FET circuit to switch power to IC?

A 555 circuit running from battery power needs to be turned on and off. A switch for this purpose cannot be added due to lack of space. The "on" signal
will be from an existing momentary pb switch (this switch controls an LED) which can also provide battery voltage (7.2v).
I'm thinking that a cap can be connected to the gate of a FET and charged when the pb switch is pressed. This FET will power the 555. sThe cap's voltage will leak off at a constant rate based on the FET's gate characteristics.
The "on" time isn't critical; something between several minutes to 1 day would be OK. I just need to preserve the battery between charges.
Questions:
1. Will the FET's gate remain high (once high, always high?) even after the cap has bled down? 2. Probably need a reverse-connected diode between the pb switch and the cap/gate (to avoid leakage via the LED)? What kind of diode should I use?
What am I overlooking?
Thanks,-- DaveC snipped-for-privacy@bogusdomain.net This is an invalid return address Please reply in the news group
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
DaveC wrote:

Just an alternative method to consider:
http://www.maplin.co.uk/Module.aspx?ModuleNo 116&criteria=mercury&doym3
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:
http://www.maplin.co.uk/Module.aspx?TabID=1&ModuleNo '07&doym3
--
Sue


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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,
--
DaveC
snipped-for-privacy@bogusdomain.net
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
DaveC wrote:

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..
--
Sue

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Palindrome wrote:

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.
--
Regards, Joerg

http://www.analogconsultants.com
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

LOL! Point taken...

So, the circuit as described previously (FET, cap, diode), now fed 7.2v by the tilt switch?
Thanks,
--
DaveC
snipped-for-privacy@bogusdomain.net
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
DaveC wrote:

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.
--
Sue










Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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,
--
DaveC
snipped-for-privacy@bogusdomain.net
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Sorry, it's at Farnell: <http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU 07056>
10A switch in such a small package. Gotta love those Brits!
--
DaveC
snipped-for-privacy@bogusdomain.net
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
DaveC wrote:

I'm slighlty puzzled why you want to use a pot at all?
Why not use a qtc pill and have finger-tip speed control?
http://www.maplin.co.uk/Module.aspx?ModuleNoD202&criteria=pill&doy !m3
--
Sue

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Because some of us are not quite as quick as you to know that some very cool and tantalizing things like QTCs exist! (c:

(How can Maplin show such a tempting item and not provide data sheet or additional specs?) Data sheet: <http://www.peratech.co.uk/pdfs/p001v004.pdf shows that resistance is logarithmic. How to map this to achieve linear motor response?

Why not, indeed... except that the QTC's min resistance (or the MOSFET's if I use the QTC to control that) still won't allow full motor speed which is necessary for full range of useful torque and in practical use is required for those really tough screws.
Most tool mfrs utilize a full-speed switch. Pull the trigger on any variable-speed tool (cordless or otherwise) and you'll see the speed increase linearly until about 90 percent speed, then a slight jump to light speed. They all short past the semiconductor for full speed.
Now if I could figure out how to mount the QTC atop a mechanical momentary PB with sufficient spring resistance... at max QTC pressure the switch would close. Voila!
A QTC is quite tempting -- at 10 amps, could be used solo, w/0 555 or such...
--
DaveC
snipped-for-privacy@bogusdomain.net
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Nice idea! But...
Resistance range: 10^-12 to 1 ohms. Not sure that this would provide much linear speed control... Perhaps a calibration circuit is needed?
--
DaveC
snipped-for-privacy@bogusdomain.net
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

basically yor have this, centre-off reversing switch amd motor .--- | ---o-------. 7.2>-|--- : | | ---o---(M)-' +--- | 0V>-' do this: .--- | ---o-------+->|--. 7.2>-|--- : | | | ---o-+-(M)-' | +--- | | | `------>|---+ | | `--|. .--+--. || | | .--|`-------------| 555 | | | ETC | | | | | `--+--' | | 0V>-+--------------------+
Bye. Jasen
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It's another very creative way to skin the cat!
Thanks!
--
DaveC
snipped-for-privacy@bogusdomain.net
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
cr500r wrote:

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.
--
Sue



Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Very cool design. I'm still entranced. (c: And I'm still waiting to see your house integration controller...
--
DaveC
snipped-for-privacy@bogusdomain.net
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
...

The way I was thinking of fixing that was two diodes hooked up to the switch, used as "steering diodes" so you get a positive voltage to one node no matter what direction is pressed.
-Jeff
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
cr500r wrote:

Where would you put the speed controller?
If at the battery, your diodes would be returning a pulse width modulated positive voltage back which would have an average voltage approaching zero at low speeds.
And, of course, the signal wouldn't be present at all if the signal was being used to actually switch on the speed controller...so you would need a seperate "start" signal.
If after the switch, then the diodes wouldn't be needed at all as the switch would act as the on/off control for the power controller. However, the controller would need to cope with reversed polarity..
--
Sue






Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

...
Right, put the power controller after the switch, and the diodes could "cope with reversed polarity" I think it would work, but I haven't seen a schematic for the tool to be sure. -Jeff
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.