Low Frequency Radio Receiver circuit project with VU Meter

low frequency radio receiver
Low Frequency Radio Receiver for radio astronomy and general listening pleasure.

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

We are still currently working on this unit and will add more photos and any circuit changes later. It does work with the parts values shown, however.

Are you looking for a powerful low frequency receiver? This powerful 5 transistor receiver works great when used with 50 ohm antennas.

This 5 transistor low frequency radio receiver allows you to hear longwave beacons, broadcasts, and other signals in the range of 20 KHz to 530 KHz. It works just like an AM radio, a diode demodulates AM signals.

The radio even includes a VU meter which helps with tuning for the strongest signal.

The receiver is designed for use with a tuned 50 ohm antenna, the antenna's tuning adjustment (via a variable or fixed-value capacitor) is what selects the receive channel.

This is a direct detection amplified "crystal" radio receiver. It does not generate RFI or use regeneration.

This radio will drive any audio amplifier (like the Radio Shack mini-amp) loudly to room-filling volume.

If you don't want to use an external amplifier, it also powers an earphone directly from the output jack.

Power the radio with a rechargeable 6 volt lantern battery. The radio will run a -very- long time, it only draws @ 10 mA at full volume. The radio will also work on 9 volts without any problem.

The PARTS:

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

  • T1 is Stormwise Part # LF Mini Balun Core-1.

    To wind the balun core: Wind 1 turn of # 22 insulated solid wire through the center hole and connect ends to J1 input jack. Wind 4 turns # 24 insulated solid wire through center hole and connect to R2 terminals like shown.

  • Q1, Q2, Q3, Q4, Q5 are all 2N4124 or 2N3904 NPN transistors.
  • R1 is a 680-ohm resistor (Do NOT use wire wound types for any of the resistors in this radio).
  • R2 is a 4.7 K-ohm resistor.
  • R3 is a 100 K-ohm resistor.
  • R4 is a 2.2 K-ohm resistor.
  • R5 is a 100 K-ohm resistor.
  • R6 is a 100 K-ohm resistor.
  • R7 is a 100 K-ohm resistor.
  • R8 is a 1 Meg-ohm resistor.
  • R9 is a 10 K-ohm resistor.
  • R10 is a 4.7 K-ohm resistor. (If using an 8-ohm earphone then bypass this resistor for louder volume. Do NOT bypass if using a crystal earphone or if using an external amplifier!).
  • R11 is a 10 K-ohm resistor.
  • R12 is a 100 K-ohm resistor.
  • R13 is a 1 Meg-ohm resistor.
  • R14 is a 3.3 K-ohm resistor (may need to adjust value for full-scale [+3dB] peak movement on strongest (overload) signals).
  • R15 is a 680-ohm resistor.
  • C1 is a 0.1 uF capacitor.
  • C2 is a 1000 pF capacitor.
  • C3 is a 0.22 uF capacitor.
  • C4 is a 1000 pF capacitor.
  • C5 is a 47 uF capacitor. Polarity marked part.
  • C6 is a 0.047 uF capacitor.
  • C7 is a 0.1 uF capactor.
  • C8 is a 2.2 uF capacitor. Polarity marked part.
  • C9 is a 4.7 uF capacitor. Polarity marked part.
  • C10 is a 1000 uF capacitor. Polarity marked part.
  • C11 is a 470 pF capacitor. Use a greater value if needed to filter out strong signals above desired reception band. 470 pF works good for filtering above 500 KHz.
  • C12 is a 1000 pF capacitor.
  • D1 is a 1N914 or 1N4148 silicon diode. Polarity marked part.
  • D2 is a 1N914 or 1N4148 silicon diode. Polarity marked part.
  • D3 is a 1N914 or 1N4148 silicon diode. Polarity marked part.
  • D4 is a 1N914 or 1N4148 silicon diode. Polarity marked part.
  • J1 is an SO239 connector to match the coaxial cable used between antenna and radio.
  • METER is MOUSER ELECTRONICS Part # 541-MSQ-AVU-AW. Large VU Meter. (This meter has 120 ohms coil movement). Polarity marked part.

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

    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 7.0 inches wide as indicated.

    Do not leave out or bypass any of the parts, all parts have some required function. You can leave out the meter and meter drivers if you don't need the signal meter, but it is better looking with it.

    How to build: Obtain some copper tacks (or copper plated weather-strip nails) from the local hardware store. Get a small piece of wood. Cut out the paper circuit diagram printout 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.

    Place the transistors and the two diodes 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.

    J1 is a an SO-239 coaxial connector. Just connect antenna directly to this connector via any type of coaxial cable or microphone cable. Don't worry about the cable length or cable impedance, unless its more than 1/8th wavelength or more than 100 feet. Best to keep cable short as possible to prevent losses, though. Connect a tuning variable capacitor across the antenna's binding posts tuning leads.

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

    The output audio signal is several millivolts, enough to drive a small audio amplifier loudly. Use Radio Shack's Mini Amp, Radio Shack part number 277-1008C.

    You should easily hear static from thunderstorms 1000's of miles away at night. To hear some types of beacon stations (that don't send AM or morse code) you will need to input the same frequency as the station from a local oscillator ( an audio generator that goes from 20 KHz to 530 KHz) to allow mixing of the signals to get a "beat note". You can input the mixing signal directly into R4-R5-C2 junction point via an additional 10 pF capacitor.

    VLF and LF signals penetrate walls of homes. The antenna and pre-amp should do very well indoors. It can be built into a wooden or metal box with jacks. Power it with a 6-volt battery.

    Meter not read 0 when no signal? (no antenna connected). Adjust value of R13 until meter bearly moves up scale (about a 0.5 millimeter) with power on and no signal input. This is the most sensistive spot.

    Receiver's status: Its currently built on a solderless breadboard, preliminary testing for radio astronomy application. We'll be transfering it to a nice wooden case. We'll show photos when done.

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

    LIST OF OTHER RADIO PROJECTS:

    VLF Receiver with Signal Strength Meter

    ELF Magnetic Field Detector

    VLF Whistler Receiver

    Copyright 2010
    All rights reserved