Replace the electrolytic capacitor with a ceramic or polyester (non-polarised capacitor) in the nano-farad (nF), and pico-farad (pF) ranges to see how much impact these values have on the signal.What if you reverse the polarity of the electrolytic capacitor, does it make any difference?. What would happen if we replace the electrolytic capacitor with a direct link to the base junction of the transistor? You will see that the transistor no longer amplifies.However if you disconnect the battery, then the sound stops. You will find that the transistor still detects the signal without the diode! This is becasue the pn junction of the transistor behaves as a diode. Remove the germanium diode, and feed the signal directly to the 10 ♟ electrolytic capacitor, and see if it still works.If you are getting a strong audio signal through the crystal earpiece, and your circuit is working, then do some experimenting. Hence it is best to start by building the crystal radio first. You can use any size single dry cell you wish for the power supply.Ĭomparing these two circuits, we can see that they have identical components, and the one transistor radio circuit is just one small step further from the crystal radio circuit. The general-purpose transistor BC549C then amplifies the AC component of the signal, allowing one to hear it more loudly through the crystal earpiece. I am using a 10 ♟ value simply because this is very common to have. A value in the ♟ range will have negligible effect on the signal, and one can use a wide range of values. Since I simply wish to block DC I chose to use an electrolytic capacitor, which is usually something all hobbyists tend to have. One good reason for blocking the DC component of the signal is that it adversely affects the biasing of the transistor and this is undesirable. We need to block this DC component and allow only AC through by using an electrolytic capacitor. The demodulation process also produces an average value, which is a DC component of the signal. The diode demodulates the RF (radio frequency) signal by removing the carrier, leaving the meaningful AF (audio frequency) part in AC form, and this is what we need to amplify using the transistor. The ferrite rod is a standard sized one, with 10 mm diameter. All the turns must be in the same direction. Should use to get a strong signal, and I have wound 45-turns on either side of the centre tap. For the coil wire, I am using Litz wire, which is the proper radio coil you Back when I was a whippersnapper, I had an old Mullard Transistor Databook for reference and I was always experimenting with different high gain transistors to make my crystal radio louder.Īs you can see in this circuit diagram, I have a centre tap in the ferrite coil from where I am taking the signal. If you have an MPSA18 high gain transistor, then that one is capable of providing maximum gain (hFE) of 1500. You could also try other transistors for experimentation, such as the 2SC3112 capable of providing gain (hFE) of 3600. However, BC547, and BC548 general purpose NPN transistors might also work as well. The transistor used in this circuit is the common garden variety BC549C capable of providing maximum gain (hFE) of 800. Although this circuit uses a transistor to amplify the signal from the crystal radio, it still requires a crystal earpiece, and you will be able to hear BBC Radio 4 very clearly if you are based in Croydon, UK. This circuit is the one that I designed and made when I was a little whippersnapper. The one transistor AM radio receiver is something all whippersnappers try out when starting out on the royal road to radio building.
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