The last post here, dealt with the basic design and code for the MCU. Today is all about the circuit. Firstly the ability of the MSP430 to source current form a port is a little complex. It can sink a total of 48 ma over all pins in a port, but a maximum of 20 ma per pin in a port. Additionally this MCU is a push pull type where a voltage drop happens when you increase the current draw on a pin. Thankfully none of that matters here as the LED’s are quite stable and are current limited by a resistor. The point we can make here is that on the 2 port chips like the 2553, we have many open pins to run additional current sinks off. The code in the previous post can be expanded to incorporate all sorts of other cool stuff.
Now on to the circuit. Its actually very simple. Battery positive attached to chip pin1, and ground to pin 20. S1 is attached to pin 6 (P1.4) and S2 is attached to pin 7 (P1.5). They are both in turn connected to ground. The LED feed is pin 4 (P1.2), and feeds a resistor (in my case I used a 100 ohm 1/4 watt, and 4 X 5mm white LED’s, who are in turn connected to ground.
The MCU is capable of running on between 3.6v and 1.8v. To handle odd behavior you should be using a low dropout circuit to stop the source from dropping below 1.8v, however in this setup its not mission critical and you’ll just notice flicker in the cabin.
On the point of power, I would have loved to used a 3.7v Lion cell but those things are just too heavy. My next choice was to use a CR2032, which I ordered for this project, however I realized that in standby it would only provide me 18 days and 3.5 days with LED’s on. Not an option when those little suckers are not rechargeable.
In its place Ill use 2xAAA rechargeable cells. HA!!! I found an even better source of power. Seems that those coin cells are varied, even more so than I had thought.
Everyone knows the CR2032, and more than likely the LR44, however there is a nice little hidden gem in the CR2477. Firstly, its 24mm in radius, and 7.7 mm high, but this little guy has 1000mah capacity. NOW WE’RE TALKING!! This is perfect. It will fit nicely in the space provided. Ok, so they are not rechargeable, but they certainly will do the job.
Another point of note, and to be honest I found this out during assembly of the test board, is don’t forget the 100k resistor between RST pin and Vcc. She just wont run right if ye dont.
Next item on the list are the tilt/vibration sensors. I settled on these little guys from Assemtech.
With a little fine tuning they should do the job nicely. The only issue is that I have to mount them on their sides, but a solid wire extension per pin should work well.
So, all thats left is to wire in the LED’s, and not forgetting the current limiting resistor, that’s about the last of it. See you on the next post.