Crystal Radio Circuits

Crystal Radio Circuits

Simple Crystal RadioOne-Transistor Amplifier/DetectorTL431 Crystal Radio AmplifierCrystal Radio RF AmplifierVery High Gain Crystal Earphone AmplifierSimple Two-transistor RadioExperiments with Detector Diodes Make sure to see the simple Reflex Receivers! A little more work yields vastly superior results.

Simple Crystal Radio

The crystal radio gets its name from the galena crystal (lead sulfide) used to rectify the signals. A "cat's whisker" wire contact was moved about the surface of the crystal until a diode junction was formed. The 1N34A germanium diode is the modern substitute for galena and most other germanium small-signal diodes will also work well. Silicon diodes are not a good choice because their much higher barrier potential requires larger signals for efficient rectification. Certain silicon Schottky diodes with low barrier potential will work well but most small-signal Schottky diodes will not perform as well as a garden-variety germanium diode.

The circuit is quite simple but many pitfalls await the novice. The first precaution is most important! The crystal radio works best with a long, high outdoor antenna but the beginner may not fully appreciate the danger of bringing such a wire into the house. Lightning strikes to the antenna will probably destroy the crystal radio but if precautions are not taken, much more damage will result. The best strategy is to incorporate a commercial lightning arrestor with a straight, heavy gauge ground wire leading down to a buried water pipe. It is not sufficient to disconnect the antenna from the receiver during thunderstorms.

Other pitfalls are less dangerous and relate to the receiver's performance. A common mistake when building a crystal radio is to load the tuned circuit excessively. The Q of the tuned circuit must remain high to give selectivity or strong radio stations will all mix together. A good design will usually have low-impedance taps on the inductor for connections to the antenna and diode as shown in the schematic. A long wire antenna with a good ground connection will connect to the lowest impedance tap whereas a shorter antenna with no ground connection may connect to a higher tap. The diode may be experimentally moved to different taps and even across the whole coil for maximum sensitivity. The antenna and diode connection may be made with alligator clips for easy experimentation.

Another potential problem area is the earphone. Not all crystal earphones are sensitive and the experimenter should test a few to get a "good" one. High impedance dynamic earphones are a bit more reliable and can give excellent results. Try an old telephone receiver or a modern portable tape player headset (some are high-Z and fairly sensitive). Low impedance earphones like those used with many portable radios will not work at all. A simple test is to hold one earphone wire between the fingers while scraping the other lead across a large metal object like a file cabinet. If static is heard in the earphone it will probably work well with the crystal radio.

The variable capacitor is often connected incorrectly. Make sure to connect the rotor to ground and the stator to the "hot" side of the coil. Otherwise, the radio will detune when the capacitor knob is touched. If detuning is noticed then try reversing the connections.

Some experimenters are tempted to omit the 82k resistor which discharges the capacitor on the theory that it wastes precious signal power. With a typical germanium diode, this little "improvement" may work somewhat but only because the diode has significant leakage and the performance will not be predictable. A dynamic earphone may be DC coupled eliminating the need for the resistor.

The coil may be wound on a 1.5 inch PVC pipe coupler as shown in the drawing. These typically have an outer diameter of about 2.2". Drill two small holes at each end to secure the ends of the coil. The wire type is not particularly critical but select a gauge and insulation so that the 65 turns cover about 2/3 of the coupler. An excellent choice is 30 AWG "wrap" wire from Radio Shack. The prototype uses this solid conductor wire with blue insulation. This wirewrap wire is available in 50' lengths on little spools and about 37' will be needed. A "loopstick" coil may be used in place of the coil shown. These coils have an adjustable ferrite core for tuning so a fixed value capacitor may be used in place of the variable capacitor shown. The coil, capacitor and a terminal strip for the other parts may be mounted to a small wooden board. (See photo of receiver with transistor amplifier below.)

If a metal chassis is used then the coil must be mounted horizontally and above the metal to prevent unacceptable loading.

Here are some alternative construction ideas:

Fahnestock clips make excellent connectors for the antenna and ground wires. The coil may be mounted above the board or chassis with angle brackets by adding another bend, as shown below. The windings may be quickly secured with a single layer of colored "Duck" tape that is now available in more attractive colors than gray or black. The taps to the coil can be located at the rear, near the bottom so that the unavoidable bulges in the tape don't show. An ordinary piece of wood may be quickly finished by applying adhesive-backed PVC film intended for kitchen cabinets. Just stick it on and trim flush with scissors.

One Transistor Amplifier/Detector

An amplifier may be added to boost the audio level as shown below. The current consumption of this amplifier is quite low and a power switch is not included. Disconnect the battery when the receiver is stored for long periods.

