Prop Pitch Motor Direction Indicator Using Op Amps

The PPM (prop pitch motors) I purchased in advance of building my current station included a home brew controller. In addition to motor control it has electronics to display the direction on a scavenged meter from a Hy-Gain rotator (appears to be from a Ham-M). It relies on a 10-turn 10 kΩ pot on the tower that turns with the mast. It was up to me to figure out how to do that.

The direction control is really very simple. There are a couple of op amps, pots, resistors and regulated ±12 VDC power supplies.

The first stage employs a 741 op amp as a differential amplifier. The tower pot connect to one input and the same pot type in the connects connects to the other input to zero the meter. Both pots are linear 10-turn 10 kΩ.There is no need to physically align the tower pot since the indicator can be zeroed at any position over its 10 kΩ range. The only requirement is that the tower pot not hit the end of its range. 

Over-rotation beyond 360° is supported provided the coax rotation loops can reach that far. The physical or electronic display must also support bearings greater than 360° and less than 0° in some fashion. A soft limit switch would be useful the operator avoid damaging the rotation loop.

The second stage uses another 741 op amp as a DC amplifier with variable gain -- dual op amp devices are available. The unlabelled resistor and pot are selected to set the gain range. The variable gain has two purposes. The first is to allow for different pot rotation ranges depending how the mast is physically coupled to the rotation system. The second is to set the voltage range to drive a meter or electronic bearing display. 

Alternatively, if the maximum voltage from the differential amplifier is directly usable, the second stage can be replaced by a variable voltage divider (pot) or software parameters in a computer controller and display. The value of the unlabelled resistor at the op amp output is selected to limit current to the meter movement. Meters are tolerant of modest and brief out of range conditions, but beyond that they can sustain irreversible damage.

There are many variations possible on the same theme. A few of these include:

• Variable supply voltages instead of a gain stage
• Add variable gain to the differential amplifier, taking care to preserve linearity across the rotation range
• Single power supply op amps to limit the output voltage range between 0 and Vcc to be compatible with an Arduino or other analogue input (ADC GPIO) of a computer controller such as an Arduino
• Centre, left or right movements of physical meters

With a little thought I am sure that readers can come up with other and perhaps better circuit variations. My plan is to begin with a physical meter and later transition to a software application for direction control and display. By using a physical meter I can get the display built and working faster, and right now my time is at a premium.

Everything I've written so far is elementary to many hams. It isn't to me. While I have many technical skills, circuit design is not one of them! I struggle with semiconductors. I still write about it since when I learn something since I know that there are others who will benefit. In that spirit of mutual learning we can proceed.

The controller I purchased along with the PPM was old and home brewed by the ham I bought them from. They were built so long ago that his memory of the design had faded. But his electronics knowledge is superior to mine and he made an effort to reconstruct the design from memory, filling in the blanks from his understanding of circuit fundamentals. That was helpful when I first installed the PPM and the controller.

It became more urgent recently when the indicator malfunctioned. I suspected lightning damage to the op amps. However, I was late noticing the problem since the tower pot was succumbing to the elements and obscuring the true cause. When I replaced the pot on the tower, along with a redesigned mast coupling system (more on this in a future article) it continued to misbehave. Since the pot checks out perfectly when measured in the shack with an ohmmeter it must be the controller.

This was a further spur to set about a redesign of the direction indicator. The basic op amp design is a good one so I stuck with it. What I didn't like was the need for dual power supplies and the voltage level. It is perfectly good for a physical meter movement. I know hams who do it with discarded Hy-Gain rotator controllers that are frequently found at flea markets from rotators that have expired. I am tempted to do the same, however I also want to prepare for computer integration and control.

With my tenuous understanding of op amps I researched alternative devices. I wanted an op amp that would operate from a single DC supply and would work well at 5 VDC. This is ideal for connection to the ADC GPIO pins of an Arduino. Risk of damage to the microprocessor is reduced when the voltage is unable to swing negative or above 5 volts, and therefore doesn't require a more complicated circuit to prevent out of range voltage presented to the analogue GPIO pin. 

