Transistor equivalent

Tony Stewart said:

An old RED LED is 1.2V
An IR LED is 0.8V threshold
Silicon LED 0.5 to 0.6
Schottky diode 0.1V is best bet, Typical examples are the 1N5711 or BAT41.

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I found the normal voltage drop of 1N5711 is around 350mV to 400mV so that it's not working in a simpler batteryless crystal radio. Power rectifier like 1N5819 (1A) has just around 170mV drop and working very nice.

I made crystal radio just for fun and for teach my younger brother. FM radio I will made a regen (or super-regen???) and superhet. Has regen or superhet make better sound and sensitive than another??? always adjust gain on regen radio not a problem

A superhet is a REAL radio. It has many tuned LC circuits for good selectivity ( no interference from other stations), has many RF amplifier stages for excellent sensitivity to pickup weak distant stations and has no overloading from strong local stations (if the radio is designed correctly). A regen or super-regen is a single transistor oscillator pretending to be a radio. The oscillation causes interference to other FM radios. It has only one LC tuned circuit so it produces interference from other stations. You must increase its gain to pickup a weak distant station (but with interference from strong local stations) then reduce its gain to avoid overloading from a strong local station. It produces distortion and a tinny sound because it is missing de-emphasis that is on all real FM radios. It actually has an AM detector (not an FM detector) so it picks up amplitude noises. You tune it to the side of a transmission for its AM detector to "slope-detect" the FM. A super-regen turns its oscillation on and off at a high frequency to adjust its gain that frequently causes whistles when the station is in stereo (super-regen FM circuits were designed before FM stereo was invented).

A super-regen receiver is used in a child's very cheap radio-controlled car.

I credit my father for making me a pickle jar crystal radio, when I was 12, so I could listen to AM radio , going to sleep and no batteries needed. I could easily pick up radio stations to the nearest city 200 miles away with megawatt output.

Not only did it sound good but the crystal earbud had great bass response when it fit tightly near the drum and also served as lightning storm monitor, hundreds of miles away. The antenna wire went outside to a tree.
Thanks to my dad for the inspiration.

Ten years later, I was married with a child and a job in Aerospace R&D, because of that pickle jar. ( the job, not the child)

Willen said:

I found the normal voltage drop of 1N5711 is around 350mV to 400mV so that it's not working in a simpler batteryless crystal radio. Power rectifier like 1N5819 (1A) has just around 170mV drop and working very nice.

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Yes that's a Schottky diode.

Maybe others can relate to this cartoon.
https://obrazki.elektroda.pl/7411381500_1446053626.png

I added a 1N5817 Schottky diode in series with a voltage stepup circuit of a solar garden light a week ago and today its output voltage and current were low.
Its forward voltage measured 150mV with the low current of the meter but it also conducted a little in reverse. Its reverse leakage current was much higher than it should be.
I do not have another Schottky diode with me so I used an ordinary 1N4148 silicon diode and the circuit works fine.

audioguru said:

Its reverse leakage current was much higher than it should be.

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Schottky diodes do have much higher reverse leakage than other types.

Schottky diodes increase reverse leakage current ;
a) with temperature (~x5)
b) with reverse voltage (~x5)
c) as Vf spec gets lower

ON Semi rate the 1N5817 as 1mA at 25'C so these would perform poorly for a garden lamp using the diodes as a charge pump or boost regulator.
But with zero bias, would be OK for a Crystal radio.

But 1n4148 worked. Signal too strong

Nikolai Petrenko said:

But 1n4148 worked. Signal too strong

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By heating it? Or biasing in a battery supply? Can you post a circuit?

At night I can see lights on the towers of a very powerful AM radio station from my home, the station is about 9km away. If I was closer and if my antenna is long enough and is in the correct direction then I betcha a 1N4148 silicon diode could detect it very well.

