This circuit takes standard 0-10V control voltage (for example from analogue light controlling desk) and outputs a standard 1-2 ms RC servo motors control pulse.
Component How many 100 kohm resistor 2 47 kohm resistor 1 33 kohm resistor 1 470 ohm trimmer 1 1N4148 diode 1 220 nF capacitor 1 100 nF capacitor 1 NE555 timer IC 1
I have constructed my prototype to a piece of veroboard. No special construction considerations.
The 470 ohm trimmer in the circuit should be adjusted so that the pin 5 of NE555 IC receives a control voltage of around 1.6V. When a RC servo is connected to the circuit, you can slightly adjust the trimmer to adjust the center position and control range.
In this circuit NE555 is configured as astable multivibrator. The main timing capacitor (220 nF) is always changed to the voltage level which is determined by the voltage on NE555 pin 5 (in this circuit that voltage is aroun 1.6V). When the timing capacitor is charged only with current coming through 33 kohm resistor, the charging takes around 2 milliseconds. When control voltage is appied to control input, the charging current increased when conrol voltage increases. When th econtrol voltage is 10V, the charging takes around 1 millisecond.
When the capacitor is fully charged, NE555 starts to discharge the capacitor through 100 kohm resistor. The discharging continues until the voltage has reached the half of the control voltage (1.6V/2=0.8V). The discharge time is determined by the resistor between NE55 pins 2 and 7. The 100 kohm resistor in this circuit makes this time to be around 11 milliseconds.
When the capacitor is discharged, the circuit start charging it again.