Original circuit for focus support by Ingo Rohrich
I've built a new box centred on a custom circuit board that has both my heat/power and the focus motor support with ST-4 and RS232 breakout on the board. See it here.
Needed parts
- Perf board
- Suitable project box
- 7806 Positive 6v Regulator (optional)
- L293D motor controller chip
- 16-pin socket for L293D
- 16-wire ribbon cable connector
- 15-pin D-shell female (crimp connect)
- an old IDE cable
- 2 x 9-pin D-shell male connectors (solder-tail)
- 22AWG prototyping wire
- 2 x RCA plug (one gold one silver... or other colours as long as they're different)
- 6k8 or 10K resistor (1/4W)
- Soldering iron
- Dremel tool (or other cutter)
- Tweezers, haemostat or needle nosed pliers
- Salve for burnt fingers
- Whiskey (for afterwards)
-
I've put a diagram of my box here. The traces in white are on the bottom of the perf
board and the coloured ones are above using prototype wire or bits of ribbon cable.
Here is the circuit diagram.
I added to Ingo Rohrich's original focus motor support circuit as I didn't want to pass 12v to the motor. While it has a 132:1 gearbox on it, it still turned too fast for my liking so I cut the voltage to the motor down to 6v using a 7806 6v positive voltage regulator. You can't cut the power down any further as the Vcc voltage has to be greater than the 5v Vc voltage. The 7806 regulator will handle up to 1A... if my math is correct, the current should stay well below that. This cuts the 'no-load' speed of the motor down from 6000rpm to 3960, which gives 30rpm after reduction. I'll be reducing this another 6x on my setup. If you want to you can just buy a higher reduction gearmotor and use the straight 12v supply.
For the RS232 connector. If you are going to use a null-modem cable connect pin 2 from the 15-pin connector to pin 2 on the DB-9 and pin 3 on the 15-pin connector to pin 3 on the DB-9. If you want to use a standard serial cable then cross-connect pins 2 and 3 on the breakout box; pin 2 on the 15-pin to pin 3 on the DB-9 and vice versa.
The breakout box and the mount MUST be receiving their 12v supply from the same source. Whether you are using a battery pack, a powertank or whatever, power both from the same 12v supply.
-
The ribbon cable
-
The first thing I did was to build the ribbon cable that will go between the mount and
the breakout box. I took an old IDE cable and ripped 15 wires from it (with the red index
wire). Crimp on your female crimp-connect end on one end, and the 16-wire board connector
onto the other. Mind that you'll have a 'spare' lead on the board connector. Place the index
wire so it becomes pin 1 on the board connector then solder both the socket for the L293D
and the ribbon connector onto the perf board.
The resistor and Vss (5v) input
-
Locate your resistor between pins 1 and 16 of the L293D's socket then connect one end of the resistor
to pin 16 on the socket and the other to pin 4 on the ribbon connector. Then you are going
to connect the other end of the resistor to pin 1 AND pin 9 on the socket.
The grounds
-
Now locate pins 4, 5, 12 and 13 on the socket on the underside of the board. Strip some prototyping
wire and using wire and solder connect all 4 of these pins together with enough wire to run off to one
side of the socket. This is your ground. Connect it using wire to pin 4 on the ribbon connector.
The motor control inputs
-
Using more wire connect pin 1 on the ribbon connector to pin 2 on the socket, then pin 15 on the ribbon
connector to pin 7 on the socket.
The motor voltage input
-
(1). If you are using a voltage regulator connect a wire from the 12v positive input to pin 1 of the regulator, pin 2 of
the regulator goes to your ground and pin 3 goes to pin 8 on the socket. Once the regulator has been located
and soldered in place, put a .33uF ceramic capacitor between the +12v leg of the regulator and ground, and a .01uF
ceramic capacitor between the +6v output leg and ground.
(2). If you are NOT using a regulator connect pin 8 on the socket directly to your 12v positive input on one of your RCA plugs. Then connect long lead to the ground you made which goes to the 12v negative input.
The motor output leads
-
Connect another pair of long wire leads to pins 3 and 6 on the socket. These go to your motor.
The serial port
-
Now, strip out three leads of the IDE cable. Connect pin 2 on the ribbon connector to pin 3 of one of the
9-pin D-shells and pin 3 on the ribbon connector to pin 2 of that same D-shell. Now using the third lead
of what you stripped off connect pin 5 on the D-shell to your ground somewhere.
The ST-4 interface
-
Lastly, strip out five leads of the IDE cable. Connect pins 10, 11, 12, and 13 on the ribbon connector
to pins 1, 2, 3, and 4 of the second 9-pin D-shell. Then using the last lead connect pin 5 of that D-shell
to your ground. You can actually use any pins that you want on the D-shell for this. If you're going to use
an ST-4 you'll have to build a cable between the autoguider and the D-shell to match anyway.
Test all your connections.
Test them all again!
Putting it all together
-
Now for your box. Shave down the top of the box near the ribbon connector so you can run the cable out
the side of the box. Then, using a template or an old header from a computer board locate and cut the holes
for the two D-shell connectors and screw them into place. Next, locate and drill two holes for the RCA plugs.
the 12v +/- inputs go to one and the motor leads to the other. I've used two different colour RCA plugs, one
gold and the other silver to try to minimize my plugging things in wrong.
As for the two D-shells, you'll just have to remember which one is which. Or, you could do some fancy lettering on the box labeling each input/output.
Again, test your connections. You're done!