I have 10 solar garden lights that use a 0.8V to 1.4V Ni-MH AAA cell and a voltage stepup circuit using a "blob-on-board" IC. The output of the IC is swinging up and down at 500kHz which is fine to light a single color LED. But I am using color-changing LEDs that have an IC inside that drives them and this IC needs a smooth supply voltage. So the output of the voltage stepup IC feeds a series 1n5817 Schottky diode and a 0.1uF ceramic capacitor to ground. These solar garden lights worked fine (half were Chinese and the other half I modified with added color-changing LEDs, diodes and capacitors) but one 1N5817 Schottky diode failed with high leakage current. I replaced the 1N5817 with an ordinary 1N4148 and it works fine.

not heatting, just cool. I found a random 600ft antenna near my house, it is an unused telephone wire. radio station to my house about 22km

Haha how easily you made 600ft antenna! And it's amazing the 4148 is working even the station is 22km far! I am thinking how loud audio could get using a 1N5819 there!

radio station has huge power,

Nikolai Petrenko said:

I am searching something can replace these transistors:
BF199, BF494, 2N4001, 2N4003, MPSA18
And there are all I have:
2N3904, S9012, S9014, S9015, S9018, 8050, 8550, C945, D965. But 8050 and 8550 seem by fit to make push-pull amplifer.
All I need are for radio receiver or RF, light... detetors.
I also want to know can 1N4148 replace 1N34 in crystal radio. I can't find 1N34, no stores I asked have 1N34 or other germanium diode. ( I can make 300ft antenna and good ground for strong signal ) ( 1N4148 heated or red LED in series with 1,5v cell will be good idea??? )

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Hi Nikolai Petrenko,

I don't fully understand your questions about transistor equivalents but here is some information about crystal radio diodes:

I haven't any hands-on experience of crystal radios, but it is clear that the detector diode characteristics are the critical issue. As has been implied by the other posts, the forward voltage (VF) of the detector diode defines the lowest amplitude signal that will be detected. This will not be a gradual thing: below the detector diode VF you will get nothing. It's only when the peak of the radio signal sine wave reaches the forward VF that you will get an out put. But another important characteristic is reverse current (IR), because it's not much good if you pump a nice charge into the detector capacitor on the positive half cycle of the radio signal only to have it sucked out again on the negative half cycle.

As stated already, the forward voltage (VF) of germanium diodes is 300mV and 600mV for silicon diodes. Schottky diode VF is also 300mV, which means that they should make good crystal radio detector diodes. But one disadvantage is their high IR. But the IR of small signal Schottky diodes is similar to germanium diodes. Generally, with semiconductors there is a trade-off in parameters. For example, you may get a Schottky power supply rectifier that will handle 10 Amps but it could have an IR as high as 5mA. This, among other factors, means that they would be totally unsuitable for crystal radio detectors.

I'm afraid to say that, in theory anyway, any form of light emitting diode (LED) simply has the wrong characteristics for this job.

The 1N4001 series and 1N5189 series of silicon rectifiers while, having some advantages, have too higher VF and IR and probably to higher capacitance. The 1N4148, 1N916 etc are high speed, small signal diodes with low capacitance but they are also silicon and therefor have a relatively high VF.

With all this in mind, I had a look at some small signal schottky diodes and put together a set of graphs from the datasheets of four likely candidates as shown in the attached image. Going by the graphs the SD103 looks particularity interesting as it has the lowest VF. Mind you, it also has the highest IR as a consequence, but I don't think that it's enough to cause a problem. Anyway, by definition, the detector reverse voltage in a crystal radio is very low, so this will keep the reverse current low in practice. The only slight problem with the SD103 is that it is only available in a surface mount package, but for experimental purposes just mount it on a small piece of printed circuit board so it can be handled easily.

Here is a list showing the VF at 100uA and IR at -2v for the schottky diodes:
DIODE___________ VF______ IR
BAT41__________270 mV___10 nA
BAT42 & BAT43___200 mV___100 nA
SD103__________180 mV___100nA

You ask about transistors. One thing you could try is to use the emitter base junction of a transistor as a detector diode. You could also make a 'super diode' by connecting the collector to the base. For an NPN transistor the emitter would then be the cathode. For a PNP transistor the cathode would be the base/collector. Its important to use a small signal transistor, like a BC109, BC184 etc. Ideally, a small signal high frequency transistor would be the choice, but make sure the leakage currents aren't high.

