VideoStar jr. is Alive!
By popular demand, I have created a small and inexpensive subset of the VideoStar system specifically to allow the computerized control of DC motor driven focusers with optical encoder position feedback. The system has the capacity to feed back values from a temperature sensor to provide temperature compensation capabilities. Finally, it contains relays to allow RS-232 control of the telescope through the guider port.
Features:
DC motor output for NGF-S focuser or home made units using DC motor drives.
Optical encoder input from standard quadrature encoders like used on the NGF-S
Stepper motor step/direction/enable outputs for use with stepper based systems
RS-232 serial interface
Temperature sensor to enable temperature compensated focusing
Focusing knob
Guider port output for standard telescope mounts
Compact size
12V power supply input
Target parts cost: <$50 (not including cabling and bare circuit board). (ok, I've exceeded this slightly)
For a cost breakdown, check out this page: VideoStar jr Costs
For more information on the VideoStar project, please visit the VideoStar homepage
Sample PC Board:
The finished diagram below is the actual VideoStar jr. Circuit board as it has been created. The actual size of the board is 1.75" tall by 4.85" wide. This slides into a small Radio Shack case (pn 270-1803). A tiny external controller is plugged in to provide manual control of focus and provide one "trigger button" - the use of such is optional.
The 4-pin connector labeled "Prog1" is used for programming the microcontroller using the Parallax inc. SX-Key device. This allows do-it-yourselfers to reprogram the entire system. In fact, the VideoStarJR unit makes a great prototyping system for all sorts of device controllers.
The system is designed modularly so that one does not need to populate the section dedicated to guider port relays or to the temperature probe if one only wants an RS-232 controlled focuser.
The circuit is powered via 7-18V @1A. This is converted through a precision switching power supply. This is much more efficient than using a simple 7805 voltage regulator.
Please see the table below for a description of what each part does.
(NOTE: circuit diagram below is the actual circuit board as I have had made)
Purchasing the VideoStarJR:
Even though this project is intended for do-it-yourselfers, several people have asked if they could purchase completed units so I've built up boxes for them. If you're interested, please drop me an email.
Now that I've built several, and have a good idea of the costs and time involved, I've set the price at $200 for single units. To make a single unit is typically a half-day project, between soldering the parts, testing the board and building the cases. The design and programming of this system turned it into a several month project, so all told, this isn't a profit making venture! But I'd at least like to pay for the circuit design software, developer's kit for the microprocessor, and cover other basic development costs.
If you're looking for a commercial product that runs the JMI NGF-S, see:
http://www.jimsmobile.com - see the SmartFocus system
http://www.astrovid.com/pc_focus__and_focusaide.htm - FocusAide focus system. This is by far the most complete system of its kind.
VideoStarJR Circuit Board
Important note: there's one important error on the circuit board! The software I'm using had the symbols wrong for transistors so the transistors actually install on the board opposite of the way they are indicated by the above diagram. That is, rather than looking like a "D", they should look like "(]". The circuit itself is ok as long as the transistors are installed "backwards".
Important note 2: The decoupling capacitors used for the temperature probe (the little 1 and 2 under the mini-stereo jacks) should NOT be installed on this board. They, along with the 2.2k pull-up resistor for the temperature probe, should be mounted right next to the probe itself. Otherwise the capacitance of the long wire to the probe will cause problems (i.e. it won't work!).
| Circuit Description | |||
| Part | Real part | Description | ~cost |
| Con1 - NGF-S | 6 pin RJ11 phone connector | Direct plug connection for NGF-S with DRO option. | $0.65 |
| Con3 - Temp probe | Mini-stereo jack | Supplies +5, gnd and signal from temperature probe | $2.50 |
| Con4 - Limit switches | Mini-stereo jack | Supplies gnd, reads two switches active when pulled to ground | $2.50 |
| Guider - connector | 6 pin RJ11 phone connector | Standard guider port output. Supplies common connection and connections for RA+, RA-, DEC+, DEC-. When relay activated, the given guiding direction is connected to the common connection. | $0.65 |
| Prog1 - connector | 4 pin connector | allows in-circuit programming of uP1. Compatible with SX-Key programming device. | $0.35 |
| exControl1 | 6 pin RJ11 phone conector | Connects to a small hand controller allowing focus control in addition to computer control. This port is active at all times. | $0.65 |
| RS-232 | DB9 female | Provides RS-232 communication to host computer. Handshaking is used to ensure reliable communication. | $0.89 |
| IC1 - Motor driver | TPIC0107B | H-Bridge power motor drive for DC motor control http://www-s.ti.com/sc/ds/tpic0107b.pdf |
$4.28 |
| Con2 - Stepper motor | 6 pin connector | Supplies power and control signals for an
external stepper motor control board. Signals present include:
Step, Direction, Enable, +5V, +12V, gnd |
$0.25 |
| Rly1-4 | NAIS micro relays | Guider port relays. Normally open. When enabled, connects a given guider output to the guider common input | $0.75x4 |
| XTAL1 | 50mhz resonator | Provides clock for microcontroller | $1.66 |
| JP1 JP2 JP3 |
2 pin connectors | Jumper that selects between motor drive modes.
Default is connected (closed). In this state, the DC motor driver will be
used in PWM mode to drive focus.
