Servos are one of the basic components used in all branches of model building. They are small, lightweight, low cost and are remarkably easy to control. Model building servos connect directly to an RF receiver unit. They typically have just three connections: positive supply (+5 V), ground (GND) and control (Pulse) lead, which supplies a control signal to move the servo arm. The signal on this lead is pulse width modulated and supplied by the receiver. Positive pulses with a length of 1 ms cause the servo arm to move fully to one end of its travel while 2 ms pulses move the arm fully in the opposite direction. Pulse widths between these limits move the arm to an intermediate position proportional to the pulse width. A pulse width of 1.5 ms centers the arm. The pulse repetition rate is approximately 20 ms i.e. 50 Hz but this rate is not too critical.
When you suspect that the model is not behaving as it should it could be a problem with the remote control transmitter, receiver or a servo motor. This handy unit allows you to quickly test the servo and eliminate it (or otherwise) from your lines of inquiry. This pulse generator design shown in Figure 1 is one of the basic bread and butter circuits known to almost all engineers.
A Two timing circuit
The pulse generator is made up of a dual timer chip type NE556, the output pulse width is controlled by the position of a potentiometer. The combination of resistor R1 and capacitor C2 in timer 1 of the NE556 produces the repetition rate of the pulse. This timer output signal at pin 5 has an approximately symmetrical mark-space ratio. The negative going edge of the output signal is used via C3 to trigger the second timer which then produces a positive going output pulse at pin 9. The width of this pulse is defined by the values of capacitor C4 and the combined resistance of R3 and P1. Pot P1 thereby gives control of the pulse width. During tests, it was found that the circuit with the component values specified here produced a pulse width in the range of 0.5 to 2.6 ms which more than covers the standard pulse width range used by these types of servomotors.
For this reason, P1 should not be turned fully to either end of its travel otherwise the connected servo will go past its intended end position and hit the mechanical stops, possibly damaging the servo. Before the circuit is powered up to ensure that the control knob P1 is the roughly mid position. The pulse repetition rate of the circuit was found to be 18 ms. The vast majority of servos operate with a supply in the range of 4.8 to 6 V. Here the operating voltage is in the range of 5 to 6 V which can be supplied by four AA primary cells or rechargeables. To make a neat job and simplify construction we have made a PCB for this design (Figure 2) which is available from the Elektor Shop . All components have standard (non-SMD) outlines so fitting the components should not pose any problems.
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