FM Stereo Decoder for Mini FM Receiver Circuit Schematic Diagram

The following circuit diagram is a VHF FM radio with stereo output. It is intended to be used in combination with the FM radio to make a stereo FM receiver. The stereo decoder we use here is a TDA7040T made by NXP (formerly Philips Semiconductors). The design of this chip has not changed since 1986 . This IC, just like the chip in the receiver, is now only available in an SMD version. The old, familiar DIP package is no longer produced. The only throughhole components are a few electrolytic capacitors. As a consequence the dimensions of this PCB are only 2.5 by 4 centimetres (approx. 1 by 1.6 inches).

FM Stereo Decoder for Mini FM Receiver Circuit Schematic

K1 of the decoder is connected to K1 of the receiver, and K3 is connected to K2 (the circuit also works without this connection). The mono/stereo switch S1 is connected to K2 (even though the schematic only shows S1, this is actually K2). Output K4 is intended to drive headphones with an impedance of 32 ? or more, but for testing you can also connect a couple of small loudspeakers. Output resistors R7 and R9 protect the outputs of IC2 against overloading. R8 and R10 ensure that C11 and C12 are always charged so that a switch-on plop is prevented when the headphones are plugged in. The output power of the TDA7050 is inadequate for serious use with loudspeakers, and it would be better in that case if you used an additional power amplifier. The stereo potentiometer for the volume control (P3) is connected to a 6-way pinheader. If the wires are kept short there is no need to use screened cable.

At 6 V the total current consumption of the receiver and decoder together is a little higher than at 3 V: 17.3 mA instead of 12.5 mA. For testing we connected two 8 ? loudspeakers to the outputs. The current consumption at highest volume peaked at 70 mA. The average was a little over 40 mA. You will have to decide for yourself whether you would like to use two or four penlight batteries. If you use four you will get more from the batteries because together they will only have to supply a minimum of 2 V (minimum power supply voltages for the stereo decoder and receiver are 1.8 V and 1.6 V respectively). [Source : Elektor Magazine]

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