Triple - Servo Motor Driver Circuit (556)


  This circuit is for driving small servo motors that are often used in Radio Controlled airplanes and cars. These motors have become popular as switch machine motors for model railroads due to their relatively low cost. The are also popular for small robot designs.

A printed circuit board is available for this circuit.

  The Servo Driver circuit allows users to experiment with servos without the need for a computer or a microcontroller device to program the travel limits. The circuits uses 556 timers to generate the control pulses and two potentiometers to adjust the widths of the pulse.

  External controls can be toggle switches, potentiometers or inputs from other circuits.

  The servo motors can be use to control turnouts, semaphore signals and other animations. One driver can control more than one servo such as turnouts of a crossover.

  Each driver circuit can control motors in one of three modes depending on how its input terminal is configured.

  1. 2 Fixed Positions.
  2. 3 Fixed Positions.
  3. Variable Position.

  The motor's rotation arc can be adjusted to almost 90 degrees each side of centre (180 degrees total). Many typical RC servo applications use 45 degrees (90 degrees total) from centre as their limit.

  The speed of rotation can be crudely controlled by slowing the output pulse rate of timer IC 1A. This is done by capacitor C3 to the common of the circuit. The motor will move in slow steps rather than a fast sweep.

  Testing showed that about 19 or 20 steps (pulses) are needed for the motor to rotate 180 degrees.

  The user can buy a few inexpensive motors to experiment with to see if they are acceptable for their needs.

  Analog and digital type motors can be controlled by this circuit.

Basic Servo Motor Driver Schematic

  The next schematic is basis of the servo motor driver circuit using two 555 timers. The Triple Servo Driver circuit uses two - 556 timers.

Triple Servo Motor Driver PCB Schematic

  The next schematic is for the Triple Servo Driver's circuit board. Each circuit board can control three servos. Each motor driver has independent controls.

  The bottom half of IC 1 provides the clock pulses while the other three timers control the servo motors.


Position Control Options

  The next diagram shows three methods that can be used to control the position of the servos. Anything that mimics these methods can be used to control the circuit.

Servo Motor Position Control Methods

  The 2 Position control uses a SPST toggle switch or equivalent. The position of the servo is set by the potentiometers on the circuit board.

  The Variable Position control uses an external, 1K ohm potentiometer to adjust the servo's position. A higher control voltage input could be used by increasing the value of the fixed 1K resistor. to compensate.

  The 3 Position control uses a SPDT / ON-OFF-ON, toggle switch or equivalent. The CW and CCW positions of the servo is set by the potentiometers on the circuit board. The centre position is set by an external 20K potentiometer although a suitable fixed resistor could also be used. The 2.2K resistor prevents a short circuit if the potentiometer is set to zero ohms.

  A multiple fixed position control could be made using a rotary switch and fixed resistors.

  Electronic controls can also be used as inputs.

2 Position Servo Driver Circuits (LM556)

  The next diagram shows 3 servo motors connected to the circuit board using 2 Position controls. Also shown is the sweep rate control.

PCB Parts List

2 556 Dual Timer IC 296-6504-5-ND
3 1N4148 Diode 1N4148-ND
1 33K/0.25W 33KQBK-ND
1 10K/0.25W 10KQBK-ND
4 4.7K/0.25W 4.7KQBK-ND
6 20K Potentiometer 399-6596-ND
4 0.1uF Capacitor 495-2479-ND
1 1uF/50V Capacitor 399-6596-ND
1 10uF/50V Capacitor P5178-ND
1 100uF/35V Capacitor P5165-ND
6 2 Position Terminal Block - 3.5mm ED2635-ND

Servo Motor Driver Circuit printed circuit boards

  A printed circuit board and parts ar available for this circuit. There are 8 switch machine drivers on each board.

  The following is an image of the circuit board which is 1.9 inches by 3.0 inches and is drilled to fit terminal blocks with a 3.5mm pin spacing.

  The board has been commercially made and is tinned.

Servo Motor Driver Circuit PCB - Assembled Example

Option 1

  Servo Motor Driver circuit boards only:

  11.00 dollars US each, Plus postage.


Option 2

  Assembled - Servo Motor Driver Circuit Board circuit boards with all parts:

  25.00 dollars US each, Plus postage.


Option 3

  Kit - Servo Motor Driver Circuit Board circuit boards with all parts:

  23.00 dollars US each, Plus postage.


  If you are interested in printed circuit boards please send an email to the following address: Subject: Servo Motor Driver Circuit

  Your message will be answered as soon as possible.

Servo Motor Driver Circuit PCB Parts Placement Diagram

Servo Motor Driver Circuit Board Parts Placement Diagram

Print the diagrams to aid in assembling the circuit board.

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Please Read Before Using These Circuit Ideas

  The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

  If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

  Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.

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15 March, 2020