UKW-Radio Bausatz FM Radio Construction Set
Imprint © 2020 Franzis Verlag GmbH, Richard-Reitzner-Allee 2, 85540 Haar www.franzis.de Author: Burkhard Kainka ISBN 978-3-645-65287-2 All circuits and programs depicted in this book are developed and tested with utmost care. Nonetheless, it is not possible to rule out all errors in the book or in the software. Publisher and author are only liable in case of intent or gross negligence according to legal regulation.
Introduction By assembling this radio, you will gain insights into electronics and experience a sense of achievement even early on. Explore the functionality of the individual components and gradually build a complex circuit. Finally, use the completed FM radio to listen to your local FM station in great sound quality! The FM radio is easy to assemble and yet offers many possibilities. There are numerous versions and options.
Contents Introduction ................................................................................... 3 The Components ............................................................................ 5 Step 1: Mounting the Amplifier ..................................................... 12 Step 2: Sound Generator ............................................................... 16 Step 3: Improved Amplifier ........................................................... 19 Step 4: Simple Radio ....................
The Components The various circuits are built on a breadboard. The centre part contains 46 contact strips with five contacts each. The two long strips with 20 contacts along the edges are typically used to provide the operating voltage.
All components are inserted in the breadboard and thus connected to each other. The individual steps are illustrated with assembly drawings, photos or circuit diagrams.
The FM board is the essential component of this radio. It contains an integrated circuit and many tiny pre-soldered capacitors and resistors. You can easily recognise two printed coils and the upright variable capacitance diode. There are six pins to connect the board with the breadboard and thus the other components. It is important to supply the radio board with an operating voltage of only 3 V. It must never be connected directly to a 9 V battery. Instead, a voltage regulator is required.
LM386 amplifier Loudspeaker The LM386 loudspeaker amplifier is an integrated circuit (IC) in a housing with eight pins, numbered from pin 1 at the bottom left corner to pin 8 at the top left corner. Pin 4 (bottom right) is the negative terminal of the power supply. The amplifier operates at 9 V and provides 0.5 W to the loudspeaker. The loudspeaker exhibits a resistance of 8 Ohm and can tolerate up to 0.5 W. The volume depends mainly on how the loudspeaker is mounted.
The loudspeaker must not be connected directly to the amplifier but requires a capacitor. Any capacitor consists of two metal sheets insulated against each other. The electrolytic capacitor (e-cap) used here contains aluminium sheets in a conductive fluid (electrolyte). You have to pay attention to the mounting direction as 100 µF e-cap the e-cap will be destroyed when the polarity is reversed. The negative terminal is the shorter leg; it is additionally marked by a white bar.
The resistors in the kit are of the carbon film type and can be mounted in any direction. The smallest one has a resistance of 100 ohm (100 Ω), the biggest one has 220 kiloohm (220 kΩ). The resistance values are shown by three coloured rings. The fourth, gold ring represents a tolerance of 5%. The kit contains four resistors in total. Basically, a potentiometer is a resistor; however, it contains a third contact, which is shifted by turning the axis.
The volume potentiometer contains an additional switch and thus has five connecting wires. By turning the axis to the far left, the radio is turned off. As a special feature of this potentiometer, the resistance curve is not linear but adapted to the human sense of hearing. Hence, the middle setting provides significantly more than half the total resistance.
Step 1: Mounting the Amplifier Required components: Breadboard, LM386 amplifier IC, 100 µF e-capacitor, 1 kΩ resistor (brown, black, red), hook-up wire Loudspeaker 100 µF e-cap LM386 amplifier Strain relief Battery with clip 12 1 kΩ resistor (brown, black, red)
The eight-legged LM386 IC is a loudspeaker amplifier suited for battery operation. Internally, it contains many transistors and resistors. Pin 4 of the IC (GND) connects to the negative terminal of the battery via a 1 kΩ resistor (brown, black, red) in order to limit the current in case of improper assembly. The positive terminal is attached to pin 6 (Vs). Pin 5 is the output (Vout). Here, the loudspeaker is attached via a 100 µF e-cap. This pin supplies an average output voltage of approx. 4.5 V.
Initially, the eight legs of the IC have a slightly widened stance and must be aligned in parallel rows. This is best done with pliers. Only now it is possible to insert the IC in the breadboard without effort. Be careful to mount the chip in the correct direction. A notch at the left side marks pins 1 and 8. The assembly drawings show exactly what contacts have to be used. Carefully observe all the drawings.
Step 2: Sound Generator Required components: 10 kΩ resistor (brown, black, orange) 16
This circuit uses the 10 kΩ resistor (brown, black, orange) to turn the amplifier into a sound generator. For natural oscillations to emerge, the non-inverting input at pin 3 of the LM386 has to be connected to the output via a resistor. This feedback generates oscillations of the amplifier, which become audible in the loudspeaker as humming or clicking. Pin 2 of the LM386 is an inverting input. When the voltage at this input increases, the amplified voltage at the output decreases.
