Electronics

I/O modules

Custom electronics I/O is performed with 3 commercial RS232 cards. The computer communicates with the cards through USB with a USB to RS232 converter.

Programmable analog input module: WTAIN-M from Weeder technologies. [manufacturer documentation, pdf, July 2010]
Digital I/O module: WTDIO-M from Weeder technologies. [manufacturer documentation, pdf, July 2010]
Solid state relay module: WTSSR-M from Weeder technologies. [manufacturer documentation, pdf, July 2010]

Communication with the modules is described in the following 2 documents;

Stackable RS-232 Communications [pdf]
Host Programming Guidelines [pdf]

Temperature sensors

10k thermistors are used to sense temperature. Using Wheatstone bridge, with 10k resistors (Rx), 5V excitation voltage.

V = E ((R2/(R1+R2)) - Rx/(Rx+R3))

Using Wheatstone bridge + resistor R4 in series with bridge.

V = E1 ((R2/(R1+R2)) - Rx/(Rx+R3))

with

E1 = E  (R1+R2)//(R3+Rx) / (R4 + (R1+R2)//(R3+Rx))

(R1+R2) // (R3+Rx) = 1/(1/(R2+R3) + 1/(R3+Rx))

With E=5V, R1=R2=R3=R4=6.666k :

V = 5V (0.5 - Rx/(Rx+10k))

With the -10V to -8V voltage range setting on the WTAIN-M, the resolution is 22 uV = 0.18 Ohm on Rx at 0C = 0.4 mK (435 Ohm / C).
With the -0.6V to +0.6V voltage range setting on the WTAIN-M, the corresponding temperature range is -10C to +8.5C.

Electronics for Mount

The original mount electronics were replaced in July 2011 by two EZHR17EN EZstepper circuits (vendor: Allmotion) to improve tracking: the original circuit had unexplained behaviors and somewhat unreliable communication issues with the host computer. The EZstepper also provides more flexibility, and homing can be done without requiring the computer polling limit switch status.

Mount electronics (New configuration from Jul 2011)
Two EZstepper circuits drive the RA and DEC stepper motors (1 circuit per axis).

More information:

EZHR17EN command set [pdf]
EZHR17EN wiring [pdf]