Tuesday, July 1, 2014

Make a PCB pt2

I received the parts for the GettingToBlinky PCB I talked about in a previous blog. And they sat there for over a week while I caught up with a few other tasks I had.

Today I felt like whiling away a couple of hours without straining my brain too much and building a Blinky device seemed like the perfect task. I love the smell of burning flux in the morning.

Despite all my mutterings about using solder paste, a pick and place tool and a heat gun, I realised the parts in this project are quite large. Certainly easy enough to see under a magnifying lamp to hand solder. So with help of some Blu-Tack to hold the board still and a bit more Blu-Tack on the end of a screwdriver to lift the components out of their containers and place them on the board I soldered it all together.

I started with the 7555 because it has the smallest pads and is in the centre of the board so I didn't want other components getting in the way. Then I added the diode, resistors and capacitor in that order. The LDR is actually a big large for the thru-holes I placed for it but long leads made it easy to get it into place.

So then it was time to solder the battery holder in place and finally we were ready to roll!

I inserted the battery after checking and re-checking the polarity and...

Nothing. No blinky!

I pulled out the battery just in case something was cooking and proceeded to measure every component and trace I could. Everything checked out.

So I re-inserted the battery and measured the voltage across pin 8 (Vdd) and pin 1 (GND) of the 7555. Nothing!

A bit closer inspection of the battery holder made me suspect that I might have some sort of short. So I partially inserted the battery and yay! blinky! But if I pushed the battery fully into the clip it stopped. But nothing was shorting. I looked at the battery and I looked at the board where it connects and it looks like the solder pad is too flat or the battery flexes slightly concavely at that point. Anyway the solution was obvious. I unsoldered the battery clip, added a large drop of solder to the GND pad on the board, resoldered the clip and reinserted the battery and yay! blinky! And I could push the battery fully into the clip now and it continues working.

Here's the "offending" centre GND pad:
I had to add a blob of solder to it to raise it's height enough to contact the battery negative. Here's the clip and battery in place:

And finally, Blinky!

Some lessons for the future

The blink rate is supposed to vary depending on how much light falls on the LDR. The GettingToBlinky tutorial left the actual value or even the range undisclosed. I picked a value at random from Digi-Key (16-33K) but this is probably way too low considering it's connected to a 470K resistor. Probably should have been 160-330K. No harm done but unless I knew the blink rate is supposed to vary, I probably wouldn't notice the slight change that does occur as I move the LDR from bright light to darkness.

The second lesson is far more serious, and potentially dangerous. The battery clip touches one of the LDR leads:
Fortuitously, this LDR lead actually does connect to the positive terminal of the battery and so no harm done. However the potential for disaster is quite obvious. And again the solution is so simple. I had plenty of room on the board to move the thru holes for the LDR away from the battery clip but I had left the outline of the original SMD version and not updated it to that of the thru hole version. As a result I didn't see the potential conflict.

Which brings me to the third lesson. I made the mistake with the LDR and battery clip placement because I purchased these components blind, relying on diagrams and data sheets. In all my years as an electrical engineer, I never did that. I always insisted on having the actual components in my hands so I could measure them myself and see how they all fitted together. Digi-Key and similar suppliers are wonders compared to the old days but the potential for mistakes seems to rise when one is relying solely on data sheets.

On the other hand, in this brave new world where I can design and have these boards built for AU$1.40 plus AU$3.00 for components each, I think I can live with the cost of ordering sight unseen. The savings in time and money is enormous.

So tomorrow I will try out reflow soldering with solder paste and a heat gun.

No comments: