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MOSFET transistor switch/latch/power circuit

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discostews

New Member
Hey,

I am looking for some help in the design of a circuit that i require for a project that i am working on at the moment.

The Basic idea is to implement a device that turns on when a strap is closed (conducting strap acting like a switch). When the strap closes, a PIC microcontroller will turn on, and the system will operate as normal. On the opening of the strap, the power to the PIC must remain on, but the PIC must know that the strap has been opened.

At this point, the PIC will use a radio transmitter to alert another system that the strap is open, wait for acknowledgement and then power itself off.

My current idea uses two MOSFET transistors to achieve this, and is shown in the diagram below. Closing the strap turns on one transistor (transistor 1) to power the PIC, which turns on another transistor (Transistor 2) to latch power through a seperate loop by turning an output port high.

An interrupt pin is used to sense the strap opening. Pull down resistors are used to keep the pins from floating. Power comes from the +ve of a battery and is to be attached to a "+5V" rail, that will power everything else.

However on building a test run of this circuit, i am gettin big voltage drops across the transistors which i cannot explain.

Is my circuit wrong? In my mind it seems great! Or do i need to redesign my whole method! I'm using MOSFETs, (ZVN3306A), and a 5V supply to test.

Any help or suggestions will be appreciated!

thanks for reading this far!!

DiscoStews
 

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eblc1388

Active Member
discostews said:
However on building a test run of this circuit, i am gettin big voltage drops across the transistors which i cannot explain.

Is my circuit wrong? In my mind it seems great! Or do i need to redesign my whole method! I'm using MOSFETs, (ZVN3306A), and a 5V supply to test.

Your idea is good but you have used the wrong type of MOSFET for the switching control. If you want to switch +5V to other part of the circuit, from a +5V source, then you cannot use a N-Ch MOSFET because it will need +8V~9V to switch properly.

The solution is simple. You must use PNP transistor or P-Ch MOSFET instead to switch the +5V. If you use a MOSFET, then by connecting the "source" pin to +5V and "drain" to your load. If you pull down the MOSFET gate to 0V by the PIC, the P-Ch MOSFET will turn ON.

You also need to reverse the software logic under this situation.
 

discostews

New Member
Cool. Thank you for the help. I think i have come up with a solution that will work. Preliminary test seem to be ok!
 

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