Light led when voltage low

[djab]

Hello, I need some advice on a project idea.

I've read that smoke alarms use MOSFETs to turn on the sound when smoke stops current flowing between a radioactive source and a detector, using the same idea, how can I have an LED light up when there is no (or preferably 'very low') current and have it not light up when there is current.

My plan is to create a safe capacitor discharger that lights up when the capacitor is discharged, it needs to be able to do this for voltages up to somewhere near 1kv (this is where I stopped thinking I could use an old smoke detector)

I already have two really big resistors (100 ohms, 10 watts each) to do the discharging

Thanks in advance, Dan.

 

[davenn]

Hi Dan
welcome to the forums

firstly it isnt a current flowing between the radioactive source and the sensor. Rather, the the alpha particles from the radioactive source ionise the air between 2 electrodes.
This allows a very small current to flow across the ionisd path between the electrodes.

I'm trying to follow how a smoke detector, a LED and a discarging HV source are going to work together, I suspect not very well.

I think you need to redefine more clearly what you are actually trying to achieve and then some of us may be able to help you figure out a circuit.
ie. what is the HV discharge supposed to achieve ?
1 -- what electronics experience do you have ?
2 -- Have you worked with HV circuits before ?

cheers
Dave

 

[Resqueline]

Try this circuit and see how it performs. I can't say at exactly what voltage it'll (start to) turn on the LED.
If you use small size resistors for the HV they may stand only 200V and if so you'll need to use 5 * 2.2M in series instead of 2 * 4.7M.
And as Dave tries to get at, 1kV needs thoughtful circuit layout (distance/insulation).