Guys i had built a circuit(servo controller) using 555IC.It has VR to adjust the position of the servo.The servo is 90 degree servo.I have to control the position through microcontroller.I need to switch the servo to 3 position only which is 90,0,-90 degree.The microcontroller will control the position but i dont know how to make it to adjust the resistance of the VR by itself.

 

It is probably easier simply to control the servo directly with the microcontroller. You can generate the -90, 0, and +90 pulse with in software.

Is there a reason you have to use the 555 analog controller?

John

 

How to control directly?
555 is just an option for me bcoz i found 555 servo controller circuit when i searched in the net.
FYI im using Atmel AT89S51 microcontroller n this MC is a must n cnt use other MC.

 

I am not familiar with Atmel, just PIC. In assembly, you can just generate the PWM pulse that is needed using on/offs and delays. You can control both the pulse width and repetition rate. If you are doing other things with the mc, you can use the internal timers and interrupts.

You can use an analogue input or digital inputs to determine which position/pulse width is selected. Some PIC's also have hardware PWM that can be used, but in general the frequency is a little too high for hobby servos. The specification for most hobby servos is 50 Hz. However, they will work at higher frequencies. I have one application in which the servo works at about 120 Hz with no problems.

Just use a common ground, as I am sure you know, and connect the mc output to the signal wire of the servo. The servo can be powered by the same 5V supply used for the mc or use its own supply.

John