Heroine Virtual Ltd: Trapezoidian Equatorial Mount

The electronics package supports full speed forward and backward movement, star tracking movement, and tracking adjustment. It uses speech synthesis to communicate with the user, allowing you to know the status while looking through the viewfinder. Take a listen.

The speech synthesis consists of 1 bit audio recordings of 10 digits, back, in, and out. Most of the microprocessor's 8k of flash memory contains sound samples. The audio is compressed the same way it was done on Commodore 64's. The time difference between each crossing of 0 in the waveform is recorded as 4 bit integers. The microprocessor inverts the speaker pin after every time difference.

The numbers are mostly samples of MARS HEROINE. 9 came from her sidekick. 0 came from a documentary about nine hundred eleven, go figure.

All functions are done by a PIC 18F1320 at 40Mhz. That was more accidental than intended, since most features were added by discovery, leading to a much better design if the PIC 18F458 was used. It would have cost too much to desolder and install a bigger chip every time a new feature was needed.

Since the corkscrew period is about 7.2 seconds and is coupled directly to the tachometer, it takes a serious feedback algorithm to hit the target speed. A directly coupled tachometer on the corkscrew allows the tachometer to produce the most accurate reading possible. The IR cutoff sensor is not highpass filtered like most IR sensors. We've had better luck using a DC LED and calculating a derivative. The microprocessor waits for a rise of a certain amount. Other delay counters prevent false positivies.

The servo is pulsed with fairly long pulses to keep it from stalling yet control speed. Massive capacitance keeps the pulses from interfering with the tachometer.

With tachometer reading and target speed, the CPU calculates a new pulse width after every rotation. It takes several revolutions for the servo to hit the target speed. Mars Heroine says "in" or "out" to notify you when the target speed is achieved.

The LM324 amplifies the tachometer cutoff sensor and amplifies the microprocessor signals to the 10V required to switch on the H bridge.

The drive speed needs adjustment in the field because temperature variations and assembly differences change the corkscrew reduction ratio.

2 buttons increment and decriment the drive speed by 1% and the microprocessor plays prerecorded sounds for each setting. Compared to an LCD panel, speech synthesis was the cheapest way to display the current drive speed since it only uses 1 pin.

The AIM switch causes the right ascention arm to move up and down at full speed. This is mainly used for resetting after an hour of observations.

The TRACK switch causes it to track using the tachometer.

8 AA batteries provide 12V of power for the servo. Additional current is bled off to power a lens heater. The lens heater is 4 5 ohm, 5 W resistors running on 12V. This has been very effective against condensation.