The Process Duration Timer has been a long term project of mine and has gone through several upgrades. Recently I have improved its utility by adding increased flexibility by adding a mechanism for it to disconnect one of its inputs when a process has finished. It has also been modified so that it will track an input voltage that is rising or falling towards a target voltage. An external interface was also designed so that it could track changes in light and temperature. I have covered these upgrades in previous blogs. While my original build remains useful I decided that it was time to build a second unit in a more professional case. Here is a picture of the original build and the new build.
The process of building a new unit helped me accomplish several goals. This time as I built the individual modules of the Process Duration Timer, hereafter PDT, I took the time to create better pictures, schematics, and notes documentation. Sometimes when I build something it is intuitive enough that I think, I will remember how I did things and why I built things in a particular fashion. Time however has shown that I seldom remember and that I almost always have to go through a big process of reverse engineering, kicking myself the whole time for not making better documentation when the ideas are fresh. Fortunately, when I built the original I made it modular and put all the boards on cables that could be unplugged. In this way I was able to remove a board from the original and study it as I build the new board for the bench model.
This picture shows the three main modules of the PDT. In the lower right corner we have the clock module. This board is a cheap clock kit from China. Besides the modifications that were necessary so that it could be stopped at the end of a process and hold its final time until it is requested and be reset to 00:00:00 at the start of each test, there was the need this time to extend the clock displays so that they could be mounted on the front panel. For this procedure I used header pins on the board itself and female bread board jumper wires which were soldered to the pins of each 2 segment display. The many colors that are available on these bread board wires are a real bonus as I wanted to have the pins coordinated from one display to the next. I also used the breadboard jumper wires as a way to color code and connect module to module. The building and modification of the clock module went very smoothly.
The module in the middle is the interface board. This is where the connections from the clock, power supply, and user interface on the front panel come together. It is also an Arduino Uno shield that plugs directly onto the Arduino underneath it. To build the interface I took the interface from the other unit and for the most part duplicated it.
This board has the 2N7000 MOSFETS that are controlled by the Arduino and in turn control the clock and the functions of the unit. It also has the resistor divider and buffer circuit that feed the 0 and 1 analog inputs of the Arduino. The unit is designed to handle voltages up to 30 + volts and this of course must be mapped onto the 5 volt limit of the Arduino. These inputs compared the voltage of a device or process that is under test and a reference voltage, called the Target, that we have selected using the variable power supply. You can see how I have used bread board jumper to make my connections. By using a combination of male and female end and by heat shrinking them into combinations of 2 through 4 I am able to make it impossible to mismatch. A color scheme has also been followed and is consistent with the first unit.
This is the power supply module. It is also a Chinese Kit and while it is capable when properly heat sunk of providing 3 Amps over the range of 0 to 30 volts for this project it is only needed to provide a variable reference Target voltage. I have modified it for this application and removed the usual current limit control and replaced it with a trimmer which has been set to 100 mA. The fact is it will not be called on to deliver any more than 400 nA. This module caused me quite a bit of trouble in this build. If you notice in the upper left hand corner of the circuit board I have installed a 9 volt Recom regulator that is used to supply power to the clock module, the Arduino, and the voltmeter and indicator LEDs on the front panel. This option for a fixed voltage regulator was placed here by the original designers as a power source for a fan to cool the output transistor. For my build I wanted to use it for module power. This worked out on the first build in the blue box but it was not working properly on this one. After a couple hours of investigation I determined that the problem was being caused by the two different grounds that the board has. There is the internal ground that is used by the power supply itself which is to the left of R7 0.47 Ohm resistor and there is the external ground that is used by the load. The power supply circuit uses the voltage across R7 to sense and control the current. As long as these two grounds are kept separate the board functions great. Unfortunately my application had brought the two grounds together on the interface board and this was causing things to malfunction. The solution wasn't too bad however. I left the R78-9.0 regulator on the power supply board but I isolated all its leads from the board circuitry. Next I installed a small 12 volt switching power supply on the back inner case and fed the 12 volts to the regulator to get my system 9 volt supply. Now the grounds for the two voltages were isolated and could be commoned without creating any problems.
Here is a picture of the 120 VAC to 12 VDC switching power supply that solved the ground conflict problem. I still do not understand why this has not been an issue for the first build but it continues to work properly at this time. It was quite a relief at 4:00 AM this morning when I finally felt that I had the mystery solved and I could crash.
Here is a picture of the unit looking from the back towards the front panel.
The biggest challenge of this project was the front control panel. I could not justify the costs of laser cutting it this time so I did it with the drill, copping saw, and lots of file work. It is done on my usual black plexiglass but I have printed a sticky on one side sheet with the labeling and then put a sheet of clear laminate over the top. Here is a picture of the unit on the shelf along with all its black paneled cousins.
Thanks for checking it out.