JP1 signals the microcontroller so that it can send the appropriate control signals to the motor driver JP2 and JP3 are used to allow control signals to go to the motor driver. While they can be left in place always, it's best to remove them when driving a stepper controller so that the microcontroller doesn't drive both circuits simultaneously. When these three jumpers are removed, an external stepper motor control board will be used for driving the focus. |
$0.25x3 |
| V12 | Input connection for power supply. Power supply wires are soldered directly to the board for best connection. | $2.50 | |
| R7-R10 | 1k resistor | Input resistors for relay drive transistors. These limit the maximum load that the transistors can draw from the microcontroller. | |
| T1-T4 | 2n2222 transistors | Relay switching transistors. Provide high current drive capacity for the relays, protecting the microcontroller from damage. | $1.20 for 10 |
| uP1 | Scenix / Ubicom SX28AC/DP | Microcontroller | $4.33 |
| C11 | 1uF capacitor | Decoupling capacitor for microprocessor supply. Helps protect processor from electrical noise. | $2.90 for 10 |
| C4 | 1uF capacitor | Decoupling capacitor for IC3 | |
| R1 | 100K resistor | " | |
| R4 | 6.8K resistor | " | |
| R12 | 10 ohm resistor | voltage drop / current limiter for DC motor to allow motor to run from 12v source. This needs to have significant capacity and will dissipate quite a bit of heat. | |
| C7 | 1uF capacitor | Decoupling capacitor for IC2 | $0.20 |
| IC2 | Max233AEPP | RS-232 driver | $4.95 |
| Case | RS270-1803 | Radio Shack Project Enclosure: 5" x 2 1/2" x 2" | $3.29 |
| Power supply | 9V power supply | Anything from 9-12V @ 1-2A should be fine. | $5.95 |
| PARTS REMOVED FROM ORIGINAL DESIGN | |||
| Temp. Sensor | LM34CZ | ||
| IC3 | XR4151 V to F converter | ||
| C1, C3 | .01uF capacitor | ||
| R2 | 22k resistor | ||
| R3 | 100k resistor | ||
| C5 | 47uF capacitor | ||
| VREG | 7805 voltage regulator | ||
| C12 | 470uF capacitor | Provides smoothing for the incoming voltage supply. | |
|
PARTS ADDED TO ORIGINAL DESIGN |
|||
| Thermometer | DS18B20 | Dallas Semiconductor Digital Thermometer | $5.04 |
| C18 | 100-500uF capacitor | Initial power supply smoothing capacitor | $0.15 |
| C13 | 1uF decoupling capacitor | $0.20 | |
| IC3 | LM2594 | 150 kHz switching power supply chip | $3.05 |
| C17, C16 | 100uF. Panasonic FC Series | precision capacitors for switching power supply | $0.92 |
| L1 | Toko 8RHB2 100uH | Inductor for switching power supply | $2.20 |
| D1 | 1N5817 diode | for switching power supply | $0.70 |
| Encoder | 36 position rotary encoder | Clarostat 510E1A48F209PC 36 position mechanical encoder. | $4.87 |
| Knob | Knob for focus control. Fits on 1/4" shaft | $1.00 | |
| PC board | Printed circuit board | $20.00 | |
|
Total bare parts cost: (does not include cables or shipping costs) Typically you'll spend about this much on shipping charges when you buy from several different manufacturers in small quantities. |
$79.50 | ||
Controller Concept Drawings:
3/23/2002
After some on-line discussion, it appears that the desire is for a small control box that has all the cables (power, RS-232, focus motor, temperature sensor) combined with an optional hand controller. This gives the best of both worlds. It also allows people who will only use computer control skip the additional cost/time of building the hand controller.
Miniature hand controller - actual size: 2"x3"x1" (Radio Shack: 270-1801) The hand controller uses a simple mechanical rotary encoder with detents. This provides a very nice feel for precision focusing. One click per focuser step. No more pushing buttons and hoping the motor goes!
This controller connects to the main box via a phone type cable. This way you can hide the main controller away, or mount it to the pier or telescope.
Items to note:
Focus knob for interactive adjustment of focus
Action button(s) - it's use will is up to whomever writes the ASCOM driver. It could, for example, be used to trigger an auto-focus operation or reset the focus position to 0 or make a set-point measurement for temperature compensated focusing.
Note: the production version has just one button.
It's been a fun project, and a great learning experience. Now it's time to refine the software and add features.
Some things learned:
1 - the NGF-S, at least the one I have, can be used well for auto-focus routines. However it hasn't been repeatable enough for reliable temperature compensations. I find that if I run a dozen auto-focus iterations, where the focuser runs through the length of its travel, that the absolute position shifts. This has made it impossible for me to accurately quantify the minute changes due to thermal expansion.
2 - While the guider relay system is a cool idea, and will allow one to drive the motors nicely, the software I use, MaxIm DL, doesn't use the guider relays for anything other than "pulse guiding". I had hoped I could use it for auto-centering objects and other operations. Unfortunately, it uses "slew" commands, which require a goto capable mount.
3 - For really critical focusing, the single step size of the NGF-S doesn't seem adequate. Running on an f/6 system, a the 7 micron step size results in a star size change of 1 micron, which is quite a large change when using a camera with 7 micron pixels. For really critical focusing, I'd prefer focusing at least 2-4x as precise.
4 - As I suspected :-) the focus knob rocks. It is so much easier to control something using a knob, which gives controllable steps, as opposed to up/down buttons. This is something I've crusaded about for decades. Moreover, even with computer control, whenever I want to focus, I grab the knob rather than use the on-screen focus controls.
