I spent a few hours testing various configurations of booster circuits and wire lengths and have come to the conclusion that no booster makes a difference here.
- None - direct from Microcontroller
- Voltage Booster - HeaterMeter v4.2/4.3 circuit with a 1k pullup to 5V on servo signal
- Voltage Booster + Inline Diode - Same as voltage booster but with a diode after the boost
- Current Booster - common collector amplifier / emitter follower / Ralph's linked circuit
- Directly to HeaterMeter (<100mm)
- End of ethernet cable (~2m)
- Long run simulation. This uses the 2m ethernet cable but also adds a 10 ohm resistor to the servo signal after the booster to simulate a longer run (~75m).
Of all the connections, the long run was by far the most problematic, with the servo going nuts all the time so most of my testing was done with this setup. The Current Boost and None setup did equally well. Both Voltage and Voltage + Diode did poorly, but only mildly better than the other circuits but I'd say none of them were acceptable in terms of performance. The Current Boost puts out a 2.7V signal with a 0.24mA current draw during the pulse followed by a -0.20mA dip as the pulse is discharged. The None was similar with a 3.3V signal, 0.24mA current, and a discharge that maxed out my current meter (-1.30mA). Voltage Boost put out a 5V signal but used almost twice the current 0.4mA and the same maxed discharge. All three had a high inrush current which maxed my meter >1.30mA.
I then spliced in a splitter to the servo end of the output and installed a socket for placing a capacitor on the power lines at the servo end. First capacitor, 10V 470uF, all circuits had zero problems. 10V 220uF - zero problems, 10V 100uF - zero problems, 25V 47uF - zero problems, 25V 22uF - zero problems. You can see a trend here. Then I tried 50V 0.1uF which had zero problems with the Current and None boosts, but showed rare glitches with the voltage boost circuit. Placing any of these capacitors on the HeaterMeter side of the 5V line had no beneficial effect. Without the capacitor, the 5V line measured at the servo varied from 4.14V to 6.18V.
No boost circuit works perfectly, but the Voltage Boost which is integrated on the HeaterMeter 4.2/4.3 boards is the clear loser in all testing. The best solution is to place a capacitor on the servo's 5V/GND pins as close to the servo as possible. Based on these observations, future hardware designs will certainly not use the Voltage Boost circuit. The Current Boost circuit as it did not show any beneficial effect above the None and given that the peak current draw and pulse current draw are the same with both of these, the Current Boost will probably not be used either.