Keith's Electronics Blog

Back in May, I got this email from my friend J who works ER:

http://www.advancedcirculatory.com/CET/resqproduct.htm

I was at work last night and we had a code come in by ambulance. The Newton fire dept. now uses these when they code somebody. I saw that it had a circuit board and some LED’s and thought that you might be interested. It has been used but it I cleaned it up. Let me know if this is something that you want to take a look at.

“Code” is code blue, no respiration and no pulse. So J saved me a gizmo from a dead person, and “cleaned it up.” This is interesting. Wonder what it could be. I read the manufacturer’s web page and it doesn’t really enlighten me as to what kind of electronics are inside it.

I visit a few days later, and the gizmo looks like this:

There’s a hole down the middle and a switch on the side. Turn on the switch and two red LEDs flash every six seconds. You plug the bottom end into a breathing mask and the top end into the CPR squeeze bag, and the lights tell you how often to squeeze for respiration for optimal (two-person) CPR timing.

A little bit of prying gets the case open, and the board pops right out.

There’s not much to the circuit — the eight-pin chip is obviously what makes it work. My first thought is that it’s going to be a 555 timer, but it’s labelled “MBAC ZC5.” Google gets me nothing useful to identify the chip, so I draw out a copy of the PCB traces to see whether I can get anything out of the pinouts.

For my first pass, I place the components on the schematic corresponding with their physical placement on the board. (I actually did this on paper; when I translated to EAGLE, I didn’t have a library component for the mystery part, so I used a connector that had the pins numbered wrong. Heed the green numbers I added, not the grey ones.)

Then I slide things around based on a logical placement.

And this is starting to look awfully familiar — it is indeed the same pinout as a 555 in the astable multivibrator configuration, which makes perfect sense. In fact, the 555 circuit should have a connection from pin 2 (TRIGGER) to pin 6 (THRESHOLD) that I haven’t drawn — and when I look closely, I can see that it’s hidden underneath the chip.

So now I have a tiny SMT 555, a couple of red SMT LEDs, and a handful of SMT passives, plus a cell that’ll probably drive this thing for a long time. Cool.

This entry was posted on Saturday, June 23rd, 2007 at 08:00 pm by Keith Neufeld and is filed under Circuits, Miscellaneous. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.