Extremely Low Frequency Receiver circuit project

ELF receiver

There is no technical help with the projects. This circuit is shown as an educational example only.

This extremely low frequency ELF receiver built on a piece of wood uses only 2 transistors but is very sensitive. It is designed for use with any of our high impedance ELF antennas or magnetic transducers. Use it with an oscilloscope or chart recorder for monitoring solar storms, earthquakes, infrasonic sound waves, etc, depending on the sensor or antenna connected.

The output waveform is a greatly magnified copy of the input waveform. Use this receiver with a chart recorder program to record data, or connect to an oscilloscope to see the waveforms. Set to 0.1 volt per division.


There is no technical help with the projects. This circuit is shown as an educational example only.

The parts and values are shown on the diagram above on this page. Standard values used. General parts available at any electronics store. Antennas available here at WWW.STORMWISE.COM

Antenna is any of our high impedance ELF antennas (impedance greater than 5 K-ohms).

The transistors Q1 and Q2 are NPN transistors 2N3904.

All transistors are available from Radio Shack stores or other electronics parts store.

How to build it: Print out the above circuit diagram. Scale it down on your printer or photoshop program so that it is the right size, large enough for the electronic parts to fit the dots. This size is about 6 and 1/8th inches long and 2 and 1/2 inches wide.

You will want to enclose finished unit inside of a sealed wood or metal case, or seal it in a glass mason jar - The transistors are sensitive to rapid air temperature changes, blowing on Q1, or touching it will cause the output to swing up or down. Placing your hand near Q1, about 1 inch away will result in an output swing. This is a result of the transistor's ability to detect thermal infra-red rays. (not the same as near-IR light). This effect is normal and found in many transistors, but not a desirable trait. If the room temperature changes are slow the circuit will not sense them, thus sealing the circuit in a glass, wood, or metal case will prevent detection of thermal infrared rays and sudden temperature changes. This shows how very sensitive this receiver is, and it is far more sensitve to what goes into the input.

Do not leave out or bypass any of the parts, all parts have a required function.

How to build: See photos below on this page.

Obtain some safety goggles, you'll be soldering and hammering nails and cutting a piece of wood (use a hand saw only). Solder is molten metal and can spatter into your eyes. Do not solder without full eye protection.

A 30 watt soldering iron was used.

If you are not familiar with soldering practices, then it -IS- possible to use wire-wrap methods to build the receiver. You can also build the receiver on a solderless breadboard. Ask at Radio Shack store for this item. You'll need the one with the grounded shield base, receiver -will- oscillate on these if not connected to the shield base. You will still need the safety goggles when clipping part's leads or working with the wires.

A square piece of wood should be obtained that is slightly larger than your print-out.

Obtain some copper plated weather-strip nails from the local hardware store. Cut out the paper circuit diagram print-out and tack it to the wood, one tack going into each dot in the diagram. Place the parts on the tacks. Solder parts to the tacks. Make sure tacks do not short to ground shield except where indicated.

Place the transistors last after all the other parts have been soldered, to prevent static damage or heat damage. It is best to just let the transistor sit on top of the tacks and solder it to the top of the tacks with a small drop of solder. Do not bend the transistor's leads too much.

Parts may contain LEAD or other chemicals so wash hands after handling. Solder contains LEAD or other chemicals so wash hands after handling.

Photos to be added after 07-21-2011

There is no technical help with the projects. This circuit is shown as an educational example only.

There is no technical help with the projects. This circuit is shown as an educational example only.

The antenna is directional so rotate the antenna for best reception.

Certianly a PC board can be etched and the receiver built that way, for those who are advanced, but the wood way works great for beginners so it's up to you.

Testing and modifications

The receiver will work on 6 to 9 volts. A 6 volt lantern rechargeable battery will power it for months before needing charging.

When first turned ON, allow the receiver about 3 minutes for the capacitors to charge up and settle down. The voltage output will swing up and down and then finally drop down to zero volts (with no antenna connected). Connect antenna. Signals should be seen when the antenna is stimulated with magnetic field movements. If too much signal is input the receiver may briefly jam, but it will self-reset once the capacitors settle again. If signals are too strong then reduce the 33 K-ohm resistor across the input to a lower value, to adsorb signals.

The output sub audio signal is from 0.001 volt to about 2 volts, enough to easily drive an oscilloscope or chart recorder.

You will need to check the receiver for balanced output swing by inputing a 1 Hz magnetic field signal into it and driving it to the start of clipping. A mechanically spinning small magnet [1 rotation per second] a few feet away will do the trick. Use a a battery powered screwdriver with a small magnet taped to it. Does the output voltage swing as high as it does low and do the high and low portions look the same or is one side flat? If not the same then adjust the 470 K ohm resistor across Q2 until perfect fidelity is obtained at maximum input drive just before clipping occurs. JUST BEFORE CLIPPING!, please!!! You will want to capture undistorted natural waveforms in your recordings. Output should swing +/- several volts. A pure sine wave (~) in must give a pure sine wave (~) out.

There is no technical help with the projects. This circuit is shown as an educational example only.

MODIFICATIONS: The 22 uF capacitor controls how high the received frequency will be. It filters out the high frequencies by passing them to ground. Good for removing 60 Hz power hum.

MODIFICATIONS: The 0.047 uF capacitors across the B - C connections of the 2N3904 transistors act as a negative feedback for the higher frequencies - they limit the high frequency response while preserving the lower frequency response. The action is that a small portion of the amplified signal is fed back into the transistor's B input. The amplified signals are 180 degrees out of phase thereby cancelling some of the received higher-frequency signals. Also some of the high frequencies bypass the input through this capacitor. Both actions limit the transistor's high frequency response, good for removing 60 Hz power hum when you are just interested in reception below 20 Hz.

MODIFICATIONS: Do not remove the 10000 ohm resistors in the front end input network, else the antenna will become tuned to one frequency due to the 22 uF capacitor. This resistance eleminates any tendency to become tuned to one frequency, keeping the system broad-band.

For recording earth's magnetic field and solar storms there are 3 standard ways to aim the antenna:

You can point the end of the antenna (not where the connections are) toward magnetic NORTH compass point, or

You can ponnt the end of the antenna (not where the connections are) toward the magnetic EAST compass point, or

You can point the end of the antenna (not where the connections are) straight down toward the center of the earth. Do not hang the antenna as motion will cause unwanted signal readings to occur. You'll need a plum bob or a level to determine the antenna end is pointed straight down.

The antenna should be mounted outdoors on a concrete slab and bolted down so it does not move or vibrate. Antenna should be placed inside of a larger PVC pipe case, padded with foam so it does not move, and given protection to keep water off of it and to stabilize any sudden temperature changes. The receiver should be mounted indoors. Use shielded cable between antenna and receiver. It could be possible to mount the receiver outdoors if it is kept dry and kept from sudden air temperature changes, then you can solar-power it with a small panel and a 6-volt lantern battery, just feed the output down a shielded cable. You'll have to see which works best for you depending on how you built the receiver. Should be built on water-resistant PC board if to be used outdoors. Keep everything dry!!!

People or animals walking past the antenna may cause weak signal indications, and cars and trucks going down the street may cause detections, so mount the antenna away from any moving traffic.

If using a kinetic magnetic transducer then mount these away from roads and sidewalks or any where undesired vibrations or sub-audio noise could be detected.

There is no technical help with the projects. This circuit is shown as an educational example only.

Copyright 2011
All rights reserved