servo tester
IC1 = LM555 Timer R1 = 2K7 C1 = 390nF R2 = 68K C2 = 100nF (0.1uF) P1 = 2K7, potentiometer C3 = 100nF (0.1uF) D1 = 1N4148, signal diode Misc: Perf board, adapter 6Vdc, IC socket, case, knob, wire, solder, soldering iron.

The normal duty-cycle for a 555 is always 50% or more. RC time to charge is always larger than the discharge time, because during charge-time the discharge resistor is in series with the charge resistor. A servo however needs less than a 50% duty-cycle so that we can vary the pulses between 1 and 2mS (milliseconds) with a repeat frequency of 50Hz. (One periode equals 20mS) so a duty-cycle of 5 to 10%. By adding one diode over R2 and changing position of the charge resistor this problem is solved.

The charge-time, and therefore the time that the output is 'high' ("1") is set by P1 & R1 and the discharge-time is set by R2. They can be choosen independently from each other. The circuit is designed in such a way that varying the resistance (over D1) from 2K7 to 5K4 a pulsewidth gives from 1.0 to 2.0 mS by adjusting potmeter P1. The output pin(3) is where the signal wire goes. For most servos this is either orange, white or yellow.
The red wire of the servo, and the battery, goes to '+' and the black or brown servo wire, and the battery,
goes to '-'.
Data sheet on the LM555 timer is available, just click on LM555 in 'Parts & Description' section.
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