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A Portable Lightning Detector

This little device could save your life...

By John Clarke

Features

Portable

Battery or external power supply

Visual and audible lightning indication

Sensitivity control

Battery condition indicator

Reverse supply protection

While most of us love the wide open spaces, they are definitely not the place to be if a thunderstorm is on the way. If there is a lightning strike nearby you could be in big danger of death or injury. And you don’t have to be hit directly – induction can kill you and so can the voltage gradient across the ground in the vicinity of a lightning strike.

Our Lightning Detector can warn you of an approaching lightning storm and provides valuable time to take shelter safely indoors. And even if you’re not outdoors it can give you warning to disconnect vulnerable electrical appliances from the 230VAC mains supply. It provides audible and visual indication to warn of approaching thunderstorms.

Lightning damage to electronic appliances

Many people do not realise how vulnerable electronic equipment can be in a thunderstorm, even if it is not close by. Service organisations report a big surge in repair jobs after storms and just about all of this could be avoided simply by switching off and removing power plugs from the wall socket.

Those appliances especially at risk include microwave ovens, TV sets, satellite receivers, mains powered computers (especially those also connected to the phone lines via a modem), washing machines and dryers.

They should not be just switched off at the power point but the mains plug should be removed from the socket. TV antenna and satellite dish connections should be disconnected too. Many ovens incorporate electronic timers and power to these can be switched off at the “fuse” box.

Apart from mains-powered computers, devices that are particularly prone to damage are fax machines and cordless telephone base stations. It is the fact that they are connected to both the 230VAC mains and the telephone wiring that provides a double whammy. During a big thunderstorm they should be disconnected both from the phone line and the mains power.

Of course, it is well known that any phones (apart from mobiles and cordless models) should not be used during a thunderstorm.

So what to do?

To get a warning of imminent thunderstorms, you need the SILICON CHIP Lightning Detector. It is a pocket-sized unit that provides visual indication using a flashing LED and sounds an audible tone whenever lightning occurs in your area. The greater the number of lightning strikes, the more LED flashes and audible tone bursts are produced.

For portable use it is powered with two alkaline AA cells, Battery life should be at least 1000 hours. For indoor use, you can use a 6V-12V DC supply, such as a plugpack. One resistor needs to be chosen according to the DC supply voltage. When the external power supply is connected to the jack the socket, the AA cells are automatically disconnected from the circuit.  

The principle of operation is based on detection of the broad-spectrum electromagnetic emissions produced by lightning strikes. This is readily detected by a simple AM (amplitude modulation) radio receiver.

If you’ve ever been anywhere near an electrical storm with an AM radio turned on, you’ll have heard the crashes (static) of lightning strikes. Very large strikes can be heard from a considerable distance away.

We use a single AM radio IC which comprises a RF (radio frequency) amplifier, detector and AGC (automatic gain control). This was originally available in 1984 from Ferranti Semiconductors as the ZN414Z but replaced by the MK484, now also obsolete.

We have used the modern equivalent, the TA7642. It operates from a 1.2 to 1.6V supply and covers from 150kHz to 3MHz. This includes the normal AM radio broadcast band (530kHz to 1.6MHz) but for our purposes, we are not concerned with listening to broadcast radio stations. We simply monitor the whole spectrum covered by the AM radio chip.

Block diagram

Click for larger image
Fig.1: the block diagram of the Lightning Detector. The early part looks similar to a radio receiver - which of course it is - but this radio receiver picks up just one thing: the RF pulse from a lightning strike within range.

The general arrangement of the Lightning Detector is shown in the block diagram of Fig.1. IC1 receives signals from a pickup coil. In an AM radio this pickup coil would normally be tuned to a particular frequency using a variable tuning capacitor.

We want to monitor a wide frequency range and so the coil is left un-tuned. IC1’s output signal is noise bursts from lightning.

Output from IC1 is typically 15mV with a tuned coil but is around 2mV with the un-tuned coil. This signal is amplified using transistor Q2 and a sensitivity control sets the level applied to the following pulse extender comprising IC2 and diode D3. When lightning is detected, a noise-burst triggers the pulse extender.

The pulse extender produces a 200ms pulse and this lights the “detect” LED1.  The pulse extender is necessary because the lightning strike pulses are too short in duration to be noticed as a flash from the LED.

IC3 is an oscillator that runs for 200ms each time the pulse extender produces a low signal and the resulting 4kHz tone burst drives the piezo transducer which is resonant at that frequency.

Specifications

Supply voltage:    3V (2 x AA cells) [will operate down to 2V]

Plugpack: 6 to 12VDC at 30mA

Current Consumption:    Battery operation 1.5mA at 3V, 1mA at 2V,

DC plugpack operation 17mA at 12V

Battery life:    Typically 1000h with Alkaline cells

IC1 supply:    Typically 1.46V with 3V supply, 1.28V with 2V supply

Battery voltage indication:    Down to 2V

Strike indication duration:    200ms

Transducer frequency drive:    4kHz

Frequency detection band:    150kHz to 3MHz

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