Sunday, July 9, 2017

Finished construction of the permissive make relay tester


My grommet supply arrived today and I finalized the holes after selecting the ideal size grommet for the LEDs. I seem to always underestimate the hole size required to fit in the grommet, but eventually got it right. After installing the LEDs into the cover, I was ready for applying the insulation - a rubber insulator that is brushed on over exposed wire junctions to form a protective coating.

Unfortunately, two things went wrong today. First, my existing bottle of connector coating is partially dried out and almost empty. Second, when I got over to CHM to check for the updated Arduino code that runs this version of the circuit, I couldn't locate it. Stan will need to hunt it down tomorrow.

Frys did have the connector coating in stock, which I ordered for pickup in the afternoon.  After lunch with my wife and a friend, I dropped them off and rode over to grab the purchase at the store. After it was applied, I loaded the out of date sketch.

I can accomplish some testing of functionality using the old versions of the sketches provided by Stan, which proves out ability to pick and hold, rattle relays and test contact resistance. First I did a test with only the USB link powering the Arduino and its relay board. All the LEDs and the relay module fired properly and the sketch communicated with the PC.

Next up was a test with the 20V supply activated in addition to the USB link. That way I can see if the relays pick and hold using a red relay plugged into the socket. The relay magnets did seem to work fine although the first flaw was uncovered. When the tester believed it was using the green relay socket, my LED lit under the red socket.

The next flaw was uncovered when I noticed that my relay module was active initially, although I believe the code thought that both Pick and Hold coils were off. My LEDs light when the drive line is high, but the relay module is off for a high voltage.

I can fix both of these flaws easily. The relay module can be changed to use the NC contact to drive the LEDs and pick/hold coils - when the relay is off, the lamp and coil will be on and vice versa. I could move the two LEDs under the relay sockets allowing the proper one to light. Dealing with the pick and hold LEDs is more complex but I can just swing the pick and hold LEDs onto the actual relay socket pick and hold wires, adding a suitable additional resistor to limit current.

Next up, I added the 5V supply and tried measuring contact resistance on the red relay. I can't check circuits 5 and 6 as these are only on green relays, but I had some interesting readings for the others. I will have to check this with Stan tomorrow when I am retrieving the correct code.

The quality of the contact test readings is mediocre as I don't have the code that makes use of the voltage reference from pin A15, but I could check out operation well enough to prove that my wiring seems sound.

I will haul this into CHM tomorrow to check it out with Stan - who will be hunting for the updated code - and I can work out any wiring changes needed before closing everything up. 


  1. Hi,

    I love following your stories and as a software-person in my day job I can very much relate to this:

    > Second, when I got over to CHM to check for the
    > updated Arduino code that runs this version of
    > the circuit, I couldn't locate it. Stan will need
    > to hunt it down tomorrow.

    I understand perfectly well how and why this happens, but it still makes me cringe a bit.

    May I suggest version-control? Try something like GitHub. It is very easy to get started with, you need very little extra tooling and if you don't mind people sharing the code it is free.

    Who knows, there even may be more people with input to the project...

    Have a nice week,

  2. Hi Marian

    I use github and similar for my projects - for example the modification to drive an IBM 029 keypunch from a PC for both reading and punching cards. However, each volunteer working on the CHM efforts has their own approach and ethos. Stan who designed this device is more casual.

    I will suggest this practice again but I don't expect to see a change. I am definitely with you on this topic, underscored by the experience with the relay tester code.