Automatic turn off

[AMay]

I have made a nifty timer for my wife's new knee rehab exercises. She often forgets to turn it off, so I have programmed it to cry (beep slowly) if it has no activity for 1 min.

But I think it would be better to rig an automatic turn off. I have tried with a transistor wired to both 5v plus and neg. so that a pin output to the base shoud keep it on for a defined term after a button starts the circuit. All attempts fail to turn it on or else to turn it off.

The unit is battery operated, so I don't want to use a relay. I have the circuit down to about 6ma, and don't want to add 30ma or so.

Any one have any ideas?

Thanks

[Melanie]

http://www.picbasic.co.uk/forum/showthread.php?t=57

[AMay]

Melanie:

Thanks for the help.

I want to eliminate a need for an "on-off" switch, only a push button on, and auto off after a period of inactivity. I will be using 9 volt batteries, therefore off needs to very close to 100% off. 9v batteries provide a rather wide range of voltages, depending on the type, alkaline, ni-cad., or nimh.

Starting with the information you provided, I developed the attached schematic, which works fine. On the way, I thought I had it worked out with an 8v battery, but the off current went way up when I tried 9 volts.

I figure the "off" current (according to Fluke) is between .1 and 1.4 uA, depending on the battery, which seems acceptable. Most of the "leak" is through the bias resistor.

AMay

[Ingvar]

If you want to save power you should get rid off the 78L05 and use a better one.

You could also run your pic straight from 3 AA cells in series(4.5V), the pic consumes only a few uA at 32kHz. The AA cells should last a few years. 32kHz should be enough for beeping and counting time.

Your project would then be reduced to.....

1 Pic
2 capacitors(for the crystal)
1 32kHz crystal
3 AA cells
1 button
1 beeper

..... if you accept a bigger timeerror you could get rid off the crystal and use the builtin oscillator of the pic. The 16F627/8 has a 37kHz(ish) oscillator, the more modern pics with nanowatt should be even better. Using one of theese would reduce the project even further....

1 Pic
3 AA cells
1 button
1 beeper

... it won't get much simpler than that.

/Ingvar

[AMay]

Thanks, Ingvar. That is just the kind of help I need.

I gave some effort to using AA cells, and decided that it was not practical. This device will be passed on to others who may not have a screwdriver, and would be hard pressed to replace the batteries. On the other hand, I can buy an enclosure with a separate hatch for a 9v battery that will be easy for anyone to change.

I am using the internal oscillator in the chip, as the time is plenty close enough.

You are right about the 78L05 using much of the power. As best I can figure, it uses about 3 ma of the 4.5 ma current. There is also a flashing led (so the user knows something is happening) and a buzzer, but they are on for such a short time their current does not matter much. But I don't know about a better voltage regulator. What do you suggest.

Thanks for your help.

AMay

[Luciano]

Hi,

For my last project I have used the LP2950. (TO-92 Plastic Package).

* * *

National Semiconductor Corporation

The LP2950 and LP2951 are micropower voltage regulators
with very low quiescent current (75 µA typ.) and very low
dropout voltage (typ. 40 mV at light loads and 380 mV at
100 mA). They are ideally suited for use in battery-powered
systems. Furthermore, the quiescent current of the LP2950/
LP2951 increases only slightly in dropout, prolonging battery
life.

http://www.national.com/pf/LP/LP2950.html
http://cache.national.com/ds/LP/LP2950.pdf

* * *

For the LED, remember that a short flash will be
enough because of the "persistence of vision" of
the retina.

* * *

Luciano