Wednesday, May 31, 2017

Waterproof Gripper

In a previous post, I described the manufacture of a waterproof electronics container.  Since then, I learned how to waterproof a hobby servo and connected it to the electronics module.  So, I now have a functional underwater manipulator.

Methods for making the servo waterproof have been well described in youtube videos such as and  Basically the servo is disassembled, the innards are filled with a low dielectric grease (Corrosion Block Grease 16OZ TB), the servo is reassembled, and then it is dipped in plastic-dip (Performix 12213 Plasti Dip Black - 22 oz. Dip Can) to form a plastic barrier.  Finally, an o-ring is sandwiched between the servo spline and the servo hub to seal the pivoting location.

The pictures below show closeups of the apparatus.  The servo gripper is the Standard Gripper Kit B from ServoCity.

Tuesday, May 30, 2017

LM393 Comparator

This post describes a test of a LM393 comparator.  This was my first soldered SMD (surface mount device).  I soldered once with some old flux using the sweep method (see for example  The comparator didn't work, so I started over with a new LM393 and simply soldered the pins carefully individually.  I used Sparkfun SOIC to DIP adaptors:

To test the comparator, I used the idea of a nightlight which turns on when a light sensitive resistor's resistance drops below a setpoint.  The setpoint can be adjusted with a potentiometer.  The circuit diagram is shown below.

The comparator compares the voltage at pins 2 and 3 and outputs a high on pin 1 if pin 3 exceeds pin 2.  If pin 2 exceeds pin 3, the output on pin 1 is low.  For example, if the LDR is at maximum resistance (in the dark), then V2 = 5V (6 Ohm / 16 Ohm) = 1.9V.  If the LDR is at minimum resistance (in bright light), then V2 is shorted to ground (0V).  Meanwhile, pin 3 may see a constant voltage of 1V (set by the pot).  So in the bright light, V3 > V2 and pin 1 is high so there is no voltage drop across the LED which stays off.  In the dark V2 > V3, pin 1 is low and the LED turns on.

The video linked below shows how the comparator works.  I really like this idea of using electronic components in place of a microprocessor.  It's a technique which may be dated, but is great to know.