One Transistor Radio

The one transistor AM radio receiver is something all whippersnappers try out when starting out on the royal road to radio building. This circuit is the one that I designed and made when I was a little whippersnapper.

Although this circuit uses as transistor to amplify the signal from the crystal radio, it still requires a crystal earpiece. The transistor will amplify the signal, and you will be able to hear BBC Radio 4 very clearly.

The transistor used in this circuit is the common garden variety BC549C capable of providing a maximum gain (hFE) of 800. However, BC547, and BC548 general purpose NPN transistors might also work as well.

You could also try other transistors for experimentation, such as the 2SC3112 capable of providing a gain (hFE) of 3600. If you have an MPSA18 high gain transistor, then that transistor is capable of providing a maximum gain (hFE) of 1500. 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.


Circuit Diagram

One Transistor Radio Circuit Diagram

As you can see in this circuit diagram, I have a centre tap in the ferrite coil from where I am taking the signal.

The diode demodulates the RF signal by removing the carrier, and then the signal becomes AF (audio frequency).

The detector diode also produces an average value, which is a DC component of the signal. We need to block that and amplify only the AC component by using the 10-µF capacitor. You could try electrolytic capacitor values down to 1 µF and it should still work.

The general-purpose transistor BC549C then amplifies the AF signal, and you can hear it more loudly through the crystal earpiece.

The power supply is a single 1.5 V cell. You can use any sized single dry cell you wish for the power supply.

Ferrite Coil

The coil wire I am using is Litz wire, which is the proper radio coil you should use to get a strong signal. I have wound 35 turns on either side of the centre tap. All the turns must be in the same direction.

The ferrite rod is a standard sized one, with a 10 mm diameter.

Have a look at the Spy Crystal Radio – Budget Cuts article for more information about the ferrite coil, because it is identical to that.

Project

If you are getting a strong reception, as a crystal radio, then you should move on to build the transistor amplifier stage.

This is one of those projects ideal for those studying GCSE Design and Technology, or Physics at school.

8.2 MΩ Biasing Resistor

The BC549C has a maximum DC current gain of 800, with Ic of 2 mA. If you want to squeeze all of that amplifying capability then the biasing resistor has to be very large.

The base junction of the BC549C transistor is biased by a positive charge from the very large 8.2 MΩ resistor. The reason why it is so large is that it requires a very tiny amount of positive charge to polarise the base junction.

When you connect the battery, it will take around ten seconds for the base junction to charge up, and then you will start hearing the sound getting progressively louder. It is almost like a valve radio warming up. :-)

If you are having problems with this then you could reduce it down to 1 MΩ, but it will not be as loud.

Germanium Diode D

The germanium diode in this circuit is for testing the reception of the tuned circuit by temporarily making a crystal radio. A germanium type, such as the OA90 that you can get from many electronics stores including your local Maplin shop (Code: QH71N) works fine. Please consult the Crystal Radio Diode article for more information.

If it is operating fine as a crystal radio, then you can progress forward to build the transistor amplifier. Do not build the transistor stage, if it does not work as a crystal radio.

Experimenting

If you are getting a strong audio signal through the crystal earpiece, and your circuit is working, then do some experimenting. Remove the germanium diode, and feed the signal directly to the capacitor, and see if it still works. You will find that the transistor still demodulates the signal without the diode!

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. What if you reverse the polarity of the electrolytic capacitor, does it make any difference?

Just experiment with the circuit and enjoy yourself, because that is what I did when I was a whippersnapper building transistor radios.


Project

If you are getting a strong reception, as a crystal radio, then you should move on to build the transistor amplifier stage.

This is one of those projects ideal for those studying GCSE Design and Technology, or Physics at school.

Related Pages

One Transistor Radio
One Transistor Radio 9 V
Crystal Radio
Spy Crystal Radio - Budget Cuts