In the previous posts heating is discussed. This is a clever approach to lower the VF but just a word of caution; most silicon transistors will only tolerate a temperature of 175 degrees Centigrade and it would be best not to exceed 110 degrees Centigrade for germanium. Heating a diode will lower its VF by by roughly 3mV for every 1 degree C rise in temperature. This means that if you heated a silicon diode from 25 to 100 degrees Centigrade the VF will drop from 600mV to roughly 375mV. The downside is that IR will go up but this may not be a problem in crystal radios.

In theory, the performance of a crystal radio could be radically improved by matching the voltage/current (V/I) characteristics of the signal from the antenna to the input V/I characteristics of the detector circuit. This would require a bit of design work and experimentation, but I suspect it would be fairly simple to do with a miniature transformer which could be made at home. This is the approach used on the more advance crystal radios that I have seen on the internet.

Another, big improvement would be to tune the input to the frequency of the radio station that you want to listen to. This could be done with a singe capacitor if you just wanted to listen to one station or a number of switched capacitors or even a variable capacitor for many stations. I suspect that tuning would not only make the crystal radio selective but could also be designed to produce a higher voltage from the radio signal. This would better suit the diode detector.

Of course, if you were prepared to use a battery, even a small 3V button cell like a CR3032, that would allow some serious improvements and more scope for experimentation.

Hope this is of some interest and that you continue with your crystal radio experiments; they sound like fun!

Willen said:

Haha how easily you made 600ft antenna! And it's amazing the 4148 is working even the station is 22km far! I am thinking how loud audio could get using a 1N5819 there!

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Hi Willen,

I see you are wondering how loud the audio would go. As I said I don't know anything practical about crystal radios, especially the matching of the antenna to the detector circuit but it did occur to me that crystal radios only seem to use the power from one half of the radio signal, so why not use both halfs. The attached image shows the idea. If anyone would like to try it out I'd love to know the results.

Willen said:

By heating it? Or biasing in a battery supply? Can you post a circuit?

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Hello again Willen,

Here is a schematic for a one transistor radio. Once again, this is just a circuit idea and not proven, but I think it has a lot of potential for development and should outperform the single diode type. By biasing the transistor on with around 20μA collector current, the diode VF and IR problems should be greatly reduced, hopefully!

Another advantage is that there will be a better drive for the earpiece and the antenna matching should be better, especially if tuning is designed in.

This approach does need a power supply, although a button cell will do. A few solar cells connected in series would be another possibility, unless you need to listen in the dark that is. Pretty much any supply voltage will be OK as long as the VCE of the transistor is not exceeded. To change the power supply voltage R3 needs to be changed in proportion but the value is not critical. Here are the values:

Voltage____R3
3V_______2M2
6V_______4M7
9V_______7M5

If the high value resistor for R3 is hard to get, let me know; I can change the circuit to suit what you have. Q1 should be a small signal type: BC109 etc. Once again not too critical, although a radio frequency type would be ideal, but it must have a current gain (hFE) of at least 20 at 20μA collector current. It would probably be best to divide the normal value for R4 used in single diode crystal radios by 10 and multiply the parallel capacitor, C4, by 10 to lower the impedance while keeping the same time constant.



ERRATA

(1) The 0.5V at the base is incorrect: I forgot to add the voltage across R4
(2) It may be better to put a capacitor between the emitter of Q1 and the earpiece, say 10nF upwards, but not an electrolytic type.

spec said:

Hello again Willen,

Here is a schematic for a one transistor radio. Once again, this is just a circuit idea and not proven but I think it has a lot of potential for development and should outperform the single diode type. By biasing the transistor on with around 20μA collector current, the diode VF and IR problems should be greatly reduced, hopefully!

