Slide Potentiometer

Purpose

Get a device that has feedback in order to implement some signal processing and use a hybrid stepper motor in order to interface using an H-Bridge.

GitHub - benhaub/sm10001slidePotentiometer: https://www.adafruit.com/product/5466https://www.adafruit.com/product/5466. Contribute to benhaub/sm10001slidePotentiometer development by creating an account on GitHub.

Hardware

Slide Potentiometer

H-Bridge

8-pin SOIC adapter

Initial Setup

Started by placing the H-Bridge ontop of the 8-pin SOIC adapter. The Motor would move but only if I pressed down on the H-Bridge (similar to what happened with the ULN2003 and he Schmart board adapter). Even when it did move it didn't seem to go in the direction I intended. If I reversed IN0 and IN1 it would only move in one direction. The output was also extremely noisy. I chalked this up to a bad connection. Perhaps I was not connecting one output or the other. I tried to solder the driver to the adapter to improve the connected but that seemed to make it worse. Now it would even budge the outputs. They were being foced low at the input, barely reaching over 2 volts. Since they were soldered I could no longer push down and move the driver to change the connection properties.

Either I had ruined the driver by soldering it (Put too much heat on the IC or accidentally run solder underneath and shorted it out on the powerpad), or I had fixed the connection and exposed a new problem.

I tried another driver and another adapter but this time I did not place any additional solder on the adapter. I simply heated up the pads and the driver stuck to the adapter. It failed in the same way as the previous driver did.

I now looked to how I had the voltage set up. My setup was probably atypical. I had to use the negative supply of my oscilloscope to achieve the required 6.8V input. I was using the ground pin of the IC for the -1.8V supply and VM for the +5V. My board was not grounded to the IC. When this setup worked previously (before I had soldered the board), I assumed that the -1.8V was probably pushing the 3.3V TTL logic up to 5V. This was fine since the datasheet says the max input for the logic was 7V. Now that I have them soldered what seems to be happening is that the negative voltage is preventing the PWM input from reach the minimum input voltage required to turn on the the H-Bridge. I could see I was sitting below a volt and my PWM signals were not moving.

I'm now thinking the only way to get the H-Bridge working is by having my board and the supply at the same ground either by:

getting a supply that can supply over 6.8V without using a negative supply.
amplifying the DC voltage coming from my board so that I can connect the board ground.

Final Setup

My main issue ended up being how my oscilloscope was powering the H-Bridge. The scope's max voltage is 10V but anything beyond 5V is only achievable using a +5V and -5V difference. This negative voltage was causing some grounding issues that stopped the H-Bridge from functioning properly.

I decided to get a DC power supply. I chose one that had a USB power output so that I could power my devkit from the same supply that was powering the H-Bridge so that everything was on the same ground plane. This ended up working.

Interestingly, the slide potentiometer works well without power the devkit from the provided USB slot. There is a difference in grounding of about 3V when the devkit is plugged into my computer and the H-Bridge is powered from my supply but I am able to drive the motor just fine.

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