel ball from a ball bearing can be propelled upward and roll back down under its own weight. If the ball is struck by a blow whose strength depends on the time of day, it will travel for a certain distance along the curtain rail. By observing the motion of the ball, you can read the time (approximately) from an hours scale marked along the length of the rail.
The previously mentioned head motor from a discarded hard disk drive is used to generate the impact on the ball. The ball rests against the arm of the motor when it is at its lowest point on the rail. The computer calculates the force of the impact and drives the motor for a certain length of time. The program for the clock is written in Visual Basic and has a simple design. The software is extensively documented. Now for some practical details on the clock:
Rail length approx. 160 cm (5’ 5”)
– Height difference (top/bottom) approx.
10 cm (5”)
– Ball diameter 17 mm (11/16”)
– Head motor coil resistance 5–15 Ω (depending
on hard disk model)
Coil voltage 5–12 V (depending on coil
resistance)
The hours scale on the rail must be determined experimentally after first adjusting the
impact for 12.00 h so the ball nearly reachesthe highest point of the rail.
