Microprocessor

The microprocessor is the heart and brain of the VideoStar. Because it had to be very fast and have lots of input/output space, I chose the Scenix (now Ubicom) SX28AC/DP.

This part runs at 50Mhz (available at up to 75Mhz) and executes one instruction per clock cycle. That means you get 50 million instructions per second. All from a chip that runs about $5!

Getting this processor to generate accurate video timing signals was pretty simple, but I had to find the exact combination of interrupt frequency and code to yield the right signals. Moreover, this had to be combined with the RS-232 timing generator, which made it even trickier. Make sure you do NOT change the clock frequency (like trying to use the 75Mhz crystal because it's faster). Everything it completely tuned to the 50Mhz crystal frequency. Jameco 76831.

Remember to get a socket too: Jameco 133006

Because this processor is so fast, it is able to run all the I/O of the system. It drives the motor, scans the LED display, reads the optical encoder on the focuser, read the encoder on the control knob, runs the guider port relays, and communicates with the host computer over RS-232. Very cool.

See Parallax inc. for information on these chips and development systems. The chip is cheap. The development system isn't.

If you want to buy just a programmed CPU, XTAL and socket from me, I can do that for around $10.

XTAL
$1.29

Socket
$0.59

Power Supply

The power supply used is a dual voltage (5v / 12v) supply that has a standard PC power supply connector and can supply about 1 amp at each 5v and 12v. You can get these for under $10 from Jameco. For safety, I might suggest one of their self-contained power supplies that runs about $20.

LED displays

VideoStar uses two two digit common cathode 7-segment LED displays. See Jameco part number 24723.

Shift registers provide a way to control scads of I/O using just a few lines from the CPU. They also provide higher drive capabilities than the CPU, allowing you to run things like LEDs without blowing out the CPU.

74HC595 - LED driver. Jameco 46105

TPIC6C596 - LED driver and Relay driver. This part I used a surface mount component. It replaces a bunch of transistors for power driving the common cathods of the LEDs and the Relays. Jameco doesn't sell this part, so you have to go to Digikey for this one. If you're making the circuit from scratch, you can use the DIP version.
Digikey 296-9014-5-ND

TPIC6C596
$0.99

The resistor network is used to control the current going to the LEDs. One can use a bunch of individual resistors, but a network is easier to work with.

470 ohm, isolated DIP resistor network. Jameco 108581

The control knob is a rotary encoder. This is a device that has a bunch of switch settings and a few outputs. Each click changes the phase of the two outputs. Using the CPU to read the switch, you can determine the way the user is rotating the knob.

 

These driver transistors are used to drive the relays if using an external circuit to run them from TTL levels. For example, if you want to use software that uses the PC's parallel port to drive the relays, you would do so by driving these four transistors.

The transistors are simple 2n2222 NPN switching transistors of the variety Radio Shack and others sell.

Jameco 28628 TO-92 case

One of the most expensive parts on the board is the motor driver chip. However, it does its job, allowing the CPU to run a DC motor without blowing up. In fact, you can run a 25 watt motor with this chip if you wanted to get extreme...

Note that this part is surface mount, as used on my PC board. You can use a DIP version if you wire it up yourself.

Digi-Key 296-10857-5-ND

RS-232 ports run at different voltage levels than a normal CPU. Because of this, you need a way to translate voltages. My chip of choice is the Maxim 233 because it reduces external chip count and is known to be reliable. Others have made RS-232 level translation circuitry much cheaper, but I chose reliability and simplicity.

Jameco 106163

The thermometer interface used is a simple A/D converter. The thermometer part outputs a varying voltage. This is then fed into a voltage to frequency converter which generates pulses that the microprocessor can count.

Note that if you're not going to run a  temperature compensated focuser, you don't need these components or the associated resistors and capacitors.

V-F converter chip: XR4151. Jameco RC4151NB

Fahrenheit temperature sensor. All Electronics LM34DZ

LM34DZ
$2.50

The low power relays are very handy little units. I use the tiny NAIS AGN2004H from All Electronics. While much higher current draw than some of the other mini-relays available, they're much smaller and less expensive. However if you're going to use them in your own circuit, be aware that you shouldn't drive them directly from a microcontroller or you'll fry the controller.

All Electronics RLY-405

Several connectors are used on the main PC board to simplify wiring. While not absolutely necessary, they'll make the project more reliable and neater.

20-pin IDC female connector for ribbon cable: Jameco 138325

10-pin IDC female connector for ribbon cable: Jameco 138376

0.100" double row male header strip. You can buy long lengths of this and break them apart. Get extra as you'll probably ruin some. Jameco 117196 - 80 pin straight male header

0.100" single row male header strip. This is used throughout for simple connections. Get one strip with a bunch of pins and break off those you need. Jameco 160881

DB9 female IDC ribbon cable connector: Jameco 12520

DB25 female 0.590" setback PC board mount: Jameco 105013

DB25 male 0.590" setback PC board mount: Jameco 189624

6 Pin modular phone jack PC mount connector: Jameco 115836

10-Pin IDC
$0.29

Header pins
$0.83

Header pins
$0.45

DB9 Female
$1.99

DB25 female
$1.19

DB25 male
$0.75

Phone conn
$0.65x2

Since I've gone through the effort to create this system and had professional printed circuit boards created, I'll be making them available to others.

As noted, this isn't a profit making venture. At the same time, I'd like to cover my basic costs. At these low quantities, the circuit boards cost me ~$45 each. To cover some of my costs, I'll be adding about $30 +shipping to this for those who want to purchase the bare boards.

I'll also make available populated boards, so you don't have to run around looking for all the parts and learn how to work with surface mount chips etc. Total cost for this will run about $150.

I'm sure some people may want a completed unit with case, labels, etc. That's actually the hard part of all this - mechanical construction and testing is time consuming! If you're interested in a complete VideoStar box, I'll probably make them available for around $300.

Populated Board
~$150.00

Complete VideoStar Box
~$300.00

Throughout the circuit, there are a variety of capacitors used to reduce noise in signals. High frequency portions use 680pF capacitors and power supply pins usually use 1uF capacitors. Exact values aren't critical as long as they're in the ballpark. Radio Shack should have what you need.

1 uF dipped tantalum capacitors: Jameco 154860 (minimum qty: 10)

47 uF 50v radial electrolytic capacitor: All Electronics

680 pF capacitor: All Electronics HV-DC680 (min. qty: 10)

0.01 uF capacitor: All Electronics RMC-103

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Resistor choices are also pretty non-critical, often anywhere in the ballpark is fine, since they're often used just to limit current. Those marked with a * are more critical. All can be 1/4 or 1/8 watt. Your best bet is go go to Radio Shack to buy the small quantities you'll need.

Value	Qty		Value	Qty		Value	Qty
1K	4		22K	3		100K	2
6.8K	1		47K	1

 

47uF Cap
$0.10

680pF Cap
$1.00/10

0.01 uF Cap
$0.12

Misc. Resistors
~$2.00