Transistor pair to switch between Voltages

[fatman57]

That's perfect.

You weren't messing me around. I was posting on a phone and some words were autocorrected to other words which didn't make any sense. In addition it makes it really hard to draw pictures so I have to describe in words. You have done a great job!

In your drawing, r1 and r2 can be about 10k. They're not especially critical.

Cheers Steve! Much obliged...

 

[fatman57]

That's perfect.

You weren't messing me around. I was posting on a phone and some words were autocorrected to other words which didn't make any sense. In addition it makes it really hard to draw pictures so I have to describe in words. You have done a great job!

In your drawing, r1 and r2 can be about 10k. They're not especially critical.

Please find final version below. Many thanks for all your help to date!!!!

 

[(*steve*)]

All good except you have drain and source sealed on your MOSFETs.

The LEDs will probably be very dim.

 

[fatman57]

All good except you have drain and source sealed on your MOSFETs.

The LEDs will probably be very dim.

Thanks Steve,

Sorry, but what do you mean by sealed? I used an online sketch service and it was what came out of the box if you are referring to the symbols.

Will the LEDs be dim because of the values of the resistors?

Much obliged

 

[(*steve*)]

AAAAGH "swapped". Bloody autocorrect.

Yeah 10k for the LEDs is a bit high. The one on the 5V input need to be about 1/3 of the value of the one indicating 12V if you want equal brightness.

 

[fatman57]

AAAAGH "swapped". Bloody autocorrect.

Yeah 10k for the LEDs is a bit high. The one on the 5V input need to be about 1/3 of the value of the one indicating 12V if you want equal brightness.

Again! I followed the same example as seen on post #19 - makes me feel a little embarrassed - not sounding too stupid what indicators should I look for to make sure I get it right (as in with regards to schematic diagrams)?

Thanks! I will use an online calculator to assist in those values.

 

[(*steve*)]

I find that drawing in the body diode on MOSFETs helps a great deal in getting them around the right way. Then, knowing that in simple switching applications the source is connected to the same supply rail as the channel type, you'll get everything the right way around.

 

[fatman57]

I find that drawing in the body diode on MOSFETs helps a great deal in getting them around the right way. Then, knowing that in simple switching applications the source is connected to the same supply rail as the channel type, you'll get everything the right way around.

OK thanks, I thought I had made it the same as post #19, does this picture correctly illustrate what you are saying:

 

[(*steve*)]

Yeah, although typically you'd draw the p channel one up the other way.

 

[fatman57]

Yeah, although typically you'd draw the p channel one up the other way.

Please see my hopefully final version below:

 

[(*steve*)]

Looks fine.

Note that they are P Channel MOSFETs, not PNP MOSFETs.

We've also spoken about the resistors for the LEDs.

Neither of the issues will stop it working

 

[fatman57]

Looks fine.

Note that they are P Channel MOSFETs, not PNP MOSFETs.

We've also spoken about the resistors for the LEDs.

Neither of the issues will stop it working

Yes thanks Steve I just wanted to check it was the right way round. I have a circuit board prototype almost ready will post a picture if anyone is interested. Comments welcome.