© 1995, Charles Wenzel

Note: You may use the transistor above as a sensitive detector eliminating the need for the 1N34A diode. Simply leave out the diode, the 0.001 uF, and

the 82k resistor. Connect the negative side of the 1 uF directly to the coil. Change the base resistor from 10 meg. to 1 meg. and change the collector resistor from 100k to 10k. Now add a 0.01 uF from the collector to the emitter and the modifications are complete. This detector is quite sensitive and will be overloaded by very long antennas! Use a shorter antenna or a coil tap very near ground if significant distortion is noticed. The circuit draws about 1/2 mA.

Crystal Radio Audio Amplifier

Here is a simple audio amplifier using a TL431 shunt regulator. The amplifier will provide room-filling volume from an ordinary crystal radio outfitted with a long-wire antenna and good ground. The circuitry is similar in complexity to a simple one-transistor radio but the performance is far superior. The TL431 is available in a TO-92 package and it looks like an ordinary transistor so your hobbyist friends will be impressed by the volume you are getting with only one transistor! The amplifier may be used for other projects, too. Higher impedance headphones and speakers may also be used. An earphone from an old telephone will give ear-splitting volume and great sensitivity! The 68 ohm resistor may be increased to several hundred ohms when using high impedance earphones to save battery power.

To use the circuit as a general-purpose amplifier, apply the input signal to the top of the potentiometer. (Leave out the diode and .002 uF capacitor.) A higher value potentiometer may be used for a higher input impedance.

© 1995, Charles Wenzel

Crystal Radio RF Amplifier

For the more experienced hobbyist...

One of the best places to add a transistor to a simple crystal radio is at the front end in the form of an RF amplifier. The circuit below is a simple but effective amplifier which will give surprising performance improvement. This amplifier can exhibit negative resistance for low settings of the 500 ohm pot which results in extra gain or even oscillation. So, the circuit can actually be considered to be a regenerative receiver with an external detector. The sensitivity is so high that no cold water pipe ground is needed and the antenna is short.

The behavior of the amplifier depends on how it is connected to the tuned circuit. When connected to a lower impedance tap as shown in the schematic, the gain will be lower with less tendency to oscillate. Higher taps or even connection directly to the antenna will give higher gain and even oscillation. The 500 ohm pot is adjusted to give adequate gain without squealing as stations are tuned. High regeneration settings will actually narrow the bandwidth of the tank enough to give the sound a "mellow" quality which sounds pretty good in a "tinny" crystal earphone! Lower settings are best when using an audio amplifier and the fidelity is quite good thanks to the linear detector (typical regens use changes in the operating point of the transistor to demodulate the RF). As with any regen, the gain

may be increased after the station is tuned in and the circuit will oscillate, locked to the station's frequency.

Current consumption is about 1 mA which may be reduced by increasing the 1.8k but the RF envelope begins to distort below about 500 uA. A 4.5 volt battery may be used if the 220 k resistor is reduced to 68 k. The transistor may be just about any NPN small-signal transistor. No ground is shown but performance is better with a good ground connected to the bottom of the tuner. Longer antennas should be connected to taps instead of across the whole coil. A ferrite loopstick will pick up stronger stations with no antenna at all but use more audio gain after the diode detector and reduce the regeneration to get adequate bandwidth or the sound will be muffled.

Very High Gain Crystal Earphone Amplifier

This simple, one-transistor amplifier provides a voltage gain over 1000 (60 dB) for driving a high impedance ceramic (crystal) earphone. The high gain is achieved by replacing the traditional collector resistor with an unusual constant-current diode that supplies 1/2 mA yet exhibits a very high resistance to the audio. This amplifier will give excellent battery life, drawing only 500 uA.

Below is a typical application using it with the first crystal radio circuit on this page. The amplifier provides good volume with a modest antenna. You may want a volume control as with the TL431 project!

Or use the Crystal Radio RF Amplifier directly above for even more sensitivity with less than 2 mA current drain.

Simple Two-Transistor Radio

Here is a simple radio that was designed to minimize unusual parts; there isn't even a detector diode! The sensitivity isn't as high as the one-transistor reflex but the simplicity is attractive. Strong stations will provide plenty of volume into a crystal earphone or an external amplifier.  The AM Loopstick was purchased on eBay but the enterprising experimenter can swipe one from the interior of a cheap radio.   If the loopstick has more than one winding, use the one with the most turns. Wind 3 or 4 turns near one end of the winding as seen in the photo. The tuning capacitor in the prototype is from an old radio and the little plastic dial was cut down such that it just fit into the back of a black pointer knob. The fit was tight so no glue was needed. All of the sections of the capacitor were connected in parallel to get the most capacitance for this loopstick.

