FET circuit to switch power to IC?

Just an alternative method to consider:

It's a " hermetically sealed tilt and motion detector ideal for use with industrial control equipment, household goods and security devices. the cm1300 (dp50e) is designed for low level switching"

There are basically two types, one that switches with almost any movement and one that switches in a particular orientation.

Combine that, as needed, with a small C and R to give the time constant you want.

Or don't use a switch at all:

Use a low power, or ultra low power 555, and you can leave the thing powered up, just inhibit it from oscillation eg:

Several minutes is a lot. I'd consider a CD4060 (comes in TSSOP if it has to be small) and a FET. If you don't need a lot of current there is the BSS123 for low side and BSS84 for high side switching. If the space is really tight there are SC-75 packaged FETs like the 2SK3019 that I have used in similar applications.

1. No, unless I'm mistaken about your proposed schematic. The FET gate is connected to it's external connection, so it's voltage is the same as the little wire coming out of the package.
2. If you mean a diode to keep the cap from powering the LED -- yes, you'll need that.

You will find, if you try this, that the FET will have a very unpredictable and soft turn-off. You won't have much leakage from the FET gate itself; most of the leakage will either be inherent to the cap or will be parasitic leakage from wet scunge on the board -- so your 'on' time will vary wildly with temperature and humidity.

You'd do much better to use a resistor and cap in parallel -- the resistor current would then overwhelm other effects. You'd do better yet to use a 556 with one side connected as a monostable multivibrator to keep the power on, and the other side doing whatever you want to do. This would give you a slightly more predictable discharge, and a nice 'snap' action to the turn-off.

Even better than all of that, consider using an 8-pin (or six; they're getting small these days) microprocessor. You'll be able to control that non-critical turn-on time to absurdly fine degrees, and for every "oh dang, I forgot to consider that" issue you run into you'll need a bit more code instead of a bit more circuitry.

Well, I might just leave the 7555 powered up perpetually (see other reply in this thread). But if I don't, the multivibrator is a good choice -- schedule power-off down the microsecond (c: Thanks.

Ah, but I thought all suggestions in this NG of using a u-controller should be accompanied by code and circuit examples (c;

Seriously, I have not used PICs or such, and have yet to make that quantum (in my universe) leap. So an electrickery solution is the sole consideration, for now.

Thanks,

It's a great idea, but the tool lives almost perpetually in the boot of my car. That means it'll forever be getting "tilted", probably.

Yeah, I was ready to do that (calculated that using the power spec for the CMOS part it would run for over a year on one battery charge). But I thought better of leaving something always powered on. No good reason, I guess, to not do it.

If I go the always-on route, why disable oscillation? Oscillation requires more power? The spec sheet says typical is 100 uA; I presume this is running current? And in this configuration the MOSFET it triggers (PWM) would also be running, although not passing current; a direction switch connects the FET to the motor when it comes time to actually use the tool. Does the FET use any current when not actually sourcing or sinking current?

(If this sounds familiar, it's my cordless drill that I asked assistance getting a PWM circuit for it last week.)

Thanks,

But that comes with penalties: 7.2V requires that there be a regulator to supply the uC. Last time I used one of those new-fangled micro-power versions we were greeted with a few explosions, leaving dead regulators with craters in them. The mfg was unable to tell us why and not willing to throw it all onto SPICE (and that raised my neck hairs). Anyhow, then I chucked all that and went CD4060. Works from 5-12V unregulated, no issues whatsoever. As a nice side effect it was also cheaper. A lot cheaper.

True. But I have seen that leading to a wee bit of slacking off in terms of design discipline. When it's pure hardware one has to be much more careful to avoid egg in the face when the boards come back from fab.

Use a position tilt switch, rather than a motion detector. If the thing goes vertical during normal driving, you have bigger problems....

Set the time constant so that physically pointing the thing vertical every few minutes is all that is needed to keep it powered up.

During normal operation it is charging a capacitor and then discharging it - both through resistors - a waste of power best avoided.

The spec sheet says typical is 100 uA; I presume this is running

Depends on the circuit but better to disable switching to save power.

Rather guessed that..

-- snip --

Agreed.

But.

I've found that if you're on a tight schedule for a low-production volume device you can get by with a careful analysis of what's really necessary from hardware, followed by making sure you put in more insies and outsies than you think you'll ever need and at least 5x the micro you think you'll need.

