The Transistor Radio. Visit our homepage for more experiments: We wish you success and joy in building your radio!

Transcription

1 CONRAD

2 The Transistor Radio The special attraction of this radio construction kit is in the circuit being built on a PCB without any soldering gun. If you aim to understand the circuit technology in detail, you should structure setup in many small steps. The components each have their own functions and can also be tested in small steps. This leads to quickly audible results. The radio is expanded and improved step by step. The function of the individual components becomes clear from this. In the end, everyone can build his own special radio or develop versions that go beyond the scope of the manual. The result is a complete AB radio with strong reception and a dedicated casing. Additionally, there are any number of circuit versions for further experimentation. Visit our homepage for more experiments: We wish you success and joy in building your radio!

3 Contents TOC Components PCB NPN transistor BC547B Speaker 8 Ω Resistor 100 Ω (brown, black, brown) 2 m wire Resistor 620 Ω (blue, red, brown) Battery compartment with Resistor 820 Ω (grey, red, brown) connection wires Variable capacitator 280 pf Resistor 1 kω (brown, black, red) Extension axis and dial Resistor 10 kω (brown, black, orange) Screw connector Resistor 100 kω (brown, black, yellow) Coil 220 µh (black) 3 ceramics capacitators 100 nf (104) 2 coils 47 µh (blue) 2 electrolytic capacitators 47 µf Receiver IC TA7642

4 The Speaker Take the speaker and look at it in detail. The font has a diaphragm. It can be carefully pushed inward a little. If you tap your finger onto the diaphragm, you will hear a sound. This shows the speaker principle: A diaphragm movement generates sound. On the rear, there is a strong magnet. Inside, there is a hidden wire coil with two connections to the contacts and soldered-on cables. The diaphragm therefore can be moved by electrical power. The learning package comprises a battery compartment for a single mignon cell. Insert a 1.5-V cell. Briefly touch the wires to those of the speaker. There will be a crackling sound. The diaphragm movement is hard to see, but can be felt with a finger. Depending on connection direction, the diaphragm is pushed inward or outward by the current. The speaker has a resistance of 8 Ohm (Ω). A very new 1.5 V battery may actually have about 1.6 V. Then a current of 0.2 A flows. The power taken up is 0.32 W, and therefore still below the permitted threshold. In later experiments, a lot less power will be taken up, but even a few milliwatt will lead to the speaker generating a good volume. The Pinboard The entire radio is built on the experimenting pinboard step by step. In the beginning, the board may be simply put on the table. This makes circuit conversion particularly comfortable. Later, the PCB should be glued into the casing. Insertion of components requires relatively high power. The connection wires therefore bend easily. It is important that the wires are inserted precisely from the top. Tweezers or small pliers will help with this. A wire is held as close to the pinboard as possible and pushed vertically down. This permits insertion of even sensitive connection wires like the tin-plated ends of the battery compartment and speaker without bending them. Take out the battery and connect the battery compartment on the pinboard with the speaker. The drawing shows the internal connections. When inserting the battery, you will hear the crackling again. Take the battery from the battery compartment again after a brief test to protect it for the following tests. Use insulated jumper wire to connect the components. Some short pieces are needed for internal connections; the long rest is later used as reception aerial of the radio. Cut matching pieces with pliers or, if necessary, old scissors and strip the insulation for about 5 mm at the ends. To strip the wire ends, it has proven helpful to cut in the insulation with a sharp knife. Attention! Do not scratch the wire, or it will break easily in this position later.

5 Another piece of advice for working with the pinboard: Cut the wires diagonally at the end to get a pointed tip that can be pushed into the contacts more easily. This is also sensible for resistors and capacitators. It prevents the connection wires from bending easily when they are inserted. Use 2 wire pieces to build a switch. Every activation will then cause a noise. Put the speaker on the sound opening of the casing. This will act as a resonance box or speaker box. The sound is amplified. Two additional short wires are installed as tension relief to protect the soft connection wires of the battery compartment and speaker. Both should remain connected at all times to avoid excessive wear of the components. Always take the battery from the compartment for switching off.

