Indicator Led General electronics

[retepsnikrep]

This is a non picaxe query just looking for ideas.

I have a 33R 25w power resistor across which a voltage of between 3-25v can develop. Current/power as per V/R but within 25W. In my application the voltage across the resistor starts off at say 25V faling slowly over several hours until is goes below 5v.

I want to monitor this voltage and use it change a bi-colour led from say red to green when the voltage across resistor drops from say 25v down to 5v. Change over point at say 5v-10v.

I want the voltage across the resistor to also power the indicator led which will be a low power 2ma or so device so it's current drain is not an issue.

Ideas? Zener diodes and transistor switch? Mosfet switch and PD?

I'm looking for the simplest/cheapest practical solution.

 

[Technical]

Why bicolour?

Single LEd on/off tells you the same visually as red/green would and is much simpler to do!

 

[Dippy]

Pretty hot then.

Can you tell us how it is wired? Is one side ground? If so that would make life easier.
Anything else in series with this resistor?
Any nasty reactives pulses kicking about?
You could probably use good old fashioned electronics.
But without a teeny weeny bit more info it makes suggestions a little more long-winded.

I guess the advantage of bicolour is that when below threshold you still know it's on - unlike an on/off LED - and it looks prettier

So, give us the ingredients and then others can make a meal of it.

And we are talking DC here are we?

 

[BeanieBots]

Simplest solution I can think of would be a comparitor/op-amp.
Use a 5v regulator to provide a 5v reference (halved to 2.5v) voltage which goes on one input.
A potential divider from the 5v-25v on the other input designed such that it produces 2.5v at the voltage where you want the change to happen.
Wire the bi-colour LED between comparitor output and another potential divider on the 5v rail set at 2.5v.

You could of course use an 08 and 5v supply with the LED wired between two outputs and use ReadADC. Comparable cost/complexity.

 

[retepsnikrep]

OK we could have two seperate leds or just one at a pinch.

You could call one side of the resistor ground as the circuit will float on the resistor, the voltage across it is the item to be measured and the power supply. No nasty transients or noise it's DC

I would like an led iluminated when above and below the threshold voltage so I know it's on and when voltage goes below threshold.

 

[Dippy]

Then use BB's solution.
Just make sure op-amp/comp output is push-pull type for ease.
Job done.