A servo's position is a function of the pulse width transmitted to it and the feedback potential from the position potentiometer (pot). We can extend servo travel by increasing the range of transmitted pulse widths or by decreasing the feedback from the position pot. Increasing the range of pulse widths sent by the transmitter requires intimate knowledge of the transmitter electronics and would cause the transmitter to produce a wider range of servo motion in any servo it controls. This may be undesirable if the transmitter is also used for purposes other than KAP. Alternatively, we can reduce the amount of feedback produced by the position pot in the servo itself. By placing 2 fixed resistors in parallel with the servo pot, we can reduce the amplitude of the position feedback signal. The servo will compensate for this reduced feedback by increasing shaft travel.
How to do it...
If you don't know the value of the position feedback pot in your servo, you may be able to measure it with an ohmmeter (measure across the outside terminals with the servo unpowered). Every Futaba servo I've seen uses a 5K ohm pot. Purchase 2 miniature fixed resistors of approximately half the resistance value of the pot (2.7K for a 5K pot). Carefully solder one end of each resistor to the outside terminals of the pot. Then connect the free ends of the resistors together and to the central pot terminal (the wiper) with a short piece of wire or a resistor lead.
With the receiver turned off, gently turn the servo clockwise until it hits the internal mechanical stop. Mark the position. Turn the servo counterclockwise until you hit the other stop. Mark the position.
Turn on the receiver and transmitter. Move the stick and trim tab together to their full travel in each direction and verify, using the marked positions of the mechanical stops, that the servo does not hit the stops in either direction.
With any luck you have increased the travel of your servo beyond 90 degrees. If full stick travel on the transmitter causes the servo to hit a mechanical stop a third resistor (same value as the first two) may be required. With the additional resistor there are three possible adjustments that may solve this problem.
By using the electronics from another servo (the cheapest you can find) it is possible to construct a pan/shutter system with a very large neutral range. Inadvertent shutter release is eliminated.
How to do it...
First the Pan servo
Disassemble the Pan servo and cut off the output gear's mechanical stops. Unsolder one motor terminal and insert a small diode between the circuit board trace and the unconnected terminal (don't worry about diode polarity just yet). Adjust the transmitter trim control for the Pan/Shutter stick to dead center and set the stick to the position desired to initiate panning (I use 1/2 of full left travel). Turn on the transmitter and receiver. Adjust the servo pot until the motor just stops. Center the Pan/Shutter stick. If the motor starts spinning again flip the transmitter channel reversing switch or reverse the diode. Mark the position of the pot shaft. The servo adjustment is now complete. Cut off the end of the pot shaft or unsolder and lower the pot so it no longer engages the output gear. If you can't cut the shaft or lower the pot you may replace the pot with a trim pot or a pair of fixed resistors selected to match the resistance measured from each terminal of the pot to the wiper. Make sure the pot is in the marked position and reassemble the servo.
Then the Shutter servo
If you intend to push the shutter button with a servo, you need only attach a second servo in parallel with the first. That servo will operate normally and may be used to operate the shutter via some mechanical linkage. If you need an electronic shutter release you must disassemble the second servo to get at the electronics.
Remove the circuit board from the servo housing and unsolder the motor from the board. Connect one side of a low voltage DC relay coil to one of the circuit board's motor contacts. Connect the other end of the coil to the other motor contact through a diode. Set the Pan/Shutter stick to the position desired to actuate the shutter (I use 1/2 of full right travel). Turn on the transmitter and receiver. Adjust the position pot until the relay contacts click. Center the Pan/Shutter stick. If the relay contacts remain closed, reverse the diode. The shutter servo adjustment is now complete. You may shorten the legs on the pot or cut the shaft if desired. Removal of the motor may leave enough roon inside the servo case for installation of the relay. Save the motor and gears as spare parts for your other servos.
In Futaba servos (and perhaps others) the circuit that drives the motor draws considerable current itself when active. A normal servo draws large currents only until the motor moves to the commanded position. The motor and drive circuit then turn off and the servo draws a small quiescent current. In a modified servo the target position is never reached because the feedback pot has been removed. The motor may not spin because of the blocking diode but the control circuit is still driving a voltage and consuming power. In Futaba 3003's the normal 10mA quiescent current increases to 44mA. For a system with two modified servos (pan and shutter) you will have an additional 66mA of current draw. Fortunately, servos do not drive their motors when the transmitter is off, so it is possible to reduce system power consumption by turning off the transmitter whenever possible. Some receivers unpower their servos when the transmitter is idle, further conserving power.
Quartz crystals are not available at frequencies above 100MHz or so. If a higher frequency of operation is desired, frequency doubling techniques must be used. Without getting heavily into RF electronics theory, suffice it to say that there are ways of doubling or trebling the frequency of a signal and that this technique has been used for decades to produce frequencies higher than available from crystals, but with the crystal's inherent accuracy and stability. Because a succession of frequencies from the base crystal all the way to the transmit frequency are present in the transmitter, care must be taken to ensure that only the final desired frequency is emitted. This generally involves tuning of numerous adjustable components in the design.
Fortunately, the development of surface acoustic wave technology (SAW) in the 1960's has made is possible to construct oscillators that operate directly at very high frequency. Since the only frequency present in the transmitter is the desired one, no tuning or filtering is required to achieve a pure transmit signal. Phase Lock Loop (PLL) oscillators are also capable of synthesizing very high frequencies from relative low reference frequencies with only a single multiplying stage. The large difference between reference frequency and transmit frequency offered by PLL tuned transmitters also allows easy generation of pure transmit frequencies. For the KAPer that is considering a video viewfinder, I suggest that only SAW or PLL tuned transmitters be considered.
Even with a well designed transmitter there may be some interference with the sensitive RC receiver. An antenna filter circuit may be required to block unwanted video signals from the RC receiver antenna. In a frequency doubler style transmitter, where subharmonics may approach the RC frequency, receiver antenna filtering may not work.
Which Transmitter Frequency?
Amateur TV transmitters (ATV) are available to operate on 440MHz, 900MHz and 2.4GHz bands. The choice of frequency depends on local interference conditions, government regulations, budget and convenience requirements. While the 900Mhz and 2.4GHz systems provide for the shortest antennae, they also provide the shortest range and are not directly compatible with television receivers. The 440MHz ATV band overlaps part of the cable TV spectrum (Ch 59, 60). This allows cable-ready televisions to directly receive ATV broadcasts. 440MHz is also just below UHF channel 14 and many inexpensive portable televisions with "slide-rule" tuners can tune down far enough to receive ATV transmissions. In particular, every hand held LCD television I have tried is capable of receiving 434-440MHz ATV. Frequency "downconverters" are required to adapt standard televisions for use with 900MHz and 2.4GHz transmitters. The downconverter receives the 900MHz or 2.4GHz signal from the ATV transmitter and re-transmits the video information at low power on VHF channel 3 or 4. A standard television receiver, tuned to the appropriate channel then receives the image. The extra cost, space and power requirements of a downcoverter must be considered when selecting an ATV system.
Supercircuits have a very small 434MHz ATV transmitter called the "PowerPlate". I have not used this transmitter, but it's SAW oscillator technology should minimze interference with RC gear. It's low transmit power (200mW) may not yield more than a few hundred feet of range, but this may be sufficient for many. It is quite small and light, and coupled with one of their tiny BW cameras should produce a very nice KAP video viewfinder.
Note that the use of any ATV system requires an amateur radio license. Just think of that as an excuse to get into yet another hobby.
I'll have more on this subject after this winter's tinkering.