There is a need to monitor the 5V supply of a SIMMStick system. The standard 5V regulator can be asked to operate outside its ratings, either from excessive current draw on a heavily populated SIMMStick bus, or voltage dropout if the unregulated supply dips below the datasheet-specified value. Higher output voltages can even result from a damaged regulator. The effects of a "misbehaving" 5V supply cannot be predicted, but are in all cases bad. I have, in the past, spent hours debugging what I thought was a software error, perhaps in timing or the like, that turned out to be a case of "Vcc = 4.6V". I do not want to play that game anymore.
The i9t300 was born. National Semiconductor makes the excellent LM3914 dot/bar display driver, and once published an app note using it for exactly what I had in mind. I wanted a simple visual indicator of the state of Vcc; in this case, we have 10 LEDs that indicate voltage from 4.5V to 5.5V in 100mV steps. Red LEDs at the extremes indicate great badness, intermediate amber for slight badness (marginal cases), and green center LEDs show minimal badness (a safer operating range). The board was quick to build, and only needed a few components. I love finishing a project.
The app note in question is National's LB-48 [pdf]; rather old, but fine for our purposes. The first circuit shown was the one used, and it was built exactly to spec, and easily calibrated with the help of a 4.5-digit voltmeter. The LM3914 IC is available from their samples program, and has a very comprehensive datasheet with many interesting applications described. It's definately worth a look. Do try not to get some in the rediculous PLCC package, though (anyone need some of those?).
What is this SIMMStick thing? It is whatever you make of it - it is a modular, extendable electronics prototyping system. The original website is a little dated, and more information is available from Dontronics (the best SIMMStick retailer).
Update: I built this someting in early 2003, and have since moved away from SIMMSticks, mainly due to their non-standard thickness (the SIMM sockets they use need 0.050 PCB stock). This is still a useful thing, though, and I will likely build another in the form factor of my personal modular system (more info coming soon).
Do you like this project? I could, if so desired, make and sell PCBs and potentially mini-kits. Let me know if you would be interested in this, at firstname.lastname@example.org.
|Last updated July 30, 2004||
Original content copyright © Christian Weagle unless otherwise indicated.