6 The Transistor Transistors are components to amplify small currents. The NPN transistor BC547B serves as speaker amplifier. The transistor connections are called emitter (E), base (B) and collector (C). The base connection is the middle one. The emitter is the right one when looking at the label with the connections pointing down. The circuit symbol marks the base with a bar and the emitter with an arrow. Observe the connections when installing. The flat labelled side points up towards the plus line. Build a wire switch between the base and collector. With the switch open, no current will flow through the transistor and speaker. Therefore, there will not be any sound when you insert the battery. Crackling is only audible when you switch a base current with the switch. Attention! The base must only be short-circuited against the collector, but never against the battery plus pole. Base Current Find the resistor with 10 kω. The resistor has coloured rings. Brown, black, orange means 10,000 Ω. The fourth ring (gold) means tolerance class 5 %. A resistor is used to reduce a current. In this case, the resistor leads to a base current of only about 0.1 ma. The transistor amplifies this small current by up to 300 times to about 30 ma. There is a switch between the base and emitter of the transistor. When it is closed, it discharges the base current. No collector current is flowing anymore either. Opening and closing of the contact therefore generates a sound.

7 A Coupling Capacitator The electrolyte capacitator at 47 micro farad (µf) serves to couple the LF amplifier to the highfrequency stage. A capacitator comprises two metal surfaces and an insulation layer. Applying electrical voltage will lead to formation of an electrical force field between the capacitator plates, in which energy is stored. The capacity of a capacitator is measured in farad (F). The electrolyte capacitator has a capacity of 47 µf ( F). The voltage must only be applied in one direction. In the wrong direction, a fault current will flow that will destroy the component after a while. The minus pole is marked by a white strip and has a shorter connection wire. The circuit symbol shows the minus connection as a solid bar. Install a second resistor at 1 kω (brown, black, red) into the circuit so that the electrolyte capacitator is charged continually. Only closing of the switch discharges it. Every opening and closing of the switch generates a sound.

8 The Integrated Circuit Now the most important component of this radio is built, an integrated circuit (IC) type TA7642. The IC has a plastic housing like the transistor and also three connections. Observe the other direction when installing. The flat, labelled side points to the board's minus side. The function of the radio IC can be imagined simplified as amplifier, similar to a transistor. In fact, the IC contains many transistors, resistors and capacitators, however. The central connection is the amplifier input. Another resistor at 100 kω (brown, black, yellow) services to let a small input current flow, similar to the base resistor of the transistor. This current then can be amplified and reduced by low radio signals. The changes are amplified and made audible. Touch the input with your finger. You will probably hear a hum, buzz or prattle. Operation of a light switch in the room should be audible as a cracking sound. The circuit therefore is already a simple high-frequency receiver that is able to take up radio interferences. A LF Low Pass Install a ceramics disc capacitator with 100 nf (print 104). This capacitator has no polarity and may be used in any direction. It is now installed as low-pass filter and leads to high-frequency signals (HF) at the output of the receiver IC being suppressed and signals in the audible range (LF, low frequency) being retained. Demodulation inside the receiver IC delivers the NF signal that was modulated onto the HF signal of a transmitter. Although the device is far from being done, you may already listen to the radio now with a little luck. If there is a strong station nearby, its signal will likely be enough to be received. Your body serves as aerial. A long wire works just as well. Put the speaker on the sound opening of the casing so that it can serve as speaker box. The sound becomes louder and fuller. Later, the board should be installed in the casing. At the moment, however, open experiments on the table are more comfortable. The Receiver Coil The small coil with 220 micro henry (µh; black sleeve) has thin copper wire that is coiled onto a coil box of ferrite and connected to the two connection wires. The coil is covered with a plastic sleeve. Install this coil at the receiver input. It short-circuits LF signals so that only HF signals get to the input. Reception interferences are suppressed. The coil acts as a filter. Another capacitator with 100 nf is used as coupling capacitator and lets HF signals get to the input without discharging the input direct current. Again, a touch of your finger may replace the aerial. Your local medium wave station will be received even more clearly now.

