The ZN415 IC, being promoted as “The World’s smallest radio” by its manufacturer (FERRANTI), is an improved version of ZN414. As seen on the Pic.3.36-a (IC’s block diagram), the 8-pin DIL package contains ZN414 IC with R1 and R2 resistors, an amplifier with T1 transistor and the separation stage performed with T2 transistor. All the necessary resistors are comprised in the IC. Someone will, perhaps, wonder why there are no capacitors integrated inside, so that the externally mounted parts would then be only the battery and the headphones? The answer is fairly simple: It isn’t yet possible to make the capacitors that would have enough big capacitance, in the monolithic IC’s manufacturing process. Regardless of this “problem”, the receiver built around the ZN415 is, really, extremely simple.
As described in project 3.10, the LF signal exits the ZN414. Through the coupling capacitor C3 it is led on the base of T1, and the amplified signal is obtained on its collector pin, and is taken directly on the base pin of T2. The remains of the HF signal are led to the ground over the C4 capacitor. The separation stage allows us to connect small - resistance (64 Ohms) headphones, which are mass - produced these days and are the easiest ones to obtain.
The 1.5 V battery is used for the power supply of the circuit, and is being connected between the pins 6 and 4. Having in mind the previous experience gained with the ZN414 IC, the author recommends to the readers to try improving the signal reception by adding the R2* resistor (helping themselves with linear 1-5 kOhms potentiometer), which is shown in dashed line on the picture.
In the upper end of the Pic.3.37 the table with basic data about ZN415 circuit is given. All these apply on the ZN414 as well, except for the last line. The smaller table given below it contains the data about the DC voltages on the pins of the IC. If something is wrong, the first thing to do is check these voltages and if they are (approximately) as those given in the table, the IC is OK and the error is therefore to be searched in the printings, the contacts, junctions, etc.
Everything that is said about the ferrite antenna in the previous projects applies here as well.
This FRT AM receiver combines the features of two magnificent IC’s, ZN416E (HF amplifier, detector and the pre-amplifier) and LM386 (audio power amplifier). Add a power supply based on 7809 voltage stabilizer IC, use a loudspeaker with membrane diameter being about 15 cm, put all this in one of the boxes described in the Appendix, and you will have a new and improved silicone - based model of an old - fashioned radio receiver, built somewhat less than 100 years ago.
The electrical diagram of this device is given on Pic.3.38. The voltage reduction (and stabilization) from 9V to 1.5V, as needed by ZN415, is being done over the R2 resistor and D1 and D2 diodes, although any circuit given on Pic.3.34 can be used instead. Universal - type diodes 1N4148 are used here, but any other low-power silicon diodes will do. With 250 Ohms trimmer TP the fine - tuning of the voltage on the pin 6 is being done (the receiver is set to some station in the upper part of the reception bandwidth (app. 1500 kHz), and the slider of the TP is then used to obtain the optimum reception). After that, the receiver is switched off, the trimmer is disconnected, its resistance measured and an equivalent resistor soldered in the circuitry. On the PCB, this resistor is marked as R3.
* Some more modern IC, other than LM386, can also be used in this device. The author recommends TDA7052A. Its main features are:
1. Extremely stable operation
2. Implemented output short-circuit protection
3. No cooler necessary
4. Small power consumption, IO=4 mA
5. No “clicking” during switching ON/OFF
6. Voltage amplification is 39 dB
7. Output power Piz=1.2 W (3x bigger than LM386)
8. The 220 mF capacitor isn’t needed if the power rectifier that has an electrolytic capacitor on its output is used, and the cables connecting it with the device are short. In that case the amplifier is practically made without the external components, if we exclude the (inevitable) 100 nF capacitor, which is in every way great advantage.
The 9 V battery is not so popular nowadays as it used to be, probably for its quite big price/capacity ratio, so it is almost never used in the pocket-type receivers today. Much more frequently utilized these days are the cylindrically shaped 1.5 V AA type batteries. Electrical diagram of one pocket-type AM receiver that uses two of these for power supply is given on Pic.3.41. Two IC's, ZN415 and TDA7050 are being used in it. The DC supply voltage for TDA7050 must be exclusively 3 V, and should be no bigger than 4 V. The voltage being supplied from the batteries gets smaller during the use, but the IC operates properly as long as it doesn't drop below 1.6 V. Besides this nice feature, this IC has very small dimensions, very low idle current (4 mA), all the external components it requires are two capacitors (C8 and C9), which makes it ideal for usage in the mobile-type devices (radios, wakmans, etc.), the purpose that is originally designated to it by its manufacturer (Philips).
* Caution must be expressed with the power supply, since this two-batteries power supply cannot be simply replaced with some common-type 3V adaptor. If you wish to use such adaptor, the pin 6 of ZN414 must be connected, over the R2 resistor and one of the circuits from Pic.3.35, to the pin 8 of the TDA7050, and the adaptor is then to be connected between the pin 8 and the ground.
* The R2* resistor, shown in dashed line on the picture, should be added only if the receiver does not operate stable. Determining its exact value is explained in several previous projects.
* The 64 Ohm headphones are connected onto the output of the ZN415 (between pin 5 and Gnd), or instead of the loudspeaker (in which case the reproduction can be much louder).
* The PCB and components layout is shown on the Pic.3.42. Connecting other components is being done in the similar manner as in the previous project. In the left corner the batteries are shown, placed in some convenient holder taken from some old device. Between the plus pole of the left and the minus pole of the right battery the tin plate is inserted, with one of 3 cables connecting the supply to the PCB soldered to it.