Another advantage is that there will be a better drive for the earpiece and the antenna matching should be better, especially if tuning is designed in.

This approach does need a power supply, although a button cell will do. A few solar cells connected in series would be another possibility, unless you need to listen in the dark that is. Pretty much any supply voltage will be OK as long as the VCE of the transistor is not exceeded. To change the power supply voltage R3 needs to be changed in proportion but the value is not critical. Here are the values:

Voltage____R3
3V_______2M2
6V_______4M7
9V_______7M5

If the high value resistor for R3 are hard to get, let me know; I can change the circuit to suit what you have. Q1 should be a small signal type: BC109 etc. Once again not too critical, although a radio frequency type would be ideal, but it must have a current gain (hFE) of at least 20 at 20μA collector current. It would probably be best to divide the normal value for R4 used in single diode crystal radios by 10 and multiply the parallel capacitor, C4, by 10 to lower the impedence but keep the same time constant.

View attachment 95052

ERRATA

(1) The 0.5V at the base is incorrect: I forgot to add the voltage across R4
(2) It may be better to put a capacitor between the emitter of Q1 and the earpiece, say 10nF upwards, but not an electrolytic type.

Click to expand...

Hi,

It looks pretty nice and simple. I will remove emitter resistor because I am going to use my pretty loud dynamic earpiece which is already around 32 ohms. (also I will try with emitter resistor with crystal earpiece with 7V or 9V supply with high swing.) Sure will share result here. Probably I don't need huge antenna here as discussed before. Thank you!

spec said:

Hi Willen,

I see you are wondering how loud the audio would go. As I said I don't know anything practical about crystal radios, especially the matching of the antenna to the detector circuit but it did occur to me that crystal radios only seem to use the power from one half of the radio signal, so why not use both halfs. The attached image shows the idea. If anyone would like to try it out I'd love to know the results.

View attachment 95051

Click to expand...

Pretty interesting! I will share result after few days!

Willen said:

Hi,

...I will remove emitter resistor because I am going to use my pretty loud dynamic earpiece which is already around 32 ohms...... (also I will try with emitter resistor with crystal earpiece with 7V or 9V supply with high swing.) Sure will share result here..

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Hi Willen,

Great to know you will be doing some experimenting.

Just a couple of points though:

The time constant, R4 x C4, helps to recover the audio signal from the radio signal efficiently so you may need a larger capacitor if you put a 32 Ohm earpiece in the emitter. In theory, it should work even without a capacitor but I'm not sure how well. By the way the impedance of a crystal earpiece is around 20k Ohms so there is a radical difference between the two earpieces.

The single transistor circuit will not produce any more voltage than the single diode circuit however high the supply line is. It's effectively an emitter follower which has a voltage gain of 1. What the one transistor circuit should do is to recover the audio from much smaller radio signals and 'not cut off' as badly as the single diode circuit.

As far as the development of the single transistor circuit goes though, it would be best to try the circuit with the high resistance crystal earpiece first and get that working. I suspect that the transistor bias current will be critical and need some adjusting to get the best result. An adjustable bias control may be useful for development and could even be worth having as a permanent feature.

Once the basic concept has been established we could then take the next step and develop a circuit that would make your 32 Ohm or crystal earpiece as loud as you like. To get the maximum voltage swing from a 9V supply you would need +-141 mA peak to drive the 32 Ohm earpiece which is quite a lot of current. It would be extremely loud though and you would need a volume control!

This is what I have in mind for the development circuit:
C5 should make the crystal earpiece sound better by removing the direct voltage and C6 is just decoupling which is always a good thing. Neither value is critical, but C6 should be a high frequency type; ceramic is a good choice.

A time constant of 100 microseconds seems about right to give an audio frequency response to about 5kHz. An R4 of 5k6 and a C4 of 3n3 would suit the transistor and will have a low impedance compared to the crystal earpiece impedance load of 20K Ohms.