I also wanted to maximize the gain to use as much of the voltage range as possible. The 10-bit ADC of Arduino and some other microprocessors can resolve 1024 points. With a 10:1 pot and no gain a full rotation only utilizes 10% of the voltage range. That leaves only 100 points, or a position resolution no better than 3°. That's acceptable for HF yagis but is often inadequate at VHF and UHF.

I settled on the LM358 dual op amp and purchased a small quantity. Like most electronic components they are inexpensive and encourage experimentation. The output range is from 0 to Vcc - 1.5 volts, which in this case is 0 to 3.5 volts. It seemed a perfect choice. Unfortunately it is not so simple. 

Rail-to-rail operation of an op amp is constrained by its design and by operating conditions. It was the lower rail limit of 0 volts that caused me grief. I didn't fully understand this and I ran into difficulties adjusting the inputs to get the range and linearity I needed. I got close but with parameters that would be difficult to maintain in practice. In particular it required precise setting of the tower pot. That is not reasonable when working 40 meters up the tower.

For computer integration the non-linearity can be compensated in software though not easily and it, too, is susceptible to the pot setting. I gave up the experiment with the LM358 and dove into my bag of 741 op amps. This approach was far more successful despite the seeming chaos of the breadboard prototype.

Tower pot rotation is emulated by the pot on the left. Degree of rotation can be eyeballed from the knob indicator. The 10 kΩ pots zero the direction pot (left) and set the gain of the second stage (right). All the resistors are 10 kΩ except the 33 kΩ current limiting resistor in series with the meter. 

The meter pulled from my junk box has a centre zero so that 0 volts output from the circuit represents due north. Negative and positive differential voltage pull the meter toward south counter-clockwise and clockwise, respectively. The batteries supply ±9 VDC.

In the picture the bearing is about 60°. That the stage 2 output voltage (that's what the DMM is measuring) bears a striking resemblance to that value is a coincidence. All that matters is linearity, gain and a suitably marked meter face. The last two can be implemented in software when the circuit delivers the signal to a computer ADC port.

Meter movement can be reversed by switching the tower pot connections at the controller or by switching the V+ and V- inputs to the second op amp. The same can be done with a DPDT switch. I probably won't bother since it's easy to switch the wires and reversal only happens by swapping wires during maintenance. Little mistakes like that are easy to make.

As built, the circuit gain is suitable for a the 1:1 rotation ratio of the chain drive PPM on the tower with the 40 and 10 meter yagi. For the upside down PPM on the other tower the rotation ratio is closer to 3:1. That is, the resistance range of the 10 kΩ pot on the first PPM is 1000 Ω and perhaps 3000 Ω on the second PPM. I will alter the design so that the circuit is compatible with both. More components will be needed for a computer interface to strictly limit the output within 0 and 5 volts.

This is the design I'll proceed with. For the first version I'll tap the ±12 VDC regulated supply in the old home brew controller and adjust the gain stage accordingly. Physical meters with a linear scale (the scale units on the face plate don't matter) will be used for the display. I will do it for both PPM rotators since diagnosing and repairing the malfunctioning circuit is more difficult due to its accessibility. 

For the next version I may, as others have, use Hy-Gain control units for both direction indication and motor control. DC power for the motors will be taken from the existing control unit's power supply. Once this is done the old control unit can be moved off the desktop.

Of more immediate concern is the pot wiper for the pot on the 15-20 stack PPM rotator. I grounded it with the expectation that I'd use the LM358. The ground wire will be disconnected on my next trip up that tower. The ugly rope I first put on for the pulley belt has already been replaced and tested.

Nothing especially novel has been introduced in this article. The circuit is simple and its usage straight forward. Nevertheless, even basic concepts are worth communicating since they are often unfamiliar to new and longtime hams. Experimenting and prototyping with a breadboard is a valuable technique to learn.