Author Topic: Circuit for MCU IO or VCC switch (help!)  (Read 1185 times)

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Offline SarcareanTopic starter

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Circuit for MCU IO or VCC switch (help!)
« on: February 20, 2019, 06:25:59 am »
I have a MCU I/O that connects directly to the input pin of another IC. I want to insert a circuit/IC that will use another 

to an input pin of another IC. I want to use a second MCU IO to control a circuit / IC that will switch between either the first I/O or a +12V power source.

So my question is, what is the best way to do this? Off the top of my head, I am thinking of putting a standard diode on the MCU pin and use a couple of transistors or is there a single IC that will accomplish this for me?

(Note: the MCU pin is not 12V tolerant).

Thanks in advance for any help and/or schematics from working projects that do this :)
 

Offline tszaboo

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Re: Circuit for MCU IO or VCC switch (help!)
« Reply #1 on: February 20, 2019, 09:20:38 am »
Sorry, but what?
 

Offline Moriambar

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Re: Circuit for MCU IO or VCC switch (help!)
« Reply #2 on: February 20, 2019, 09:50:36 am »
I have a MCU I/O that connects directly to the input pin of another IC. I want to insert a circuit/IC that will use another 

to an input pin of another IC. I want to use a second MCU IO to control a circuit / IC that will switch between either the first I/O or a +12V power source.

So my question is, what is the best way to do this? Off the top of my head, I am thinking of putting a standard diode on the MCU pin and use a couple of transistors or is there a single IC that will accomplish this for me?

(Note: the MCU pin is not 12V tolerant).

Thanks in advance for any help and/or schematics from working projects that do this :)
you mean something like a SPDT circuit?
I found this:
https://datasheets.maximintegrated.com/en/ds/MAX381-MAX385.pdf

which perhaps can meet your standards. Otherwise check for multiplexers or that kind of circuits on digikey, etc... and you will have parametrics to narrow down your search. Please do mind that in general the V of the commuted input is different from V+
« Last Edit: February 20, 2019, 09:52:58 am by Moriambar »
 

Offline SarcareanTopic starter

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Re: Circuit for MCU IO or VCC switch (help!)
« Reply #3 on: February 20, 2019, 11:01:55 pm »
That won't work because the output has to be either a signal or load current.

What I am thinking is a general purpose diode on the MCU pin and a MOSFET between diode and input pin. The transistor when on will flow current into the line and when off, the signal will pass directly through the diode to the input.
 

Offline SarcareanTopic starter

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Re: Circuit for MCU IO or VCC switch (help!)
« Reply #4 on: August 08, 2019, 07:42:31 am »
I thought I would update this post and share the verified solution that I came up with.

The 74LS595D output pin (5V CMOS) connects to a schottky diode (30V Vrrm 0.37 Vf) and then to a 22K ohm resistor and also the main output pin A9. The other end of the resistor goes to ground. Between the diode and resistor, a connection is made to the output (L1) of a solid state relay (CPC1017N). The input (L2) of the relay is connected to the 12V rail, the CTR_N connects to ground and CTR_P connects to a MCU GPIO (configured as output, default logic low).

When the MCU is in the default state, the GPIO pin is low and the relay is OFF and the output from 74LS595D can drive the A9 pin. When the GPIO pin is HIGH, the relay is active and will allow the 12V to flow to the A9 pin while the diode will protect the 74LS595D from reverse current.

805908-0
 


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