[Dennis G]

The attached Focus Circuit Fig 10 uses Dave Johnson’s ONE SHOT Fig 11 with a added transistor and a few modifications such as a reverse slide switch for focus in and focus out and a change over switch to provide a course and fine focus rotation. Please note. My focus has 10: 1 fine adjustment knob and the belt drives on this small knob. You may need to change the two capacitors to suit the style of focuser on your telescope.
The circuit is designed for standard Vero Board. Check you have the lands as shown in the diagram. Use an IC base and do handle the IC with great care when you come to install it. It is a C moss and an electro static charge will destroy it. The method I use has never let me down. I lay the completed circuit board on the kitchen stainless steels sink and discharge any static on my body by grasping both the SINK and METAL WATER TAPS warning! modern plumbing can have plastic knobs and never work over carpet. New ICs have expanded points, which need to be slightly squeezed to fit the base. After being discharged squeeze the points until you can push the IC into the base without bending a point. Double check that the IC is the correct way round. The dot on the device indicates pin 1. Once the IC is installed it is quite safe to handle the circuit board.
The circuit requires sections of the copper lands to be cut through, shown as a cross, use a 5mm drill in a hand drill and drill through the copper but not through the backing board and check that all the copper has been cut with a hand magnifying glass. Likewise when soldering, double check every part for solder bridges and sound soldered joints and when all has been soldered, check it again before the IC is installed. Tip. Clean resistor leads with steel wool and presolder flying lead ends before soldering. This will overcome dry solder joints.
Resistors are stood on end, which are represented by a circle with a top lead. Flying leads are thin plastic covered wire with colour coding and a tip that will prevent breaking the soldered leads. Pass the wire through a hole in the Vero Board from the copper side and then into its location hole and solder it. These flying leads that attach to the transistor, leds and switches, etc., need to be given sufficient length to permit easy assembly. The case is made from 6mm clear Perspex glued up with chloroform. A 3mm aluminium lid acts as a heat sink Add extra thickness of Perspex at the corners of the case to accommodate self-tapping screws to hold the lid. All the power and control leads on my scope are connected with stereo RCA cables and bases, colour coded. Red for 12v dc power input and yellow for out put to the DC motors. Single individual cables supplying fans and the Finder illumination and Laser Finder use RCA fittings with colour coding such as green and silver for reduced voltage (3v dc) and red if its 12v dc. RCA fittings are positive and provide both the plus and ground in a single cable. Salvage multi RCA bases and cables from discarded sound equipment.
The small geared head dc motor is attached to the telescope with a simple aluminium bracket .A single 15 mm dia grooved pulley grub screwed to the motor shaft is all that is needed with the O ring belt slipped over the focuser knob. No modification to the focuser is needed. I have fitted a simple cone clutch to the pulley, but it is never used. The O ring will slip sufficiently should you require to turn the course knob You can find Dave Johnson’s circuits on the Webb discovercir@aol.com I’d recommend you look the One Shot circuit up and then compare it to the attached circuit layout. You will need to connect the output to the base of a transistor and a 1 meg resistor from the emitter to ground to provide power to the motor and use a 12v dc supply to the circuit. If you send Dave an email do not add an attachment, he will not open it.