Servo Control


Hi Folks. Some 15 years ago, Clive Seager presented a fairly comprehensive series of articles entitled 'Picaxe in Schools' as an intro to Picaxe programming, and designed to be used with a Picaxe dedicated PCB. This was my intro to the 08 and Picaxe programming per se. Included was a short servo dissertation and two programs for demonstration with the board, which I never tried out at the time, not having any use then for a servo. This was a bare cct which used pin 1 in parallel with a 330R and a LED, the latter forming part of a traffic light demo in the articles. I recently tried out the servo section, and it worked fine (of course!).

My problem is that pt 3 of the Picaxe manual relating to servo control, and the descriptions for servo and servopos in pt 2, specify a 330R in the line from the selected Picaxe pin to the servo initiation input. At first I thought that this was probably for current limiting, but on further consideration, Mr Seager's PCB did not include such (as the LED and resistor are in parallel i.e no series resistor). So I would be interested to know why the 330R is specified in the manual.

Could anyone elucidate, please.

Ben Baker
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Senior Member
Just a bit of basic protection for the PICAXE chip, in case that pin gets inadvertantly
grounded or connected to V+.

It will work fine without it, but offers no protection for the chip. (I personally never use a resistor when driving servos).



Senior Member
The "high end" download circuit in the Picaxe manuals includes a 180 ohm resistor on the SEROUT pin for the same reason. Most Picaxe pins are datasheet limited to about 25 mA, and that resistor keeps the pin intrinsic diodes from exceeding their ratings (which is larger than 25 mA).

For the same reason, the standard download circuit includes a 10K shunt resistor to ground, and a 22K series resistor to the SERIN input. This is NOT intended to be a voltage divider. The 10K resistor ensures that the SERIN pin is pulled low at start up, so that the PICAXE can recognize that programming is intended. The 22K resistor limits the current into the SERIN pin, so that even old style serial ports (which may reach 25 volts in signal amplitude) will be clipped by the Picaxe input protection diodes, without overloading their current capabilities.