All the other parts are common. The transistors can be just about any small-signal type.  The prototype uses the metal can 2N2222, primarily for looks. Some transistors may have too much high frequency gain; if the circuit squeals, try adding a small resistor in the emitter of the first transistor, maybe 47 ohms, the smaller the better as long as the circuit is stable. The large 47 uF could be smaller in most cases but the circuit can pick up hum if the wires are too long. Don't leave out the large capacitor across the battery, it provides needed low power supply impedance.

The circuit is built on a piece of 3.8" x 2.7" x 0.5" stained and varnished oak. The terminals are copper-plated nails used for weatherstripping. These nails are commonly available in home improvement stores and are also available in brass which is also solderable. Predrill the holes to make nailing easier and use a nail set or larger nail turned upside-down to make it easier to hit only the desired nail. The loopstick is held in position by an ordinary nylon cable clamp and the battery is mounted in a spring holder.  The front panel is aluminum that was polished to a nice shine. First sand off all scratches with fine sandpaper. Then remove the sanding marks with ordinary kitchen steel wool. Now polish the surface with the finest steel wool in the paint department, usually "000". Then, for the real shine, polish the surface with a polishing compound like rouge. By the way, those paper-wrapped sticks of polishing compound are easily dissolved by lighter fluid (naphtha). Just put a few drops on a tissue and rub it on the end of the stick to load the tissue with compound. These polishing steps go quickly and you can have a mirror finish in a couple of minutes.

The front panel has a couple of scales for the tuning and volume printed on high gloss report cover stock and sprayed with a protective clear spray. The feet on the bottom are recessed in the oak using a forstner bit. The corners on the aluminum and oak were rounded on a belt sander.

Experiments with Detector Diodes

When building crystal radios or other simple receivers, the experimenter often wonders about the relative performance of the different diodes in the junk box. Here are the results of several experiments using the typical types available to the hobbyist. The source is a low impedance and the load is a fairly high impedance. A particular diode will behave differently with different impedance levels but for low received signal levels these measurements are fairly predictive of the relative performance in most circuits. The diode types include germanium, silicon, Schottky, and even a light-emitting diode! The

test setup uses an accurate RF synthesizer, a homemade AM pin diode modulator driven by an audio generator, a simple test fixture, a DC power supply for adding bias current, and a sensitive audio voltmeter. The setup shown below was used to test the diodes at several frequencies with a low modulation index (about 20%) and the near optimum bias current was determined by varying the DC supply.

Initially an RF level of about -15 dBm was used but this level was dropped to -25 dBm without much change in the relative results. The best performance was provided by an H-P (Agilent) diode, the 5082-2835 with a tiny 10 uA of DC bias. The -25 dBm (35 mV p-p) results are shown below using the 5082-2835 Schottky diode at 20 MHz as the 0 dB reference point. The dB numbers are the audio level at the audio meter for the different RF carrier frequencies. Some variability is due to the test setup.

Diode Part Number

DC Bias

20 MHz

60 MHz

100 MHz

130 MHz

Notes

5082-2835 Schottky

10 uA "0" dB 0 dB -2.5 dB -4.5 dB quite good!

1N5711 Schottky 10 uA-0.5 dB

-0.5 dB

-2.0 dB -3.5 dB better at high freq.

1N4454 silicon(similar to 1N914)

20 uA-8.5 dB

-9.5 dB

-10.5 dB

-11.5 dB

pretty bad!

1N277 (Ge.) None-3.0 dB

-4.0 dB

-6.5 dB -8.5 dBnot great at high freq.

1N34A (Ge.) None-3.0 dB

-4.0 dB

-6.5 dB -8.5 dB .

1N32 10 uA-1.0 dB

-1.0 dB

-3.5 dB -5.0 dB microwave diode

Red LED 10 uA-4.0 dB

-4.5 dB

-8.0 dB -11 dB not bad for low freq.

Note: -3 dB means that the audio voltage dropped to about 0.7 of the 0 dB level and -6 dB would be a drop to about one half.

The Schottky diodes are the all-around winners if bias is used but they do not perform as well as a germanium diode without bias. Other small-signal Schottky diodes gave nearly the same results as the 1N5711. Other germanium diodes were tried but the results were nearly identical to those shown. (The 1N60 was not available for testing.) The 1N4454 is similar to other ordinary silicon diodes and the results were poor, as expected. Several LEDs were tried and a bright type red led gave fairly good results as shown. Zeners were tried with dismal results in both directions. Large rectifiers like the 1N4001 were similarly poor. 

