We explore lightning detection, and present two very different methods for warning of approaching storms.
BUILD TIME: AN AFTERNOON
DIFFICULTY RATING: Intermediate
Most people find lightning either fascinating or terrifying, with little in between. From those who hide in the cupboards in a storm at one end of the scale, to those who chase storms at the other, our response to our feelings varies a lot more than the feelings themselves. Many people who are fascinated by lightning are content to watch it from a sheltered position safe in a building or car, but some will try to get out and see it when possible. This tends to work best when the lighting is distant, and the sound may not even be audible anymore. Lightning out to sea, visible silently from the coast, is a great example. Anyone who has ever visited somewhere like Coober Pedy and seen lightning roll in (or out) over the Moon Plains will never forget that experience.
However, lightning is more than a visual spectacle. It generates huge amounts of electromagnetic energy, which can be very problematic for certain equipment. It can cause power surges in power lines, by induction when close, or by direct strike. It can start fires in dry bushland, and for these reasons, there are a myriad of people who want to know in advance if lightning is on the way.
You may need to have equipment turned off, move people inside, or get the volunteer fire crew on standby. While there is some sophisticated equipment around for critical applications, there are a bunch of DIY options, too; and at DIYODE, we love DIY so much that we modified our name to say so!
Much of the energy of lightning is radiated as heat, close to the bolt of lightning itself. There is a strong light content,
and quite a portion of that is in the Ultraviolet spectrum. However, a significant amount of energy radiated as radio waves. The spectrum is huge, and studies vary as to where across the RF spectrum that lightning has the strongest emissions. The likely explanation for that is that it probably varies due to factors science hasn't discovered yet, or the answer is just beyond anyone who is not a physicist and so has not made it into general reading.
One thing that is certain, is that many lightning detectors function around the 100kHz to 350kHz band. The reason for this is not so much that the radiated energy is strongest there (though many sources say so), but rather that it offers the best combination of strength from the RF radiation, and lack of human interference. There are less human sources of RF in this band than many other sections in lightning's radiation spectral pattern.
Another characteristic of lightning less often used for detection, is the static charges involved. Lightning occurs because the movement of moisture in the air carries charge from one place to another, leading to a big imbalance. This exists in the form of a static charge before lightning becomes a bolt.
When the charge is great enough, the air ionises, which provides a conductive path for the electric charge to take. This may be cloud to cloud, or between the cloud and ground. This charge can, with varying degrees of success, detect the approach of a storm or lightning event before the first bolt has occurred and sent a radio signal for further warning.
SUPER SIMPLE
One of the ways of detecting this charge has been around since well before the internet. We haven't been able to verify it, because there has been no favourable weather or storm events since we set one up. There has only been rain, lots of rain. However, it is capable of detecting the static field around a plasma ball, so that's encouraging.
This 'lightning detector' makes use of a quirky property of neon globes. They will emit light when presented with a static field, and do not need their full operating current to do so. If one is held near a plasma globe, it lights up. Those who read our Slayer Exciter article back in Issue 41 will have seen this in action.
To make this very basic detector, all that is needed is a ground connected to one side of the globe, and an antenna on the other. This is not a tuned circuit, so the longer the antenna, the better. One of the DIYODE team set one of these up several years ago, with a wire running the entire length of the tile roof of the house, down the wall, and into the workshop. We have no idea if it worked, because the Cockatoos shredded the wire before we could find out.
We have seen pictures of this setup used successfully. The one major drawback, in some circumstances, is that the user needs to be looking at the globe at the time. That may not be as bad as it seems, especially if the end-use is purely curiosity. If used as a genuine warning device, the light will glow constantly as long as a big enough static field is nearby. However, it would be better if the device could be made audible. To do so, we propose enclosing the neon in a light-proof case with a Light Dependent Resistor (LDR) inside it. The LDR and a suitable resistor form a variable voltage divider, connected to one input of an operational amplifier (op amp). The other op amp input is connected to a potentiometer to set the reference voltage.
In the circuit diagram presented, R1 is the LDR, while R2's value depends on the LDR chosen. There is a great variety of light/dark resistances for LDRs. The junction is connected to the non-inverting input, while the reference is connected to the inverting input. The output drives a transistor with a piezo buzzer attached, which provides an audible warning when the globe illuminates. This system can be set to be very sensitive, with the globe barely lit to trigger the sound; or not sensitive at all so that the globe has to be glowing very brightly to trigger the alarm. The alarm itself could be connected via further circuitry to have a timer cut-out, or pulse on and off. Further, the circuit here lacks hysteresis for the op amp, which may or may not be a problem. Some op amps have a degree of hysteresis built in.
SOMETHING MORE SERIOUS
The neon circuit may or may not be good for warning of developing storms. We would love to know but haven't been able to verify it ourselves yet. One thing is certain: once a storm is active, radio detection is a far more reliable way of detecting an approaching storm, and from further away. Our main build for this project, then, is a radio-based detector circuit, tuned to around 300kHz.
It is based around a resonant circuit, formed by a resistor, capacitor, and two inductors connected in series to produce an unequal tapped inductor. An antenna is the input to this, and the output is taken via a 100pF capacitor. The antenna would ideally be a telescopic one like on a radio, but they're not so easy to get hold of. Instead, we have used a metre of hookup wire. The output from the resonant circuit is fed to a transistor, Q1, which has its base biased by the trimpot to be close to saturation. When a pulse arrives from the resonant circuit via capacitor C2, the transistor saturates and conducts. The resonant circuit, often called a tank circuit, extends the duration of this pulse as the current moves back and forth in it. Only when the current falls far enough does the base of the transistor fall below saturation. The oscilloscope screenshot shows this well.
