Using modern technology..........
Build this simple crystal receiver getting experience and fun with the interesting CMOS Integrated Circuit 4011.
This radio receiver has been implemented to demonstrate the capability of CMOS NAND gates to amplify analog signals up to a frequency ranging to one megahertz. Usually these gates are always driving digital circuits, but you can use them as small power amplifiers with excellent results in the audio frequency range.
The number of components to build an audio amplifier with a NAND gate is actually only three, or even two...which is a great advantage compared with the number of components needed to mount any other small power audio amplifier.
Here is the basic schematic to build an audio amplifier with a NAND gate
There is an input capacitor of 100nF and two resistors. The amplification can be regulated choosing the resistors values. The input resistor R1 must be of 1000 ohms, or even suppressed in case of faint audio signals, like those obtain from a germanium detector. Resistor R2 can be a potentiometer from 5 to 10 meghoms.
THE INTEGRATED CIRCUIT 4011
We obtain the amplifier from one of the four NAND gates inside the 4011 IC. This chip comes in a 14 pin capsule. The three other gates in the IC are not used. Though we use this integrated circuitry as an analog amplifier, its normal functions are those of a digital circuit. That means that in any of the four gates when the inputs are high (about + rail voltage) the output goes low (near the zero volts), and when the inputs are low the output goes high.
The operating voltage of the 4011 IC is from 3 to 15 volts applied to its # 14 pin. Among the outstanding characteristics of this IC is the extremely high input impedance of its NAND gates that goes to about a million megohms, which in the building of our small radio receiver means an exceptional selectivity. The different transmitter we tune are perfectly differentiated and without any kind of overlapping.
Precautions need to be taken to avoid damaging the 4011 IC internal circuitry
1) Never connect the polarity of a power supply the wrong way. Plus voltage goes to pin # 14, and minus or zero voltage to # 7 pin.
2) Before connecting any signals to the inputs, the integrated circuitry has to be plugged to its feeding source.
3) The voltages of the input signals can never be greater than the voltage applied to pin # 14.
The Middle Wave NAND Gate Receiver Schematic
COMPONENTS FOR OUR NAND GATE RADIO RECEIVER
|1... A ferrite rod antenna with coils to tune the medium wave broadcast band.||6... A potentiometer of 4.7M.|
|2... A variable capacitor of 460pF's.||7... The 4011 CMOS IC.|
|3... A detector germanium diode like an OA90/1N34 or its equivalents.||8... A high impedance earpiece (crystal earphone).|
|4... A 100nF ceramic or milar capacitor.||9... A nine-volt battery.|
|5... Another 100pF capacitor.||10... A socket for the earphone.|
I took the ferrite rod antenna from a discarded transistor radio. These antennas have two coils; one of them with the greater number of turns has a resistance about 2.2 ohms and other with less turns has only about 0.6 ohms. There are usually three terminal leads in these transistor antennas. I separated carefully the terminals to differentiate clearly the coils, so I have got four terminals, two for the 2.2 ohms coil, and another two for the 0.6 coil.
The terminal leads of the 2.2 ohms coil connect in parallel with the tuning capacitor. One terminal lead of the 0.6 coil goes to the diode anode; the other is plugged to ground.
BUILDING THE RECEIVER ON A PROTOTYPE BOARD
To explore the behaviour of a NAND gate as an amplifier in this radio I used a prototype board . Now that the value of components chosen allow a good performance of the circuit, the radio can be built on a more permanent set and even enclosed in a small box.
Below is a picture of the 460pF variable capacitor used in the project. Most of the components plug directly on the board. But the variable capacitor, the ferrite rod antenna and the socket earphone needed some help. So I have built for them small panels with appropriate pins to be inserted on the prototype panel.
Thus this radio receiver shows as a compact set and you can move it easily among the rooms in your house to detect the place of better reception...or take it outside and experiment in the open field. Because its ferrite rod antenna, there is a strong directional effect that allow you to increase or diminish the audio volume ( sometimes too strong ) turning the receiver left or right in a horizontal plane. The 4.7 meghoms potentiometer is also the volume control.
The receiver works an ample range of voltages from 3 to 15 volts. The greater amplification is obtain with the higher voltages being the most appropriated... 9 or 12 volts. With 9 volts the current drawn from the battery or power source is only about 4 miliampers.
Working with digital CMOS IC's it is a good and necessary practice to join the unused inputs to the ground rail to avoid spurious electromagnetic and static influences as well as spontaneous oscillations. So in this receiver the unused input pins 5, 6, 8, 9, 12 & 13 must be connected to the ground rail in case you are mounting the set in a more permanent way than temporary on a prototype board.
You may note in the schematic...a feedback capacitor of 18 pF. This capacitor can be up to 100 pF. The same happens with the potentiometer used as a variable resistor which can be increased up to 10 meghoms, though if you advance its' shaft to the highest values, the amplifier enters into oscillation.
Finally if you decide to experiment with digital NAND gates as analogue amplifiers, we (Patrick and myself), are highly interested in knowing your discoveries.
|This special application of NAND gates is not frequently found in electronic books or even on the web!|
With this project, I wish you success and hours of fun at your workbench ! ! !
...your friend, Pedro
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