At that point "we'll fix it in software later" is actually achievable (unless you underestimate, in which case you have that egg on your face).

"The customer is always right" rarely applies when the customer is writing a specification. Yet the designer is expected to provide what the customer needs, not what the customer says he wants. Requirements change, even after the unit has entered service and has been in use for several years. Being able to field-program the entire system, with a down-time of minutes rather than days, is worth quite a lot. With the recent hike in energy costs, a whole lot of systems are now being re-programmed for economy, rather than performance. Even F1 cars have an eocnomy button, these days..

True. And then there is feature creep. "The boss just came up with this idea that if we made a tiny change we could also use it for...". So yeah, there are clients for whom I keep things rather flexible. Because I just know that it'll be coming. When they call in for the umpteenth change and apologize I can tell them that it's perfectly ok.

With many of my designs the units are just chucked and replaced. Of course, that isn't necessarily good from an environmental point of view but technicians would often refuse to touch things that have been used in a catheter lab for a long time. It can be quite gross.

Wow. I definitely didn't know that.

A friend in Botswana told me that in some areas this could be a quite normal modus of vehicle operation. T'is why they all have winches with them.

That reminds me of a stunning stock car race that I watched a couple of decades ago. Richard Petty went into economy mode about 50 or 100 miles short of the finish. He suckered the other leaders into not going in for pit stops, then drifted across the finish line about the same time he ran out of gas. They ran out earlier.

There was much wailing and gnashing of teeth afterward about what a tricky bastard he was. It made for good theater -- but he still won.

LOL! Point taken...

So, the circuit as described previously (FET, cap, diode), now fed 7.2v by the tilt switch?

Thanks,

Not quite. I can't remember the whole thread but assume that you are using the 555 as a variable duty-cycle oscillator, driving the FET? If so, don't connect the top of the timing chain directly to the supply, but connect it via the tilt switch and a small R, and put a C between the top of the chain and ground. The small R is there to limit the C charging current to the tilt switch Imax.

When the tilt switch operates, C is charged very quickly via the small R and will slowly discharge via the timing chain, keeping the oscillator running until it is discharged (the tool will have to be given a quick flip to recharge it and keep the tool running). Set the C to a value that will give a couple of minutes running with the values of resistors in the timing chain.

In the horizontal position, the tilt switch won't close, the oscillator won't run and the FET will stay off. The CMOS 555 and FET will draw a (negligable) amount of leakage current.

Put the tool vertical, the tilt switch operates, the oscillator runs and will continue running for a couple of minutes and then stop - unless the tool is momentarily turned to be vertical every couple of minutes.

That's a creative solution, but why can't the 555 be powered from the switch that would normally turn on the drive motor?

Jeff

Trying to minimize modification/hacking of the tool.

The switch is a 2-pole (+ and - battery conductors), 3-position design (fwd-off-rev). The MOSFET output from the 555 circuit is fed to the switch which connects it to the motor for driving in both directions. How would you suggest the 555 circuit be powered by this in both fwd and rev modes? How to drive the motor in both modes? More switches? More FETS?

I don't want to hack the existing assemblies any more than I have to. The 555 and components can easily be placed between the battery and the direction switch with minimal intrusion into the tool's form factor. I just need to find an elegant means to disable it when it's not being used. Sue suggested such a creative solution, as you noted.

Basically I think the proposed design integrates variable speed nicely and minimizes intrusion into the tool's existing form without having to redesign it from the ground up.

Now if I could just find that *$_)#! pot with push-switch here in USA. Bourns makes exactly what I need, but none of their distributors stocks it, and Bourns is asking minimum 10,000 units for an order. Anyone interested in

9,9999 pots? Guess I'll have to order that one from Maplin in UK. Are there issues with customs when mailorder electronic components hit our borders? Any recommendations when Yanks order from Maplin?

Thanks,

Sorry, it's at Farnell:

10A switch in such a small package. Gotta love those Brits!

Because the switch that normally turns on the drive motor is usually a two-pole, centre-off - wired one way to swap the supply connections in order to reverse the motor.

Whilst there are many ways of dealing with that, the ones I can think of are fairly complicated, compared to a simple, cheap, reliable, extremely small tilt switch. Which has the added advantage of automatically switching almost everything off whenever the tool is laid flat for a few minutes.

Very cool design. I'm still entranced. (c: And I'm still waiting to see your house integration controller...