Temporary current solution

dev/null

Hi all you creative minds!

I am planning on building electric doorlocks for my old Volgswagen, controlled by a Stamp.

I don't want to draw any (or very very little) current when the carkey is not used. I want a temporary current supply to the Stamp. The current should be switched on when I turn the key in the door, and stay on long enough for the Stamp to turn a couple of servos and then switch off.

I was thinking about using super caps, but I don't know how to avoid transients. Maybe there is a circuit I can use?

Another option is to have a 555 timer circuit constantly monitoring the key switch, and then turn a relay on to power the Stamp. But then I'd have to supply current to the 555 circuit at all times, which I don't want to.

Any ideas greatly appreciated.

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dev/null

Would an inductor in parallel with the cap work?

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Mike Green

Don't worry, be happy!

The BS2 already is designed to spend long periods of time in a low power mode drawing maybe tens of microAmps, then occasionally waking up and drawing a few milliAmps for an average current still in the tens of microAmps range. Why complicate things?

You will want some filtering in the +12V power source because automotive systems are very noisy. A series inductor (a couple of hundred uH) and parallel capacitor on the order of 1000uF (rated at 35V minimum) would work.

The Stamp would SLEEP most of the time, wake up, check the keyswitch, and, if off, go back to sleep again.

In this setup, you're using the Stamp's regulator and the amount of current available is limited. Don't try to draw much more than 20-30mA total from it (like for a couple of LEDs). The regulator has no real heatsink and it's dissipating about 8V x ??? mA.

dev/null

I'm also gonna have lots of other stuff in the car, like thermometer, accelerometer and things. Would you recommend to have another Stamp controlling them, and keep a dedicated doorlock Stamp? The thing is, when the ignition is on, I don't want my Stamp to go to sleep. But I suppose I could just check another switch for that. Hmm. This project is gonna be fun (maybe I'll put my Javelin in one of the weels to measure speed! ! Thanks Mike.

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Mike Green

"Would you recommend"? No, you'll figure it out. It all depends anyway on what you want to do. If I were doing something similar, I'd use a Propeller, but I'm biased.

dev/null

Ye, but I got three Stamps laying around, want to use them. Got the Javelin also if I need some fancy stuff. Another issue is the stepper motors I'm gonna use for the window levers. They are rated at 24 Volts. Not sure if I'll get enough torque with 12V, and it seems not easy to step 12Vdc up to 24.

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dev/null

If was planning on using an LM317 in front of the Stamp regulator, because I need 7.5 for a PSC as well.
How does this look?



(nevermind the resistor values, I haven't calculated them yet)

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Mike Green

I would put the large electrolytic on the LM317 side of the inductor and be sure to put a fuse first in line after the on/off switch.

I would keep the 100nF capacitor (now in parallel with the 1000uF).

I would increase the output capacitor value modestly if you're running servos off that voltage, maybe 10uF.

dev/null

Thanks Mike. More like this then:

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Beau Schwabe

Replace the 180uH inductor with a diode with the band facing the regulator and you will have much less headache.

As is, the 180uH and the 1000.1uF (<-1000uf + 100nF = 1000.1uF) capacitor forms a resonant circuit at about 375 Hz ... something your alternator could easily introduce into the system. Even a 50mV ripple could be substantially multiplied with the proposed LC configuration.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

dev/null

Thanks Beau. Like this:


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Don't worry. Be happy