# Wondering which PT785-S ratio is best for you?

The stock Hitec HS-785HB servos that drive the pan and tilt can rotate several turns in factory form. By coupling the servo to an external gear set, the amount of rotation is decreased by the ratio but the torque and precision is increased by the ratio. In order to tailor your pan and tilt system to the controller you wish to use, we offer a 3.8:1 ratio as well as a 7:1 ratio.

• If the servo controller you intend to use transmits a standard PWM range of 1050-1950usec, the 3.8:1 ratio can be selected in order to achieve just over 360 degrees of rotation on each axis. On the same controller when running a 7:1 ratio, each axis would rotate approximately 196 degrees.
• If you run a servo controller capable of a 675-2325usec signal range, you can achieve just over 360 degrees on each axis running the 7:1 ratio or approximately 662 degrees with the 3.8:1 ratio.

*Figures are calculated by multiplying the PWM range by 1.525 to get the number of degrees the servo will rotate.

Example – I am going to use a servo controller with a 1000-2000usec PWM signal range and a have selected the 7:1 ratio on PT785-S pan and tilt:

2000usec-1000usec = 1000usec

1000 x 1.525 degrees = 1525 degrees. This is the amount of rotation the HS-785HB servo itself will rotate.

1525 / 7:1 ratio = 218 degrees. This is the amount of rotation that each axis on the pan and tilt will rotate given the stated signal range.

The speed, torque and precision of the PT785-S are altered by the external ratio. The 7:1 ratio will yield higher torque and higher precision but will be slower than the 3.8:1 ratio. The 3.8:1 ratio will have more rotation given the same signal but will have less torque and less precision when compared to the 7:1 ratio.

• 6V max speed with 3.8:1 ratio: 68 degrees/sec
• 6V max speed with 7:1 ratio:  37 degrees/sec
• 6V stall torque with 3.8:1 ratio: 695 oz-in
• 6V stall torque with 7:1 ratio:  1281 oz-in
• Precision with 3.8:1 ratio: 1 pt every 2.8 degrees
• Precision with 7:1 ratio:  1 pt every 1.5 degrees

### 7:1 RATIO   ### 3.8:1 RATIO   ##### Shop Secure 