**LOW** Power applications

In article , Giorgis wrote: [....]

I have repaired equipment that went through the crash of an aircraft. I have also fixed things that have jumped out of the back of a truck as it went over bumps. From this I learned few things that you may find useful.

(1) There is no way you can make a box or chassis stiff enough that it won't flex when hit like this. You are far better off making one very tough than very stiff and allowing it to flex a little.

(2) No connection that is made by a spring's force will remain in contact. The time they open for is made briefer by increasing the springs pressure but you should include a large enough ceramic capacitor across the Vcc to hold the power up when it happens.

(3) Large value surface mount capacitors are easy to crack. Don't use

(4) Those small surface mount 10 turn pots come clean off the PCB if the shock has any high frequency components.

(5) Small silicone-rubber O-rings make great little shock mounts. They don't change much with temperature and it is quite easy to get a 0.010" travel out of them. This can be enough to kill the 10KHz component in the shock.

One mans noise is another mans data. He may only need to know about how hard the head long crash was. It such a case, just one number is needed.

I suspect that recording it would take much less than a milliamp. Digitizing it would consume more power than storing. That would be the place to try to minimize the power. If averages rather than snap shots are needed, perhaps a home made delta-mod ADC would be better than any other way:

The op-amp can be very slow since it only needs to work at a few 100Hz.

The "Bit in" input of the CMOS logic could be only active when the bit is being latched so that a partly biased CMOS circuit would not draw a lot of power. Since the op-amp integrates, the O/1 boundary doesn't matter so no comparitor is needed here.

I have tried charge pump circuits from cable signals (DTR and RX). They work for low speeds and duty cycles, but fails under certain conditions. The Max232 is more reliable, although for the price of more PCB spaces.

My Vcc goes down to 1.8V, when the BOD kick-in and shut it down.

Two points:

The Max232 you can't get is by its nature not reliable at all.

These were circuits you designed. Did you consider the posibility that you did something wrong.

I assume the micro isn't expected to do RS-232 when shut down. The 1.8V level is a bit low to use directly but there are still ways to deal with it.

Why? I have no problem getting the Generic Max232. I got 5 TI and 10 Sipex on hand for testing. I don't see any problem getting enough Sipex, TI or ST Max232 for productions.

Yes, it is possible. I just don't want to waste more time when there is a easy alternative. The charge pumps have 3 transistors to store energy from incoming signals. It is fine for prototypes (work Ok at

It will try to transmit data on the way down (i.e. dropping Vcc) as much as possible. And here could be a problem, the micro is transmitting most of the time. There is not enough receving signal transitions to trigger the charge pumps.