How do I control 2 servos with 1 switch (using 555 timers)?

Currently I am working on a project that requires 2 servos to move in opposing directions; going from 0º to 180º on one and 180º to 0º on the other. After looking at the useless machine instructable, I decided that 2 555 timers attached to 1 DPDT toggle switch (ON-ON) would work best, however I need help with the exact resistance and capacitance values for the resistors.

To get the servo to be at 0º (technically -90º) it needs a 1ms pulse every 20ms. To get it to go to 180º (technically 90º) it needs a 2ms pulse every 20ms. The servos run off of a max of 5V, so I am going to be using 3 AA or AAA for 4.5V. 


For most servos, frequency isn't all that critical, only the pulse length. Faster frequency will allow the servo to respond faster, but most servo's work fine with
a frequency above 20Hz. For some older servo's 100Hz may be too fast for the circuitry to lock onto.

The 555 calculators on this page should help you figure things out: I generally look at what value caps I have on hand, plug those values into the calc to see what value of resistance is need to give the desired pulse time.

You can buy an inverting cable, which will make the servos move in opposite directions, from a single source for under £10. I've not used a 555 before, but there are some fairly simple equations online to determine the component values needed.

AA batteries are preferable, as servos can draw a reasonable amount of current, depending on the load. Also, most servos are designed to run on up to 6V, which will give better performance than 4.5V, although if the spec. says 5V, stick to that.