Photo of motor box
For details about each component
click on it in the diagram.
Diagram showing components of motor box
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The antenna shaft is a 1.5m length of 25mm diameter aluminium tubing. The size of 25mm was chosen to fit tight into the available bearings. The bearings are mounted at the top and bottom of the box where the antenna shaft passes through. The
index disc and the drive sprockets are attached to the antenna shaft using stainless steel hose clamps. The antennas are attached at the top of the shaft with TV antenna style brackets. The 432 MHz beam is on top while the 144 MHz beam is about 150mm below it. A diplexer is also fastened to the antenna shaft. The main coaxial cable passes through the antenna shaft from top to bottom.



Circuit diagram of infrared transceiver The infrared transceiver consists of two main parts.
  • An infrared Light Emitting Diode
  • An infrared photo transistor
The infrared diode shines onto the photo transistor only when the index hole in the index disc is in position. This position is adjusted to be the rear of the beam antennas (6 o'clock).
When the infrared light is applied to the photo transistor, it conducts, causing a LOW signal to appear at input 8 and pin 11 of the 25 pin D connector. This LOW signal is then detected by the 'ROTOR HUNTER' software.

To view a close up image of the transceiver
click here.



The purpose of the index disc is to assist in the detection of the rear of the beam antennas. By detecting the same position in each rotation 'ROTOR HUNTER' is able to provide results relative to the current vehicle direction. The rear of the antenna (or index position) in detected when infrared light passes through the hole in the index disc. This light is seen by the infrared receiver and the index signal is then passed on to the computer.
The index disc I use is made from aluminium, is about 150mm in diameter with a 9mm index hole. The center hole is 25mm in diameter to accommodate the antenna shaft.
Diagramatic representation of the index disc
To view a close up image of the index disc
click here.



Diagramatic representation of position indicating resistor
'ROTOR HUNTER' uses a variable resistor to indicate the position of the motor drive shaft. The motor drive shaft is connected, by a chain, to the antenna shaft and both rotate together. The variable resistor is a special type constructed to give a full 360° rotation, not 270° as a normal potentiometer would.

The unit I use was purchased at a garage sale for $3.00 and is the only one I have ever seen. It measures about 60mm in diameter and the shaft is about 6mm in diameter. It is mounted using two M6 screws which attach the body of the resistor to an aluminium plate which is then held in line with the sprockets and the motor shaft.

It may be possible to build your own resistor using resistance wire and some type of round former.

To view a close up image of the resistor click here.
A 100k resistor is connected between +5 volts and the center (or wiper) contact to prevent an open circuit being seen by the parallel port interface during the transition from full resistance to zero resitance.
The position indicating resistor is connected to the regulated 5 volt power supply from the parallel port interface with the center contact being connected to input Ø.
Circuit diagram of position indicating resistor
Because the
sprockets used in the drive train were taken from a child's toy tractor the quality is not 100%. This means they are not identical and they do turn at slightly (1-2° / revolution) different speeds. The index disc and a software routine take care of the difference.



Diagramatic representation of shaft adapter The connection betwen the gear box shaft and the drive sprocket is made with a shaft adapter. This consists of a length of 12mm diameter mild steel tube which slides over the gear box shaft and the sprocket shaft. The adapter is held by two steel bolts which go through each shaft and the adapter. Early on in the project I did it another way. To see how it was done before click here.

The position indicating resistor is connected to the drive sprocket by a length of PVC tubing. The tubing is held in place with two hose calmps and prevents any flex within the drive shaft from exerting force upon the position indicating resistor.

Diagramatic representation of PVC tube


The transmission of power from the motor to the antenna shaft is done using a chain drive. The drive sprockets I use were scavanged from a discarded toy tractor belonging to my eldest son. He was quite pleased to see them get a 'second life'. The sprockets are about 60mm in diameter. One sprocket has had a 25mm hole drilled through it to allow attachment to the antenna shaft. The sprocket has a key hole filed in one side which aligns with a piece of 3mm rod. This rod is held in place with two hose clamps and ensure the sprocket remains attached to the antenna shaft. The other sprocket is joined to the motor shaft via a home brew shaft adapter. The sprockets are slightly different sizes which initially caused the displayed signal to rotate 1 or 2° each rotation. This problem was solved with the introduction of the index disc and a short routine within the 'ROTOR HUNTER' software.

To view a close up image of the drive sprocket click here.
Diagramatic representation of the sprocket


The power train makes use of a chain drive between the two
sprockets. The chain, like the sprockets, was scavenged from a childs toy tractor. The chain is about the same size as a normal bicycle chain. The chain is tensioned by the use of two M10 bolts between the motor bracket and the side of the box. The tension was adjusted for about 15mm of free play at the center point between the sprockets.



Diagramatic representation of the motor and gear box The DC motor and gear box used in the 'ROTOR HUNTER' are the type normally found driving windscreen wipers. The one I use has a parallel output shaft and is rated for 24 volt operation. Although I run it on 12 volts it performs very well. The output shaft rotates at about 25 RPM giving one sample every two and a half seconds. I tried a faster motor but the resulting pattern was not as sharp due the limitations of the receivers.

UP BUTTON home page rotorhunter vk3zpf@qsl.net

Copyright 1999 - Peter Fraser - all rights reserved.