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10 Casing Assembly All previous experiments were performed in the open setup. Now everything is installed in the casing. This will get you closer to a proper radio. Stability and operability improve. Experiments and changes continue to be possible because the casing can be opened comfortably with its magnetic closure. Install the speaker by pushing it into the matching slot. The connections should point down so that short connections lead to the board later. The speaker is sufficiently tight in the intended slot. You may, however, apply an additional drop of glue or hot glue. Place the extension axis onto the variable capacitator and screw it on with the long 2.5 mm screw. Do not turn the axis too hard against the stop and use pliers to maintain the axis. The variable capacitator is installed in the casing only later using two small screws and the matching washers. The variable capacitator has a medium wave side with twice 280 pf and an ultra-short-wave side with twice 20 pf. Additionally, each segment has a small trimming capacitator with approx. 20 pf each that can be adjusted with a screwdriver. The two trimmers have their own terminal lugs on the medium wave side. Including the ground connection in the middle, this side therefore has five connections. The variable capacitator is connected to the pinboard with a double screw terminal. Use the ground connection and the two right connections (variable capacitator and trimmer), which should be screwed in together. The pinboard must be installed in just the right position. It has a double-sided adhesive film on the bottom. Screw the variable capacitator connections on and plug the screw terminal into the correct position of the pinboard as a test. If everything is fitting, pull the protective film from the adhesive side and push the pinboard firmly in position. As compared to the previous experiments, all components now have to be placed a little differently. For reasons of space, the battery is placed on the right side. The battery connection wires are equipped with a tension relief because the battery compartment is lying loosely in the casing. The speaker wires do not have to be secured because it is installed stationarily.

11 The Oscillating Circuit Previously, the receiver only had a simple filter that was effective for the entire medium wave range. Now, individual frequencies are to be separated. A coil and capacitator together act as an oscillating circuit and filter out the signal on the resonance frequency. The variable capacitator with 280 pf is used to make it possible to adjust the frequency. The lowest frequency that can be set with the coil of 220 µh as present is about 640 khz. Turning to the right reduces capacity and reconciles frequencies up to more than 1,600 khz. Use the remaining jumper wire as aerial. You may receive several stations, in particular in the evening.

12 HF-Suppression Another capacitator of 100 nf (print 104) serves to improve interference suppression. Insert it between the emitter and collector of the LF transistor. This suppresses residue of the amplified HF signal that may cause interferences. Reception becomes even clearer.

13 The Loop Aerial The remaining wire should be about 180 cm long still. Use it to build a wire loop that is as large as possible as aerial. The loop is located in series with the oscillating circuit coil and forms an oscillating circuit with it and the capacitator. Slowly turn the reconciliation dial until your local station becomes audible. You may also operate a light switch as test. Its transmission signal is a broad-band signal and should always be audible as a crackling sound from the speaker. In the evening, you may receive several European stations. The loop aerial delivery a larger signal voltage than the wire aerial and is also less sensitive to interferences from the domestic environment. Therefore, remote reception is clearly improved. The wire loop has a clear directive effect. Turning the loop makes it possible to hear the desired station more strongly or to suppress an interfering station.

14 Coil Tap The learning package also contains two coils with the lower inductivity of 47 µh (blue sleeve). Put one of these coils in series with the present oscillating circuit coil. The entire inductivity now increases to approx. 267 µh, the lower reception frequency drops to approx. 580 khz. At the same time, the oscillating circuit coil is tapped. Applying the input of the receiver IC to this tap will lead to less suppression of the oscillating circuit. This increases resonance voltage and selectivity. Precise adjustment makes it possible to receive even more stations in the evening.

15 Improved Adjustment The second coil with 47 µh (blue sleeve) is now also switched in series to the oscillating circuit coil. This leads to a coil with two taps and a total of approx. 314 µh. Now adjustment is possible down to 540 khz. Best adjustment of the receiver IC is usually present at higher tapping. If an overly strong station nearly overrides other stations, the lower tapping may be better. Vice versa, weak signals below 700 khz often are uncoupled better at the upper end of the oscillating circuit. Experiment with different couplings.

16 Uncoupling The more sensitive the receiver, the higher the risk of interferences from undesired feedback from the LF amplifier to the receiver input. One possible way of coupling signal feedback to the input is the voltage supply. If the battery is no longer all new, its internal resistance increases. This may lead to unpleasant side noise. The best solution for this is uncoupling with a large capacitator. Insert another electrolytic capacitator of 47 µf between the two supply connections. This improves interference resilience.

17 RC-Filter Even better uncoupling between the LF amplifier and the HF stage is achieved with an RC filter (resistor R + capacitator C). The matching resistor at 100 Ω (brown, black, brown) is put in the supply line for this. If your receiver tended to show interferences before, this will be improved with the RC elements at 100 Ω and 47 μf. If there is no difference, you at least know that the receiver will now work rather well even with a weak battery.

18 Higher Frequencies If you want to mainly receive frequencies in the upper medium wave range above 1,000 khz, it is better to reduce coil inductivity. Take out the large 220 μh coil and use only the two 47 μh coils with tapping in the middle. The radio now can be adjusted up to 4,000 khz and reaches the lower short wave area. In the evening, you may receive interesting stations in the 75-m band.

