After being successful in duplicating FLAâ€™s Bluetooth adapter for the AllView Skywatcherâ€™s hand controller, I was inspired to see if I could build a homemade Bluetooth adapter that would plug directly into the AllView mount. Yes, I know that Panogear already makes a fine BT adapter but I like to tinker!
There are several articles in the forum that describe portions of the Merlin head interface as well as older SkyWatcher heads but I didnâ€™t want to assume that the AllView interface was identical in all respects. The short ending to this story is that for all practical purposes the interface is the same although the details are somewhat different than previously described. I was able to build a working BT mount adapter for the AllView!
In the process of disassembling the mount to trace the wires I discovered three things that could impact all owners of the AllView mount. First, the two screws holding the case together nearest to the arm were loose. I later discovered that there is not a hard stop for these screws, the more you tighten them the more the case flexes. So these screws cannot be tightened securely, youâ€™ll just have to check them periodically. Second, when the top of the case was originally assembled the back of the shutter release jack caught a couple of connectors on an internal circuit board and bent them at an angle. It didnâ€™t do any immediate damage but it could have been a problem in the future. Lastly, the power jack had a cold solder joint on one of the three contacts. The contact must have been associated with the internal battery since Iâ€™ve never had a problem using an external battery. A bit of flux and the hot tip of a soldering iron fixed this problem.
For those who are interested here are the interface details from analyzing the mount, cable and hand controller:
The RJ11 connector at the base of the AllView mount has six pins, of which only 5 are connected. Pin 1 is not connected in the mount and the white wire coming from that pin is cut off on the end of the cable that plugs into the hand controller. Pin 5 is ground and pin 3 is 12 volts from the battery or power supply. Pins 2 and 4 are connected together on the mount but are separate data paths from the cable all the way through the hand controller circuit board to the microprocessor. When the hand controller is first powered up it transmits a signal on pin 4 to see if itâ€™s connected to the mount. If it doesnâ€™t get a response it assumes it is not connected and goes into standalone mode. Pin 6 is a TTL level signal that is pulled high (5 volts) by the hand controller. Disconnecting this wire does not seem to have any effect on the operation of the mount.
As a result of my investigation I decided to design my BT module to emulate the existing AllView hand controller as closely as possible. I included a pull-up resistor for pin 6 and I kept the RXD and TXD lines separate. I also added a couple .1mf capacitors to the 78L05. In a previous life building homebrew computers in the 1970s, we always put bypass capacitors on the 7805 regulators because of their tendency to self-oscillate and I still have an ample supply of them in my junk box!
If you are going to try this yourself be very careful in your selection of the Bluetooth module. Many of them boot up in a random configuration and have to be configured each time through the use of serial commands. These boards are unsuitable for this application. You must get a Bluetooth module that is TTL (not RS232), boots up as a Slave, and has a default serial port setting of 9600, N, 8, 1. The one I used was http://www.ebay.com/itm/261035765875