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Inverter question

throbscottle

Well-Known Member
I came up with a simple idea for indicator lights for breadboarding purposes - connect a bi-directional, 2 colour LED (or two LED's in anti-parallel) and it's limiting resistor from the output to the input of an inverter. The assumption is that the DUT can sink or source the LED current, so one or the other lights depending on the polarity of the input, and the inverter provides the return path.

So the only thing I'm concerned about with this is, what happens when there is no input? There's no adequate current path to light either LED, but would the circuit oscillate? If so, what will prevent it?

DUT connects to PL1.

led-test-1.png
 

Diver300

Well-Known Member
Most Helpful Member
It would probably oscillate with no input. The LEDs will have capacitance, so it could lead to high frequency oscillation.

With a 3 V supply, the input threshold voltage would be around 1.5 V. The input could sit a bit below that, so the output is high, without any current flowing in the LED, so it might be stable in that condition.

Also, with no input connection, the LEDs may generate voltage from ambient light, so the behaviour may depend on the ambient lighting.

I would suggest that you put another inverter between the DUT and PL1, and then you can put a high value resistor between the input and ground to stop it oscillation if the DUT is open.
 

throbscottle

Well-Known Member
That was my first thought too. But then I lose the open input = no lights condition. Or am I missing something?

I was going to say ambient light voltages would cancel out, but then the LEDs are different colours so they won't completely... Never even occurred to me, that one!
 

Diver300

Well-Known Member
Most Helpful Member
If you want open input = no lights, I suggest this circuit:-

1617830106929.png

You can adjust the number of diodes depending on the supply voltage so that the total voltage of the LEDs and the diodes add up to more than the supply voltage, and with a floating input, no current will flow.
 

throbscottle

Well-Known Member
Ah, the old classic, improved! But I need to change it a bit so I can use my funky bi-directional, bi-colour LED's that I haven't got yet! So putting that in series with the input instead of each end of the PD will do the job. It's only ever expected to run on 5v so I can put 3v zeners in for the diodes.

I also realised that with a normal 3 pin 2 colour LED, the inverter circuit will work with 1 anode to it's input and the other to it's output, common to 0v, high value pullup on the input.

Since I'm making 6 or 8 of these on a small perf-board I'll just have to see what fits best.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Basically you're wanting a logic probe - have you tried googling for that?.

Here's one of many examples.

 

throbscottle

Well-Known Member
Oh I've googled for logic probes lots because I've been meaning to build an actual logic probe.
No these are just a group of status indicator lights for breadboard purposes, but I want something where I don't have to worry about whether they are common anode or common cathode - just plug it in and it works, instead of plugging in a bunch of separate LEDs.
The no lights = no input was purely incidental but I decided it was worth trying to keep.
 

Visitor

Well-Known Member
Would two comparators do the trick? One active if input > x, one active if input < y, and no output if between those bounds.
 

throbscottle

Well-Known Member

Attachments

throbscottle

Well-Known Member
Interesting, but overkill and expensive (especially needing 8-ish) for a glorified LED indicator!
 

throbscottle

Well-Known Member
Right, so a variant of the inverter based version which would actually give me a tri-state indication. This is the most complex I'm prepared to go with it. Also these LEDs seem to be a bit cheaper than the 2 pin dual ones. (I already have more 74HC04 than I will ever need)
led-test-1.png
 

throbscottle

Well-Known Member
i've just realised, that's not going to do what I want, is it? Damn.
 

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