19 Higher Amplification Install the 220 μh coil again to receive the entire medium wave area. In particular in the evening, you may receive many European stations. Radio sensitivity greatly depends on the size of the aerial loop. Another important factor is dampening of the oscillating circuit by unavoidable loss in the wire resistance and the ferrite core. The more HF energy is lost, the lower the volume and the lower selectivity will be. The receiver-ic TA7642, however, provides active de-attenuation, i.e. part of the amplified HF signal is returned to the oscillating circuit to balance out this loss. The amplification of the TA7642 and therefore also de-attenuation, may be adjusted via the operating current within a certain range. The receiver output therefore drops a little as the battery weakens. Vice versa, amplification can be increased by using a smaller initial resistor. Replace the 1-kΩ resistor by a 820 Ω resistor (grey, red, brown). Even higher amplification can be achieved with the 620 Ω resistor (blue, red, brown). Caution: If the amplification is too high, the circuit will start natural oscillation. You will then hear a loud interference whistle. In this case, increase the resistor again.

20 Higher Selectivity Even better sensitivity and selectivity can be achieved by changing the circuit. The 100-kΩ resistor is now put at the "cold end" of the oscillating circuit with the coupling capacitator. The resistor now no longer dampens the oscillating circuit. Therefore, the overall amplification and sensitivity of the receiver increase. Also dry out different coil tappings. The circuit diagram shows connection at the upper end of the oscillating circuit.

21 Scale Reconciliation The radio is now completed. Only the scale needs to be set. The printed-on figures should correspond to the frequency set as precisely as possible. "55" means 550 khz, "160" means 1,600 khz. For reconciliation at the upper end of the scale, use the corresponding trimmer on the variable capacitator. Find a known station at upper end of the band and set the trimmer so that the scale is as precise as possible here. At the lower end of the band, reconciliation is possible by adjusting inductivity of the oscillating circuit coils. This is possible when all three coils are as close together as possible on the pinboard. Magnetic coupling and mutual influence changes inductivity slightly. You may try out reversing polarity of the smaller coils with 47 µh to change inductivity by a few percent. Fine adjustment is possible by slightly adjusting the distance between the coils. Adjustment at the lower end slightly moves the upper band end. Therefore, repeat reconciliation with the trimmer. The dial was put on initially so that the left and right stop are on a horizontal line. If reconciliation is not perfect, you may put on the dial slightly offset to improve accuracy. This is a matter of dexterity. The inductiveness of the entire oscillating circuit coil also slightly depends on the size and placement of the loop aerial. If you increase the aerial to improve reception, the reception frequency will be moved slightly downwards. Reconcile the scale again.

22 Dear customers! This product was produced in alignment with the applicable European directives and therefore bears the CE sign. Intended use is described in the included instructions. For any other use or modification of the product, you alone are responsible for compliance with the applicable rules. Therefore, build the circuit just as described in the instructions. The product must only be passed on together with these instructions. The icon with the crossed-out waste bin means that this product must be recycled as electronic waste separately from domestic waste. Your community administration will inform you about the next free-of-charge collection site. Imprint 2011 Franzis Verlag GmbH, Haar bei München, Produced at the order of Conrad Electronic SE, Klaus-Conrad-Str. 1, Hirschau Author: Burkhard Kainka Art & Design, Typesetting: ISBN All rights reserved, even including photomechanical reproduction and storage on electrical media. Generation and distribution of copies on paper, data carriers or online, in particular as PDF, is only permissible with the express consent of the publisher and will be prosecuted under criminal law. Most product designations of hard- and software, company names and company logos named in this work are usually also registered trademarks and should be observed as such. The publisher essentially follows the manufacturers' spelling for the product designations. Any circuits and programmes presented in this book were developed, tried and tested with the greatest care. Nevertheless, errors in the book and software cannot be fully excluded. Publisher and author are liable according to the statutory provisions in case of wilful intent or gross negligence. Apart from this, publisher and author shall only be liable according to the Product Liability Act for violation of life, body or health or due to culpable violation of essential contractual obligations. Damages claims for violation of any essential contractual obligations shall be limited to the foreseeable damage typical for the contract except in case of mandatory liability under the product liability act.

23 Legende Bilder 1,5V 1.5V LS LS B B C C E E BC547B BC547B 10k 10k 47μ 47μ 100k 100k 1k 1k TA7642 TA7642 Finger Finger 100n 100n 220μH 220μH Antenne Aerial 280p 280p IC IC 100R 100R 820R 820R