'Soft' On/Off Power Switch
Hi Guys,
These links show examples of on/off circuitry commonly used in calculators,
mobile phones, and other battery-operated devices.
The p-channel MOSFET serves as an electronic power switch.
They look ideal for pic based circuits, so I gave the first example
a shot.
Description:
http://www.reed-electronics.com/ednm...=0&rme=0&cfd=1
Figure 1:
http://www.reed-electronics.com/ednm...es/54571f1.pdf
Figure 2:
http://www.reed-electronics.com/ednm...es/54571f2.pdf
The 5K1 resistor in series with the switch S1 in Figure1
only appears to short the battery.
I have constructed the circuit on a small PCB, and had no luck.
I tried connecting another 5K1 resistor between Q1 gate, and
the switch end of the other 5K1 resistor because I thought it
might have been left out.
I found that the P Channel MOSFET was powering the 7805 reg
no matter what I did to the gate pin. It even provides power when the gate pin is disconnected.
Any suggestions?... I don't have any experience with MOSFETs :(
Art.
1 Attachment(s)
Try an LM2941 Linear Regulator
Hi,
Try using an LM2941 linear regulator. They need to be configured, but I've attached a VB program that does all of the calculating for you. You should still take a look at the datasheet though.
They are great little linear regualtors and can also be used as a 1 amp switch.
They have a control pin that must be tied low for the unit to work.
I designed a circuit that tapped a signal off of the RS232 TX line from my PC. The RS232 line drove a transistor (2N3904) which charged a capacitor that, when it reached a certain voltage, drove another transistor that brought the regulator's control pin low. Then, when the regulator turned on, the PIC would start up and would drive one of it's pins high. This pin would take over from the RS232 signal and would hold the PIC on.
You could then turn off the PIC and the entire circuit with a simple command to make that same pin low.
Super easy!
Good luck.
See Auto-off under schematics
My final circuit, sort of like Melanie's, works perfectly as far as I can tell. I have no quick button problem, perhaps because I put a decoupling cap., .1uf, in the circuit.
AMay