Rotator Controller

An Antenna Rotator Controller

A very simple controller circuit, with fairly smart performance!

Can't afford a fully automatic computer controlled rotator, but would like more capability from your basic rotator? Would you like a simple to use "point and shoot" computer controlled antenna? You've come to the right place!

What it does -

What it doesn't do - The ZL1BPU Rotator Controller has been designed as an add-on unit for the popular Kenpro KR-400 and Yaesu G-400 rotators. The controller consists of a small circuit board which fits inside the rotator control box (see picture).

With this unit the rotator gains automatic heading seeking, computer control, and even heading control by clicking on a map or selecting a country prefix! The ZL1BPU Rotator Controller should work with any rotator with AC motor drive and a 500 Ohm feedback pot, such as the CDR CD-44 and the HY-GAIN HAM IV. It will also operate with both North-centred and South-centred rotators, as only the PC software setting changes.

Computer rotator controllers tend to be expensive - however you can build this one for under $50! The firmware supplied supports a rich specialized command set and also responds to SARTEC, ORION and YAESU commands, making it compatable with most popular multi-function logging and digital mode programs that feature rotor control.

The controller could also form the basis of a self-contained replacement controller, for rotators with faulty or missing controllers. Just add the power supply circuitry shown in the G-400/KR-400 Rotator Schematic - all you really need is a 24V AC supply, a 6V DC supply, and a 70 - 100 uF AC motor capacitor.

The small circuit board can be fitted inside the rotator control box, and operates from the controller's power supply. No holes need be made in the box, and all modifications are internal and reversible if the need arises. There are only five wires between the unit and the existing rotator box. The serial (PC) control cable can sneak out the back panel through an existing hole.

The suggested circuit board is single sided, 60mm x 60mm (without the relays), and can be etched or engraved. See the picture to the right, which is larger than life-sized. The board contains an inexpensive micro controller, a 5V voltage regulator, a simple transistor RS232 interface, and two relay drive transistors to activate the rotator. The relays are interlocked with the existing LEFT and RIGHT switches, so there is no possibility of motor or controller damage.

The unit is given heading commands via the PC serial port, from a special program, a digital modes or logging program with rotator control, or a simple terminal program. The commands are in the form of hexadecimal or decimal numbers, depending on the protocol, and the controller in effect "pushes the buttons" on the associated rotator control box until the requested heading is reached. This means that it will go to the correct heading without the need for a button to be held down to keep the rotator moving. It also allows for sophisticated computer applications to control the rotator.

How it Works


Positioning Accuracy

Accuracy is limited mostly by the host rotator and control unit. Typically 2 to 3 heading resolution and about 5 positioning accuracy. The smallest command that will cause movement is about 5. A heading approached from clockwise may differ from the same heading approached from anticlockwise by 5. Hardly a problem with HF beamwidths of 30 or more, but not appropriate for some VHF and UHF terrestrial and satellite systems.

The Rotator Controller uses hysteresis through the use of a window comparator technique, to prevent hunting and poor stability, which results in the heading uncertainty specified. The hysteresis could be reduced to improve positioning accuracy, but diminishing returns result due to poor positioning pot supply regulation and overshoot in the motor, since these simple rotators generally lack effective brakes. No braking control is provided.


The Rotator Controller operates from the host controller's power supply. It requires +12V (9 to 15V) at 10mA for the micro controller, and an additional 60mA while active (rotating). Operation from higher supplies would be possible if the relays were changed. The Rotator Controller includes its own +5V regulator. No heatsink is required.

The computer interface is a simple RS232 type. Idle is about -9V (derived from the PC TXD line), and active is about +4.5V. Passive pull-down allows future multi-drop capability. A DB9 female connector is used, and configured for DCE, allowing direct PC connection with a modem (pin-for-pin or "mouse") cable. Communications are full duplex, 9600-N-8-1. Only three connections are used (TXD, RXD, GND), with no handshake; thus the handshake lines on the PC port are available for other purposes such as PTT, if the application software permits it.


The unit is assembled on a 60 x 60 mm single sided PCB. The relays can be added, requiring an extra 20 - 25mm in one dimension, depending on the relays used. (The KR-400 controller has 94 mm width inside, and conveniently fits a 60 x 90 mm PCB slung from the top rails). No extra holes need be drilled in the outside of the controller, and no non-reversible modifications need be made.

Command Protocol   Control Software   Embedded Firmware   Construction