The transistor is an NPN BC547, and has a 1kΩ R4 resistor connected between the supply rail and its collector. The emitter goes straight to ground. In standby, when there is no radio pulse, the resistor keeps pin 2 of the NE555 held at the supply rail voltage, because the input at that pin has a high enough impedance that almost no current flows to cause a voltage drop in the transistor.
The NE555 is connected as an astable multivibrator, with the timing set by R5 and C2. As is, this runs for about half a second. When Q1 does go into saturation and conduct, R4 limits the current, and the path through the transistor is almost an open circuit compared to the high impedance of pin 2 of the NE555. Therefore, no current flows here anymore, and the internal flip flop now trips and starts the timing cycle. This way, a very short pulse of radio frequency from distant lightning can be extended to a pulse length as long as you like. Within the limits of the NE555, of course.
The output of the NE555 is connected to go high during the timing interval, by the loads connected to it being connected to ground. You could also add another LED, if desired, and a resistor to match, between the supply voltage and pin 3. When not triggered, the output pin 3 is sinking current and so this LED can act as a status LED, confirming both power and the timing status. We chose not to, however.
Our build has just a high-brightness white LED, and a piezo buzzer. The LED should be as bright as practical. In the right circumstances, the buzzer may not be needed: The bright flash of the LED would throw enough light in most household situations to catch your peripheral vision anyway. In brighter ambient conditions, the buzzer will be needed unless you are looking directly at the LED when it goes off.
The Build:
| Parts Required: | ID | Jaycar | Altronics | Pakronics |
|---|---|---|---|---|
| 1 x Solder Breadboard | - | HP9570 | H0701 | ADA1609 |
| 1 x Packet Breadboard Wire Links | - | PB8850 | P1014A | SS110990044 |
| 1 x 150Ω Resistor | R6 | RR0552 | R7538 | DF-FIT0119 |
| 2 x 1kΩ Resistors | R3, R4 | RR0572 | R7558 | DF-FIT0119 |
| 1 x 270kΩ Resistor | R1 | RR0630 | R7616 | DF-FIT0119 |
| 1 x 360kΩ Resistor | R5 | RR0633 | R7619 | DF-FIT0119 # |
| 1 x 1MΩ 25-Turn Trimpot | R2 | RT4658 | R2394A | - |
| 1 x 10pF Capacitor | C1 | RC5312 | R2810 | DF-FIT0118 |
| 1 x 1nF Capacitor | C2 | RG5010 | R2700B | DF-FIT0118 |
| 1 x 100nF Capacitor | C3 | RM7125 | R3025B | DF-FIT0118 |
| 1 x 2.2µF Electrolytic Capacitor | C4 | RE6042 | R5028 | DF-FIT0117 |
| 1 x 10mH High-Frequency Choke | L2 | Element14: 1693340 | Z1040 | - |
| 1 x 100mH High-Frequency Choke | L1 | Element14: 1693340 | Z2755 | - |
| 1 x BC547 NPN Transistor | Q1 | ZT2152 | Z0876E | ADA754 |
| 1 x NE555 Timer IC | IC1 | ZL3555 | S6109 | ADA160 |
| 1 x High-Brightness White LED | LED1 | ZD0290 | W2250 | ADA3111 |
| 1 x Piezo Buzzer | - | AB3462 | H080A | - |
| 1m Light-Duty Hookup Wire for Antenna | - | WH3010 | ||
| 5 x PCB Pins | - | HP1250 |
# Use nearest value * Quantity shown, may be sold in packs. You’ll also need a breadboard and prototyping hardware.
Assembly is straightforward, with a few points to note. Regular construction practices apply: Work from low-profile to high-profile components; make mechanical connections to the board (bend the legs); work a few components at a time so you can access each solder joint and don't miss any; and double-check at each stage.
The inductors are high-frequency types, and are far smaller than toroid-wound high-current varieties commonly used at low frequencies. They are measured in milliHenries, not microHenries. This can be a challenge with inductors, as some sources use mH for microHenries in fonts which lack the greek letter 'mu', which we know as µ. microHenry values are more common in retail ranges, and we had to get these milliHenry inductors from Element14. We have fitted them to the PCB, but they only just fit. Be careful not to break the legs out of the fragile ferrite base.
Ceramic or greencap capacitors are fine for the smaller values. We used what we had in stock, which is why we have one of each. The parts list will therefore show ceramics only. We used PCB pins for the buzzer output, power input, and antenna connection. You might like to use a wire link and connect it from the antenna input, to a bolt on the board on the opposite side to Q1 if you can get your hands on a telescopic antenna.
This would provide electrical and mechanical connection to the antenna, which usually have a threaded receptacle in the very base. Some are metric, while some are imperial, and there is little standardisation.
The build is designed to work at 5V but will work easily at 12V. This means you have a variety of power options. We intended it to be used with the vast quantity of USB device chargers and power supplies that most people have around the house. We also envisage portability with a USB power bank, if you want to have it easily moved between locations or even take it in the car for your next lightning photography expedition.
USING IT
Adjust the trimpot until the monostable output section triggers, then back it off slightly until triggering stops. You may wish to disconnect the piezo buzzer while this happens. The antenna length also adds to sensitivity. Mounting the unit near a window or even with the antenna outside will help improve sensitivity, too. Anything that blocks radio signals will impact the performance of the device to weak or distant lightning.
The device will respond, at close enough range, to the spark from the piezo igniter in a gas match or barbeque lighter. We tested ours with an old Dusk jet candle lighter which was otherwise dead. The oscilloscope screenshots were produced this way, too.
If set properly, the alarm will also respond to the static sparks from taking off a knitted jumper in the right conditions, or even the static from sliding along a wooden floor in socks. You don't have to wait for a storm to test out your creation!