Misc.:

The optimum bias current is tiny and a very small battery can be permanently wired into a "crystal" radio without a switch. A little photocell battery like those on tiny calculators could do the job. Schottky diodes will work better without bias if higher source and load impedances are achieved. Germanium diodes are hard to beat for most low frequency crystal radio designs if no bias is desired. Schottky diodes don't need bias when heated by a soldering iron! (Hardly practical information.)

Lately, I've been building crystal radios like the early radio pioneers used. Crystal radios require no outside sources of power to operate, just the radio waves themselves. The first crystal set I built was a "variometer" radio. In this design, one larger coil slides over a smaller coil to change the coil's inductance, eliminating the need for a variable capacitor. The radio I'm building has two smaller coils, making it more selective (?). I got the design from the May 1995 issue of Popular Electronics. The coil forms are made from PVC pipe. The smaller, longer coil which holds the two smaller coils is 8 inches long and 1 inch inside diameter. The shorter, larger coil which slides over the other pipe is 3 inches long and 1.5 inches inside diameter. The other components are installed using a long terminal strip. At first the radio didn't work because the detector diode, which should be a 1N34 germanium diode, was a Zener diode. Luckily, someone sent me some 1N34A germanium diodes, and now it works fine. Since I live in a "fringe" area for AM signals and my only set of high-impedance headphones has an intermittent connection in the "junction box" where the headphone cord is split to the two 'phones, I've added an onboard one-transistor amplifier, whose design I found in the Boys Second Book of Radio and Electronics. Unfortunately, it has some problems. When using a crystal earphone as a microphone, the output is soft and unintelligible. The problem could lie with the transistor, since I'm using a 2N217 (ECG102A) when the original circuit calls for a CK722 (ECG102). The ECG102A is a more rugged version of the ECG102, but the voltages are different. Reducing the bias(?) resistor from .39 Meg to 27K and doubling the voltage from 3V to 6V hasn't made a difference. A schematic for both can be found below.

My second crystal radio is a much-simpler "slider" set. This type of crystal set uses a single coil and a

"slider," which is a piece of metal which slides over the tuning coil to tune the radio. Like my other crystal set, all the parts are mounted on a terminal strip. Unlike my other crystal set, this one uses a square wooden coil form mounted diagonally. I found the design for this set in The Boys First Book of Radio and Electronics. This radio worked on the first try. This surprised me since the coil, which called for 130 feet of 26-gauge wire, has roughly 51 feet of 22-gauge wire (I had no 26-gauge wire so I used what I had, which was 22-gauge wire). This crystal set isn't quite as selective as it could be, but it works just as well as my variometer crystal set (possibly better). A schematic of this set can be found below.

My third crystal radio project is a simple ferrite coil and variable capacitor design. I was originally planning to build this radio in an "Altoids" mints tin. I salvaged the coil and variable capacitor from a failed AM/FM radio kit I once tried to build. The FM section, which came on a prebuilt circuit board, would tune from what I believe is below the FM broadcast band to the very beginning of it, while the AM section, which didn't even have a detector diode, would only get one station, with the tuning capacitor only varying the volume of the station. Instead of the 2SC transistor-based "preamplifier" the AM section of the AM/FM radio kit used, which I suspect is what caused the tuning problem, I'm going to be using a true detector diode. I was hoping the metal tin wouldn't cause problems with the antenna coil. Unfortunately, mounting the ferrite coil in the tin seemed to be causing the coil to act like a wavetrap, blocking AM signals and only allowing shortwave signals (I was surprised to hear CBC and BBC World Service drifting in and out of my headphones). Also, the variable cap did nothing, and it got two or more signals at once. I transplanted the innards to a Radio Shack plastic enclosure, hoping it would fix the problem, but it didn't change. I may have to redesign the circuit. A schematic of this set can be found below.

The fourth crystal radio project is a short-range FM crystal set. This is the most complex crystal set I've built to date. No easy-to-work-with wooden boards or small plastic enclosures involved; just a small tuning capacitor, a 12" piece of #10 copper wire (for the coil), a diode and a capacitor. I mounted all the parts on the tuning capacitor, including the coil (which was extremely difficult to attach). I bought the parts as a kit from Bill Turner. I haven't tried it out yet. A schematic can be found at http://www.somerset.net/arm/reprints/fm_crystal_set_1.jpg.

So far I've built three crystal sets and I'm in the process of building another. All I need is a better pair of headphones and possibly a better antenna (my current antenna is a 25 foot long bare copper wire) and/or ground (currently I use a baseboard heater). More later!

