Electronics Projects: How to Program a Servo in PBASIC

By Doug Lowe from Electronics All-In-One Desk Reference For Dummies

To create motion in your electronics project, you can add a very useful device, called a servo, which lets you control mechanical motion with a BASIC Stamp program. A servo is a special type of motor that is designed to rotate to a particular position and hold that position until told to rotate to a different position. Hobby servos are frequently used in radio-controlled vehicles, but there are many other uses for servos.

The BASIC Stamp Activity Kit comes with a servo that you can use to learn how to write programs that control servos. You can also purchase servos directly from Parallax or from most hobby stores.

The easiest way to control a servo from a BASIC Stamp microcontroller is to use the PULSOUT command. This command sends a pulse of any duration you specify to an I/O pin of your choosing. The syntax of this command is as follows:

PULSOUT pin, duration

You specify the duration in units of two microseconds. A microsecond is one-millionth of a second. There are one thousand microseconds in a millisecond. Thus, to send a 1.5 ms pulse with the PULSOUT command, you must specify 750 as the duration, like this:

PULSOUT 0,750

Here, a 1.5 ms pulse is sent to pin 0.

Here are the PULSOUT duration values you should use for a typical hobby servo for various angles.

Angle	Duration	Angle	Duration
0	250	95	778
5	278	100	806
10	306	105	833
15	333	110	861
20	361	115	889
25	389	120	917
30	417	125	944
35	444	130	972
40	472	135	1000
45	500	140	1028
50	528	145	1056
55	556	150	1083
60	583	155	1111
65	611	160	1139
70	639	165	1167
75	667	170	1194
80	694	175	1222
85	722	180	1250
90	750

For example, to move the servo on pin 0 to 75°, use this command:

PULSOUT 0,667

Remember that to hold its position, a servo needs a constant stream of pulses approximately 20 ms apart. Thus, PULSOUT commands are usually contained in either DO loops or FOR-NEXT loops. For example, here’s a bit of code that keeps the servo on pin 0 at 45° indefinitely:

DO
 PULSOUT 0,500
 PAUSE 20
LOOP

Here is complete program that moves the servo to 45° when SW1 (a pushbutton on pin 14) is pressed and 135° when SW2 (a pushbutton on pin 10) is pressed.

' Servo Control Program
' Doug Lowe
' July 15, 2011
'
' This program moves a servo to one of two when SW1 is pressed
' and returns the servo to center position when SW2 is pressed.
' {$STAMP BS2}
' {$PBASIC 2.5}
Servo PIN 0
SW1  PIN 14
SW2  PIN 10
Position VAR Word
Position = 500
DO
 IF SW1 = 1 THEN
  Position = 500
 ENDIF
 IF SW2 = 1 THEN
  Position = 1000
 ENDIF
 PULSOUT Servo, Position
 PAUSE 20
LOOP