Simple Servo Controller

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Servo motors have many uses in everything from robotics to puppetry to photography and beyond. These little motors can position their output shaft to any position on command and hold that position. Most servos have a range of motion to about 210 degrees and thankfully are very easy to control with a simple circuit such as the one presented here. Using just a 555 timer and a few support components this circuit can control a servo through it's full rotation based on the position of a pot. This circuit was originally published in the Think Tank column of the October 1995 issue of Popular Electronics.

Schematic

Parts

Part	Total Qty.	Description	Substitutions
R1	1	820 Ohm 1/4W Resistor
R2	1	68K 1/4W Resistor
R3	1	10K 1/4W Resistor
R4	1	1K 1/4W Resistor
R5	1	1K Linear Taper Pot
C1	1	1uF 16V Electrolytic Capacitor
Q1	1	2N3904 NPN Transistor	2N2222, Most Small Signal Transistors
U1	1	555 Timer IC
MISC	1	Board, Wire, Knob For R1, 8 Pin Socket For U1

Notes

R1 adjusts the position of the servo.

Connect the servo to the circuit as shown in the schematic. For common Futaba servos, the red wire is power, the black wire is ground, and the white wire is control.

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Comments

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Tony Simple Servo Controller Friday, August 06, 2010 2:14:50 PM

I hooked up the circuit on a proto board--it works pretty well, but I found it sort of noisy. In "Practical Electronics for Inventors," by Paul Scherz, it shows a 1000uF cap across the positive and negative inputs to the motor (page 933 of 2nd Edition). I tried a 5uF one and it cured the noise.

john albert fuller Simple Servo Controller Tuesday, July 20, 2010 3:18:06 PM

graig asked can he make the servo go from 0-90 and 90-0 i have a similar problem on a railway signal but wish to use a double throw switch help please john

zaxx Simple Servo Controller Saturday, March 06, 2010 8:36:39 PM

I built this circuit,and it does not work.Nothing at all happens.

anonymous Simple Servo Controller Friday, January 29, 2010 11:22:58 AM

Thanks for posting this circuit, it works great. I am using it to adjust the skew on a C band satellite dish. For my particular application, I found the maximum and minimum values of R5 to be ideal: Rmax: 2.947K Rmin: 909 Any resistance outside of that range, and the circuit attempts to drive the servo motor past it's mechanical limits. The values, in your particular application, can be found experimentally.

zack simple servo controller Thursday, January 28, 2010 7:20:04 AM

R3 should be change to 1K resistor to get the minimum time of positive pulse equal to 1.4mS and the maximum time is equal to 2.1mS...so it suitable for servo motor running...

tedd Simple Servo Controller Thursday, September 10, 2009 4:56:48 AM

I like to spend much time in the Internet, but, unfortunately not often find something worthy and interesting to read. I liked the way you covered the topic and depth with which you explain these important things. Hope you will like it.

J.J. Simple Servo Controller Friday, August 28, 2009 8:20:19 PM

At first I didn't get any movement, then I disconnected the linkage between R5 and pin 2 of the IC, and C1. Now, like someone else said, the servo just twitches once. Any ideas, anybody? Jerry, how'd you manage to get this to work?

Jerry Miller Simple Servo Controller Tuesday, May 19, 2009 7:55:26 PM

I have built this circuit and it works well. Did you know that you can connect pin 4 to pin 5 instead of pin 8. This just simplifies the PC board layout.

tacal Simple Servo Controller Saturday, April 25, 2009 8:32:38 PM

if the value as ff r3=10k, r4=1k, r5=1k n c=0.000001F this should be result to Freq=103.07153164296022 Tlow=0.001s Thigh=0.008s.. assume the values are correct.. the servo feeds on positive pulse. The out put from pin 3 of 555timer will trigger the transistor in positive pulse that is 0.008s? how can the servo work with that? If im not mistakern the servo need :1ms for 0 degree, 1.5ms for 90 deg, n 2ms for 180 deg.. can someone explain this? I need help to analyze this prob thanks

anonymous Simple Servo Controller Monday, April 06, 2009 9:04:37 AM

Question: I turn the circuit on and the servo it twitches once or twice. Then it doesn't do anything. What could be causing this? One comment says R1 and R5 need to corrected. Is this refering to there position on the Schematic?

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Tony	Simple Servo Controller	Friday, August 06, 2010 2:14:50 PM
I hooked up the circuit on a proto board--it works pretty well, but I found it sort of noisy. In "Practical Electronics for Inventors," by Paul Scherz, it shows a 1000uF cap across the positive and negative inputs to the motor (page 933 of 2nd Edition). I tried a 5uF one and it cured the noise.
john albert fuller	Simple Servo Controller	Tuesday, July 20, 2010 3:18:06 PM
graig asked can he make the servo go from 0-90 and 90-0 i have a similar problem on a railway signal but wish to use a double throw switch help please john
zaxx	Simple Servo Controller	Saturday, March 06, 2010 8:36:39 PM
I built this circuit,and it does not work.Nothing at all happens.
anonymous	Simple Servo Controller	Friday, January 29, 2010 11:22:58 AM
Thanks for posting this circuit, it works great. I am using it to adjust the skew on a C band satellite dish. For my particular application, I found the maximum and minimum values of R5 to be ideal: Rmax: 2.947K Rmin: 909 Any resistance outside of that range, and the circuit attempts to drive the servo motor past it's mechanical limits. The values, in your particular application, can be found experimentally.
zack	simple servo controller	Thursday, January 28, 2010 7:20:04 AM
R3 should be change to 1K resistor to get the minimum time of positive pulse equal to 1.4mS and the maximum time is equal to 2.1mS...so it suitable for servo motor running...
tedd	Simple Servo Controller	Thursday, September 10, 2009 4:56:48 AM
I like to spend much time in the Internet, but, unfortunately not often find something worthy and interesting to read. I liked the way you covered the topic and depth with which you explain these important things. Hope you will like it.
J.J.	Simple Servo Controller	Friday, August 28, 2009 8:20:19 PM
At first I didn't get any movement, then I disconnected the linkage between R5 and pin 2 of the IC, and C1. Now, like someone else said, the servo just twitches once. Any ideas, anybody? Jerry, how'd you manage to get this to work?
Jerry Miller	Simple Servo Controller	Tuesday, May 19, 2009 7:55:26 PM
I have built this circuit and it works well. Did you know that you can connect pin 4 to pin 5 instead of pin 8. This just simplifies the PC board layout.
tacal	Simple Servo Controller	Saturday, April 25, 2009 8:32:38 PM
if the value as ff r3=10k, r4=1k, r5=1k n c=0.000001F this should be result to Freq=103.07153164296022 Tlow=0.001s Thigh=0.008s.. assume the values are correct.. the servo feeds on positive pulse. The out put from pin 3 of 555timer will trigger the transistor in positive pulse that is 0.008s? how can the servo work with that? If im not mistakern the servo need :1ms for 0 degree, 1.5ms for 90 deg, n 2ms for 180 deg.. can someone explain this? I need help to analyze this prob thanks
anonymous	Simple Servo Controller	Monday, April 06, 2009 9:04:37 AM
Question: I turn the circuit on and the servo it twitches once or twice. Then it doesn't do anything. What could be causing this? One comment says R1 and R5 need to corrected. Is this refering to there position on the Schematic?