Step 3: Improved Amplifier Required components: 100 nF disc capacitor, 100 µF e-capacitor, hook-up wire 19
Capacitors are often used to transfer sound frequency signals. Here, we use a ceramic 100 nF disc capacitor (labelled 104). This amounts to just a 1000th of the capacitance of the 100 µF e-cap. A 100 nF capacitor is ideally suited as a coupling capacitor at the amplifier input. The 1 kΩ protective resistor is replaced by a piece of wire because after the successful initial test, there is no longer a risk of a faulty circuit. Later, you will insert the main switch of the radio in this place.
Step 4: Simple Radio Required components: TDA7088 FM IC receiver board, HT7530 voltage regulator, 10 kΩ resistors (brown, black, orange), 1 kΩ resistor (brown, black, red), 100 Ω resistor (brown, black, brown), hook-up wire 22
The receiver board with the TDA7088 FM IC is the heart of your FM radio. Apart from the IC, the board contains many tiny capacitors, a variable capacitance (varcap) diode and two printed coils. For the first experiment, only four pins are used. The stabilised 3 V operating voltage is supplied via GND (-) and BAT (+). Caution: The radio board must not be attached to 9 V but requires an operating voltage of 3 V. For this reason, the type 7530 voltage regulator is used. Its three legs are not interchangeable.
While there is still much to do to complete the radio, you may already be lucky and be able to receive a radio station. As the tuning contacts are not yet attached, you will receive a random frequency. However, by momentarily touching the +, S, R, and - contacts on the receiver board, you can switch to a different station.
Step 5: Tuning Required components: Hook-up wire 25
The radio IC has a scan input (S) to start the station scan. The corresponding push-button is placed between the positive operating voltage and the S input. To build the push-button, you use hook-up wires that will connect the BAT terminal to the S input when they touch each other. Bend the wires so that they make contact when you push them slightly. Momentarily touching the push-button causes the radio to search for the station of the next higher frequency. After reaching the last station, the scan ends.
The receiver board also contains a variable capacitance diode (varcap) whose capacitance changes depending on the applied DC voltage. The smaller the capacitance, the higher the frequency. The R pin of the radio board is connected to the varcap diode. By its connection to the BAT pin, the reset button causes the voltage at the diode to be cut off. In this way, the minimum frequency of a little lower than 87.5 MHz is set.
Step 6: Mounting in the Housing Required components: Volume potentiometer, tuning potentiometer, 220 kΩ resistor (red, red, yellow), washers, cap nuts, hook-up wire, both rotary knobs, radio housing 28
The kit contains two potentiometers. The first one is intended to control the volume and additionally has a switch to turn the radio on and off. The second one will be used for tuning. Mount both potentiometers with their washers and cap nuts in the radio housing. The small lug of the potentiometer slips into a hole at the side and thus prevents the component from twisting. Slide the loudspeaker into the provided bracket. You can additionally secure it with a little glue.
Connect the volume potentiometer instead of the previously used voltage divider made of two resistors. Also integrate the switch of the potentiometer with the negative battery connection, where previously a resistor or a wire had been attached. The second potentiometer will be used for scanning the frequencies. When the potentiometers are attached, you can close the housing and control the radio from the outside.
The tuning voltage set at the left potentiometer is applied via a 220 kΩ resistor (red, red, yellow) to the reset pin and thus to the varcap diode. When the wiper of the potentiometer is placed near +3 V, the resulting frequency is low. Accordingly, a maximum frequency will result from a setting of 0 V. The 220 kΩ resistor increases the influence of the automatic frequency control (AFC) on the tuning process. An improperly adjusted station will automatically be tuned more precisely.
The antenna consists of a wire loop as big as possible and is attached between A and GND. Use a wire with a length of 1 m. Thread it through the designated holes so that the antenna loop remains outside the housing. Twist the wire at both ends so that the antenna remains stable and does not slip out of the breadboard. Finally, screw the rotary knobs on the potentiometer axes. Put in the battery and test the completed radio. You should be able to receive the more powerful stations clearly.
Troubleshooting While assembling the radio, you might make a mistake that is not so easy to spot. If this happens, do not give up but accept the challenge! When the radio doesn’t work, you should first check all connections. Compare your setup with the diagrams and photos wire by wire. Make sure that all wires are firmly attached. The most common problems are: • A wire is attached at the wrong position. • The end of a wire is too short and does not reach the contact point inside the breadboard.
Repeat the first radio experiment without a particular tuning. It is sufficient to remove the middle connection of the potentiometer (blue). When you turn on the radio, it should find a station, except when there is a problem with the voltage supply. Reattach the potentiometer. If it is now impossible to find a station by means of the potentiometer, the problem resides in the area of the potentiometers and the attached resistors. For further troubleshooting, a voltmeter can be useful.
Measured voltages 36