Other crystal radio pages

The Xtal Set SocietyCrystal Radio Resources

A "Free-Power", Batteryless, One-Transis or AM Radio that

6/1/2007 by Rick Andersen, KE3IJ

Here's something that might intrigue you Crystal Radio builders out there: A one-transistor (Silicon, yet!) AM broadcast receiver that derives its power from ambient AC hum and 'spherics [static crashes]!

Gotta give credit when I can: The germ of the schematic was borrowed from Darryl Boyd's "Stay Tuned" website which features over 170 articles and plans on crystal radio construction. The URL for the site is http://www.crystalradio.net/crystalplans/ and my No-Power -- or, more accurately, Free-Power -- circuit was derived from #2 of the 3 circuits found in the site's project #153, "How to Build Free-Power Radios." That website has three jpeg pages which are copies of the original article, by Terry L. Lyon, from an old Popular Electronics magazine issue. The 2nd page is the page containing the 3 schematics of single-transistor, free-power receivers. My version has some modifications of the original circuit, which I will describe below.

Terry Lyons' original circuit was designed around a common-emitter amplifier, with emitter grounded; mine has a 100K ohm potentiometer inserted between the emitter and ground. The wiper of the pot connects to a tap on the antenna coil, 5 turns up from ground, with the pot determining how much of the emitter current is permitted to flow through the coil winding in a Hartley Oscillator feedback configuration, so that a fraction of the incoming RF signal is fed back into itself after having been boosted by the transistor. With enough battery voltage present, advancing the pot to full clockwise would connect the transistor's emitter directly to the coil tap, causing the circuit gain to increase through positive feedback (regeneration) until the circuit "plops" into oscillation at the tuned frequency. In our "free-power" receiver, there isn't enough voltage or current to achieve stable regeneration to the point of self-oscillation; instead, we get "close": the volume comes up on our radio reception, to a point where we begin to hear a lot of clipping distortion that sounds like "clicks" on voice peaks. If we back the regen ("Sensitivity") control a little, we clarify the sound but lose a little volume. This is the optimum setting of the Sensitivity control. Your mileage may vary.

Lyons' circuits were predicated upon the fact that using a 10 Megohm resistor as the collector load kept the small power supply voltage -- the charge accumulated, rectified and stored on the filter cap -- from being bled away too fast. He also used a very high value resistor between collector and base. I found out, by trial and error, that using a 1N914 silicon diode in the resistor's place, gave a louder signal. Also, I found that reducing the collector resistor from the original 10M (!) down to 470K, gave me quite a bit more sound volume--although it did hasten the bleeding away of accumulated charge. But I found I could live with it. (By the way, I did most of my experimenting with a Radio Shack Amplified Speaker hanging off the audio output. The crystal (ceramic) earphone I had lying on the bench also worked, as did a pair of stinky old 2000 ohm magnetic headphones... just make sure that the .47uF blocking cap is in circuit. If you try to connect the magnetic phones directly to the transistors's collector and ground, the phones bleed the charge away very quickly and you end up with a dead receiver!

I tried using Germanium diodes as rectifiers-- and, contrary to what I expected [they have a lower turn-on threshold than Silicon diodes -- .2v vs. .6v-- so I thought they would be more effective than the Silicons. Wrong. They only seem to generate tens of millivolts, whereas Silicon diodes in the same circuit provide a fairly constant 450 - 600 mV.

The circuit can actually be brought close to the edge of oscillation, but this attempts to pull more current than the "atmospheric supply" can deliver, so you will hear a sharp, clicking or rattling distortion rather than plopping into oscillation, when the Regen (Sensitivity) is at max.

One thing that enhances performance a lot is a series L-C in the antenna lead (another ferrite bar coil of around the same inductance [240uH or so] in series with another 365 pF tuning cap, which acts as a resonant antenna tuner, and boosts signals very nicely at the low end of the tuning range.

Very interesting (if theoretically dangerous) how the +600 mV rectified DC that powers the receiver, fluctuates during lightning storms, if you monitor the 10uF filter cap with a DVM connected across it!

I won't claim that this receiver is perceptibly 'louder' than a good single-diode crystal receiver with a Germanium 1N34, but it's amazing to me that this circuit works at all! I'd like to try it with a Germanium transistor but I don't have any yet. Meanwhile, it's neat to just leave the receiver "on" all the time, with no battery, and it sounds pretty nice through a Radio Shack Amplified Speaker.

It is essential that the antenna be nice and long and that an EARTH ground be used (or a cold water pipe). We're trying deliberately to pick up as much AC hum as possible, since it is the power source!

Let me know if you build this, and how it works for you.

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