How To Sense Quick Neon Bulb Flashes w/Phototransistor

djsfantasi

Joined Apr 11, 2010
8,577
Looking back at the first post in this thread, we are leaning over backwards to detect a 600 volt pule from an electric fence. I would seem much more efficient to sense a 1kv to 6 kv pulse than to try an note the glow of a neon lamp. I think we are on the wrong track.
I’ve been basing my responses on the stated objective (TS has mentioned it more than once) of maintaining isolation from all HV components. He has a commercial test unit that outputs the fence status using neon lamps and wished to monitor that. Detecting a 1kV to 6kV does not isolate from HV.
 

DickCappels

Joined Aug 21, 2008
9,326
No question that we are working on getting the TS what was asked for, but there are much easier ways to detect a 1 to 6 kV pulse than by looking at a neon light. Such as using a tiny bit of capacitive coupling.
 

Thread Starter

prairietech

Joined Mar 12, 2013
24
Sorry for my absence as life got in the way of this project.
I built the 2N3904 circuit (try googling this transistor and see how many different pinouts you get) and it naturally increases the voltage swing of the phototransistor. I now get a nice voltage transition sweeping a flashlight across the phototransistor. However it still doesn't detect the neon bulb flash.
As some of you have suggested, I think the light from the neon bulb just isn't bright enough or long enough.
Would a photoresistor work better than the phototransistor or am I barking up the same tree?
 

Reloadron

Joined Jan 15, 2015
6,984
I look at things this way. photo transistor, photo diode, and LDR response times vary. The fastest is likely a photo diode. What needs also considered is sensitivity with a focus on sensitivity to the neon lamp color. So sensitivity to how much light to get response and the color of the light. Again, neon lamps are not exactly brilliant bursting with lumens so consider that.

Ron
 

Thread Starter

prairietech

Joined Mar 12, 2013
24
. Again, neon lamps are not exactly brilliant bursting with lumens so consider that.

Ron
Ron, I fully agree regarding the neon bulb. I was just hoping it would be a nice solution to keep the microcontroller away from HV by using optic detection. Any other suggestions would be appreciated since I'm on the edge of circuit design.
 

Reloadron

Joined Jan 15, 2015
6,984
Problem with an optocoupler(s) comes down to a few things. If we go back to the beginning my guess as neon bulbs in a picture like I linked to. A long resistive divider and neon bulbs. Here is a few notes on neon bulbs.
neon lamps.png

The above picture shows three lamps. Left to right the first two bulbs are seeing DC only one electrode lit and the lit electrode is the DC negative. The bulb on the right is AC so both electrodes appear to be glowing. The gas in a typical neon bulb like for example a NE2 bulb will ionize (bulb glows) at about 80 volts, maybe a little more of less. As the voltage at the electrodes increases, once the gas is ionized more and more of the electrodes glow. Once the gas ionizes the bulb is basically a short. As voltage decreases eventually the gas will stop ionizing. Sop a neon bulb has a turn on point and a turn off point. I am guessing your circuit is a basic AC circuit like posted at the beginning.

They do make AC opto-couplers but their use would involve hacking into your indicator box and placing an AC optocoupler with a series resistor on each across each neon lamp. Also opto-coupler response time would matter. Beyond that I am pretty much lacking a solution. :(

Ron
 

twohats

Joined Oct 28, 2015
327
I haven't read the whole story, but have you considered using a magnifying glass/ device between the neons and the sensor?
Is the sensor enclosure a light colour inside? e.g. white or silver foil.
Good luck..........
 

Thread Starter

prairietech

Joined Mar 12, 2013
24
I haven't read the whole story, but have you considered using a magnifying glass/ device between the neons and the sensor?
Is the sensor enclosure a light colour inside? e.g. white or silver foil.
Good luck..........
The phototransistor is positioned touching the neon bulb. I'm not sure any other method would recover more light than that?
 

twohats

Joined Oct 28, 2015
327
Thank you for the reply.
At the risk of another silly question, have you tried lying the sensor on its side?
I had to stoop pretty low on one project, similar to yours, to make it work/ sense the too low light. I fitted a very dim led in the same enclosure that was permanently lit, but not enough to trigger the sensor, so that when the neon was lit,
the combined light worked the sensor.
Good luck...........
 

Thread Starter

prairietech

Joined Mar 12, 2013
24
Thank you for the reply.
At the risk of another silly question, have you tried lying the sensor on its side?
I had to stoop pretty low on one project, similar to yours, to make it work/ sense the too low light. I fitted a very dim led in the same enclosure that was permanently lit, but not enough to trigger the sensor, so that when the neon was lit,
the combined light worked the sensor.
Good luck...........
I never thought about it like that. But there are 6 neon bulbs I would like to detect. Each bulb is in it's own compartment. There is room through the enclosure window of each bulb to recess the phototransistor through that opening.
 
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