Lightning detector oscillates in heat

[Lee697]

I've built the 'beginners' lightning detector from techlib - http://techlib.com/electronics/lightningnew.htm  (first schematic shown), without any changes, using the sensitivity pot in place of the fixed resistor.
It works well, so far, (without any lightning for real testing), it reacts to a spark from a gasmatch, and my Jaycar jacobs ladder, and even if you pull a polarfleece jumper off within a couple of metres.....
I've strapped an arduino off the rear that watches for the low pulses, counts them and puts it on a webserver for monitoring - I don't have time to sit and watch for flashes!

The issue is - I have this outside in my observatory (away from home wiring/switches) and it is all quiet until the day warms up - then it starts to fire off fairly rhythmically, where I end up with 50-80 pulses every 5 minutes. On any given day, the number per 5 min seems quite stable as the temp plateaus, then as the afternoon cools off, settles back down again....
Winding back the sensitivity does help at a given temperature.

I'm keen to get to the bottom of this, even if only to advance my knowledge.....

Does anyone have any pearls as to where the issue might be? in the LC tank, transistors? I've read up on capacitors changing capacitance with temperature, so I've ordered some C0G ceramics in the hope this might help.
The circuit author recognises the circuit is temperamental in the heat at higher sensitivity, but didn't suggest why, and I've asked him so many questions lately, I thought I'd try this forum......

Thanks for any suggestions.....
Lee

[MikeK]

I'm guessing that it's the gain of the Darlington that is varying with temp.  I would try a potentiometer at the base and experimenting with various settings.  Also trying the same with the base of the middle 2N4401.  Just a guess.  And "fiddling" tends to provide interesting results.  Hopefully you'll get more informed help.

[Lee697]

I'm guessing that it's the gain of the Darlington that is varying with temp.  I would try a potentiometer at the base and experimenting with various settings.  Also trying the same with the base of the middle 2N4401.  Just a guess.  And "fiddling" tends to provide interesting results.  Hopefully you'll get more informed help.

Thanks - I'll give that a try, I might stripboard up a new one with plenty of room for 'breadboarding'.... I fear that I'll be able to stop the oscillation at the expense of sensitivity though.....

[MikeK]

I have a book on weather instruments ("Weather Toys"), but the lightning detector is a little different than yours.  It uses an NPN Darlington with a base resistor and an optoisolator to separate the detector from the output signal.  I'll take a pic of the circuit if you want.

[Lee697]

I have a book on weather instruments ("Weather Toys"), but the lightning detector is a little different than yours.  It uses an NPN Darlington with a base resistor and an optoisolator to separate the detector from the output signal.  I'll take a pic of the circuit if you want.

Sounds great - I'd be keen to see it if you don't mind..... thanks....

[floobydust]

It's likely there is nothing wrong with your circuit and the problem is EMI from something daytime like a solar inverter, or PC computer SMPS or AM radio, garage door openers etc.
You can connect a signal tracer/LM386 and headphones or loudspeaker and listen to the output of the first transistor. Maybe go for a walk with it and you'll be surprised at what it is picking up.

I've built a bunch of different lightning detectors and the Charles Wenzel original circuit has an extra inductor for high-pass filter (as well as the 300kHz tank circuit) so it might do better rejecting some noise. Depending on your tuning, I found AM radio is picked up and a particular talk show will trigger it lol. Even though a low band station is say 550kHz, the antenna tank still picks up a lot on either side of its tuning. You can add another 10pF cap in parallel and shift tuning down to see what happens.
I'm skeptical about the whip being the actual antenna, compared to the coil. Your coil orientation will also affect things. Sheet lightning vs ground strikes are 90° different, so it will not trigger for all types of lightning I find.

If you think it's temperature dependence in the circuit, measure the collector voltage on Q1 and see how it does after warming it up with your finger.

[Lee697]

It's likely there is nothing wrong with your circuit and the problem is EMI from something daytime like a solar inverter, or PC computer SMPS or AM radio, garage door openers etc.
You can connect a signal tracer/LM386 and headphones or loudspeaker and listen to the output of the first transistor. Maybe go for a walk with it and you'll be surprised at what it is picking up.

I've built a bunch of different lightning detectors and the Charles Wenzel original circuit has an extra inductor for high-pass filter (as well as the 300kHz tank circuit) so it might do better rejecting some noise. Depending on your tuning, I found AM radio is picked up and a particular talk show will trigger it lol. Even though a low band station is say 550kHz, the antenna tank still picks up a lot on either side of its tuning. You can add another 10pF cap in parallel and shift tuning down to see what happens.
I'm skeptical about the whip being the actual antenna, compared to the coil. Your coil orientation will also affect things. Sheet lightning vs ground strikes are 90° different, so it will not trigger for all types of lightning I find.

If you think it's temperature dependence in the circuit, measure the collector voltage on Q1 and see how it does after warming it up with your finger.

Thanks,

That was certainly my first thought, the observatory has 2 small solar panels, but obscuring them made no difference, our house has panels/inverters x2 as well, but previously with extensive testing indoors never saw anything like this (seeming temperature effect).... I did email Charles and he commented that "Yes, the circuit is a bit sensitive to temperature when setting the threshold for maximum sensitivity."

Given the temperature rises I'm talking about are maybe 10deg C from quiet to banging away, I was surprised.....

I have made his sferics receiver, I'll wander up there and see if it is picking anything up.....

[magic]

Gain drift of the amplifier could be reduced by eliminating the upper 330k and increasing 5.1k so as to restore the original operating current. Gain will become higher in general if it isn't limited by parasitics of the transistor already, hopefully not a problem.

I'm not entirely sure if I fully understand how the level detector works, but it looks like a sort of circuit that could be sensitive to temperature too and also to battery voltage. You could replace that part of the circuit with the opamp version, but replace the topmost diode with LM385. And not sure what's the point of the bottom two diodes, it's a single supply opamp which works with inputs near ground.

[MikeK]

I have a book on weather instruments ("Weather Toys"), but the lightning detector is a little different than yours.  It uses an NPN Darlington with a base resistor and an optoisolator to separate the detector from the output signal.  I'll take a pic of the circuit if you want.

Sounds great - I'd be keen to see it if you don't mind..... thanks....

For educational purposes only:

[Lee697]

I have a book on weather instruments ("Weather Toys"), but the lightning detector is a little different than yours.  It uses an NPN Darlington with a base resistor and an optoisolator to separate the detector from the output signal.  I'll take a pic of the circuit if you want.

Sounds great - I'd be keen to see it if you don't mind..... thanks....

For educational purposes only:

Thanks very much - looks a fairly simple one to give a try as well.....

[MarcOhm]

Hi guys, I have the same problem with my circuit but the solutions that worked for me is a little weird.
Potentiometer is turned in low sensitivity position but led still blink.. I have tried solutions readed in this post but nothing changed.
The only thing that worked for me was solder another pot, I don't know why but this works for me 

[floobydust]

Which one did you build, and what is the schematic for the second pot you added?

[MarcOhm]

The problem arose both for the "new lightning detector" and the "magnetic antenna version".
I notice only with the magnetic antenna version the phenomen of oscillating with heat but the "new lightning detector" circuit oscillated despite not being exposed to heat.
About the second pot, I wrongly wrote my last post, I have not added a pot but replaced the one already mounted with another one (before replacing the pot I tried solder with a wire the pot case with battery GND but nothing changed).
The reference link is the same posted by the topic owner:
http://techlib.com/electronics/lightningnew.htm
Thank you for support and I apologize for not repling fast.