Various Information on Servos


[Document Version: 2.01] [Last Updated: 6/17/96]

1. Servo-Motor 101

Before you begin, you really must read the Servo-Motor 101 (TLJVERERR) (TLJFILE!FOUND) by Chuck McManis. It covers the basics of servos that you will find in the modeling hobby.

2. Servo Cross-Reference Chart

(From Julio C. Chacon, Jr)

Someone kindly asked me to post this, so here goes.The Novak Input Plug System is very useful if you need to swap plugs to use different servos on different radio systems. The different plug styles in the kit are individually marked. In addition, the plugs are marked "WHT, RED, BLK" in the appropriate order that they should be installed on the plug. for reference, I'll place these at the top of the chart:

                RED(+)  BLK(-)  WHT(signal)
                ===     ===     ===
Airtronics      Red     Black   Brown
Sanwa           Red     Black   Black <strong>[*]</strong>
Futaba J        Red     Black   White
KO Propo        Red     Black   Blue
Kyosho/Pulsar   Red     Black   Yellow
Japan Radio(JR) Red     Brown   Orange
[*] I've never seen one, so don't ask!

And there it is! All you have to do to replace the plug is make sure that you place the corresponding wire in the proper location as marked on the connector. For example, if you have a Futaba servo that you'd like to use on an Airtronics receiver, simply take the Airtronics connector from the kit, and and stick the Red in RED, White in WHT, and Black in BLK. If you don't have the input kit, but you have two servos, you should write down the order of the wires as they appear on the Airtronics servo, then remove it. Next, you would remove the plug off of the Futaba servo and install the Airtronics plug by inserting the White wire where the Brown one was, then the Black to Black, and Red to Red. I Hope you make use of it!

By the way, this chart is included with the N.I.P.S.

3. Servo clock source

                                                    o  5VDC
        5k pot             4.3k                     |
 |                                   ________|______|_______
 |                                  |        4      8       |
 x----------------------------------|7                      |
 |      1N457 diode                 |                       |          output
 x---------&gt;|---------------x-------|6                     3|------------------
 |                          |       |      NE555            |
 |       68k                |       |                       |
 x---/\/\/\/\/\/\/-----x----x-------|2                      |
                       |            |_______1_______5_______|
                       |                    |       |
                      _|_                   |      _|_
                      ___  0.33uf           |      ___  0.1uf
                       |                    |       |
                                 ---  Gnd

4. Servo Driver Circuit needed

Here's a circuit that's older than dirt, but it works quite well.

+V  o-------o--------o----o-------------o----o-------------o-----o +V
            |        |    |             |    |             |
Gnd o    R1-100K     |    |             |    |       R4--&gt; R3    o Gnd
    |       |        |    |             |    |      4.7K   10K   |
    |       |      |--------|         |---------|     |    |     |
    |       |      | 4    8 |         | 5   14  |     |          |
    |       o------|7       |         |       11|-----o          |
    |     R2-15K   | (555) 3|-----o---|3        |  C2-0.22mF     |
    |       o------|6       |     |   | (74121) |     |          |
    |       |      | 2    1 |     +---|4      10|-----o          |
    |       |      |--------|         | 7    6  |                |
    |       |        |    |           |---------|                |
    |       o--------+    |             |    |                   |
    |       |             |             |    +--------- Servo    |
    |  C1-0.22 mF         |             |               Signal   |
    |       |             |             |                        |
    +------ o-------------o-------------o------------------------+

  1. R3 is a 10K pot, connected as a variable resistor
  2. Increased value for R1 -> less angular throw @ R3
  3. Servo signal (Pin 6 of 74121) is for 'positive-going' servos (Use signal @ Pin 1 for negative-going servos)
It would be simple to connect an external pot instead of the internal 'feedback' pot (most servos use a 1.5K pot), but you'd still have to provide a means for the pot to be rotated by the gear train. Remember, the servo is a 'closed loop' feedback system.

5. Servo Controller Schematic and Code

This is a PIC16C55-based servo controller project that reads a joystick and adjusts the servo accordingly. Includes source code and a schematic drawn with a free package. Only catch is that the schematic program and all the required PIC programming utilities will ONLY run under MS-DOG.

Servo Controller Schematic and Code (TLJVERERR) (TLJFILE!FOUND)

6. Futaba Channel Extender

Author: Erasmo Brenes
Date: 12 Jun 1995

Based on reading this newsgroup since the 80s, that a large percentage of the readers are somewhat electronic literate. If not, here are the instructions on how to use the three extra channels in your Futaba/Airtronic/anything that uses the Phillips NE5044. (Some older boxes may have older Signetics number CD8081, which is the same part).

To enable these additional channels you need to free pins 5, 6, and 7. These pins are grounded by Futaba to disable channels 5, 6 and 7. Once lifted/freed, the needed circuit for each channel is as follows:

As you can see, it is a tough and very tricky "hack" to gain your extra channels. BTW, this modification applies to the AM boxes as well. Some of the Futaba ATTACK are based on the NE5044 and thus you can "upgrade" them as well (except that unless you have a 7 channel AM receiver, you'll also have to buy an RCD receiver for about $65). The cost of the above "upgrade" is under $10, with the switches/pot taking the bulk of the cost.

>As far as patents go, the channel extender adds the 3 new
>channels in what I thought was a rather novel way, so I

So, you thought of a novel way :-). Let me guess, you have some simple logic which syncs to the frame coming out on pin 2 of the DIN connector. Then when the last channel has gone by, you insert channels 5, 6 and 7 from your box back into the frame stream. I guess that a PIC with an A/D (ie. 16C71) is all the circuitry that you need (plus misc. caps/res). And yes, you don't need to move the trainer switch to do such task. Now, is this a patent material?

7. Frequency Formula

Author: Don Ayers

Hello R.C. Nuts!

Ever have the problem of people showing up at your hill with unmarked channels on their transmitters? You ask for their channel, and they say, "Um, 72.810?"

The frequency is not readily useful in preventing a disaster, is it?

Here's a interesting conversion that, once memorized, can me done in a few seconds in your head. You can then amaze your friends as you call out their correct channel number from just hearing the frequency!

Here goes:

  1. Take the hundredths from the frequency (72.810; use just "81")
  2. Subtract 21 (81-21=60)
  3. Divide by 2 (60/2=30)
  4. Add 21 (30+21=51)
  5. You're done. The answer is channel 51!
This can also be reversed, and yield the frequency when only the channel number is known, although this is less useful at the field:

  1. Take channel number (51)
  2. Subtract 21 (51-21=30)
  3. Multiply by 2 (30x2=60)
  4. Add 21 (60+21=81)
  5. Put 72. in front of the answer (72.81)
By remembering these conversions, the answer is immediately available when needed, and no one has to scramble for a full page channel chart when some yahoo is threatening to turn on an unmarked transmitter!

Hope this is of interest to some out there - happy flying!

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