Zener kinda turns on

or turns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.

F until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be of f at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.

That 12V zener is not doing very much...

But look at an I-V curve for the 5.6V zener, I'd guess there's still some current flow at 5.0V. If the FET gate is at 1 V then you have ~1uA of current flowing through the zener.

George H.

On a sunny day (Wed, 17 Apr 2013 08:42:50 -0700 (PDT)) it happened panfilero wrote in :

Differential?

At link is below

turns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.

until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.

maybe, but why is D1 turning on at all? Is it leakage current that's causing this circuit not to work at 5V? D3 the 12V LED is definitely off at a 5V supply (as it should be) and the circuit works at 12V... but not at 5V...

You need a lower R2 or a different circuit. Low voltage zeners don't have super sharp knees.

turns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.

until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.

maybe, but why is D1 turning on at all? Is it leakage current that's causing this circuit not to work at 5V? D3 the 12V LED is definitely off at a 5V supply (as it should be) and the circuit works at 12V... but not at 5V...

Err...it takes about 4V to turn on a "typical" LED, so use of a 5V (or even 3V) zener will guarantee no light from the LED.

"Robert Baer"

The dopey OP has failed to allow for leakage current in the 5.6 V zener and is trolling us as usual.

... Phil

Call it leakage or what you will -- zeners do not have sharp corners in their I-V characteristic.

_Always_ study the data sheet of the part you're going to use. In the case of two-terminal parts, _always_ know what the I-V characteristic is. It's _never_ as simple as the basic books would have you believe.

turns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.

until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.

maybe, but why is D1 turning on at all? Is it leakage current that's causing this circuit not to work at 5V? D3 the 12V LED is definitely off at a 5V supply (as it should be) and the circuit works at 12V... but not at 5V...

A wee bit overkill...

but Perfectionist 'R' Us >:-} ...Jim Thompson

turns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.

until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.

maybe, but why is D1 turning on at all? Is it leakage current that's causing this circuit not to work at 5V? D3 the 12V LED is definitely off at a 5V supply (as it should be) and the circuit works at 12V... but not at 5V...

That is, as they say, "component rich."

This should work:

It could have a bandgap in the "13K" leg, but it really doesn't need it.

turns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.

until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.

maybe, but why is D1 turning on at all? Is it leakage current that's causing this circuit not to work at 5V? D3 the 12V LED is definitely off at a 5V supply (as it should be) and the circuit works at 12V... but not at 5V...

Here' in the trailer park, we'd have just painted the 5V power plug red and the 12V one green.

Over in town, they'd have used a dual-color LED, put a zener in series with the red one and called it green and orange.

Or use rectangular LEDS and call it a bar graph. Probably works better for the color-blind.

I've hosted a bunch of design reviews. Seen a lot of "solutions" that were WAY out of proportion to the root problem being addressed. Often comes under the heading "dimininishing returns". ;-)

KISS 'R' US...

turns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.

until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.

maybe, but why is D1 turning on at all? Is it leakage current that's causing this circuit not to work at 5V? D3 the 12V LED is definitely off at a 5V supply (as it should be) and the circuit works at 12V... but not at 5V...

Well! I _did_ say that it was a wee bit overkill, but it _does_ perform _exactly_ as requested >:-} ...Jim Thompson

Le Wed, 17 Apr 2013 08:42:50 -0700, panfilero a écrit:

The idea is good.

Change your zener for a 7.5V one (which being an avalanche one will be much 'stiffer'), lose R3, and as you said make R2 10K to be safer and you're done.

You don't need D2.

Hey, I just invented the LED differential pair!

nt

at

OK I'm most likely just slow this morning, but how does that turn off the one LED when the transistor turns on? And doesn't it turn on the first LED at ~ 2volts or so. How about if you put a second LED in the leg without the transistor? Then it would turn on at ~4 volts and turn off with current through the transistor.

I like Jan's circuit.

George H.

.highlandtechnology.com  jlarkin at highlandtechnology dot com

Or simply a common diode.

You also get varying currents between 5V and 12V due to the resistor setting the current.

Incwww.highlandtechnology.com  jlarkin at highlandtechnology dot com

...Jim Thompson

That works, but it needs seven parts to drive the LEDs, which is a lot better than Jim's 19. Here are a few more with 4 or 5:

I really like "A" for its elegant symmetry.

This takes 6 parts, but it's power efficient, as is Jan's circuit, with a little tweaking.

This is fun, like crossword puzzles. Doesn't anybody want to play?

Different color LEDs have different forward voltages. Use, say, blue on the right and red on the left.

And doesn't it turn on the

Sure.

Or pick the LEDs properly.

It's supposed to show if the supply is +5 or +12.

Not a particularly good circuit...

A & B waste current, conducting 2x LED current at 12V. C is wishful, let's see some numbers.

little

That works, dunno why you need a FET though.