In Book 7/8 of Practical ELECTRONICS methods for simple control of various electrical devices by computer were discussed. Practical realization of various interface circuits and sensors was described, by which the computer is being connected with the outside world, so that it can turn on/off the heater, light, fan, TV set or some other electrical device at the desired moment, based on data comprising temperature, light intensity, humidity etc. In this chapter we shall present, in short terms, one of the projects from the aforementioned book, that deals with simple computer-aided turning on/off of the radio receiver at the desired moment.The radio is connected with the computer via parallel port, the one where the printer is also being plugged. It is a 25-pin female connector, called Sub D-25, which is given on Pic.5.3.With appropriate programme, logical ones (voltage +3.6 V) and zeroes (0 V) can be sent to the outputs marked as DI-1, DI-2,...DI-8, that are located on the legs marked with numbers 2 to 9. Electrical devices that are being controlled are connected to these outputs over the interface circuit that is given on Pic.5.4. Two connected devices are shown on the picture, their maximum number is 8.Pic.5.5-a shows the electronic diagram of an extremely simple interface circuit, which can serve to connect to computer the radio receiver, that can then be switched on or off at certain time, with adequate programme. The low-power transistor BC547 can be used for the consumers that use the current from the battery that is not greater than 100 mA. In case you have bigger power consumers, some stronger transistor or two transistors in Darlington junction can be used instead of BC547. The transistor bas is connected to the pin No.9 of the mail Sub D-25 connector over the R1 resistor, while the emitter and minus pole of the battery are connected to the pin No.25, i.e. to the computer ground. As long as there’s a logical zero on the DI-8 output, the base voltage is zero and the transistor is locked and no current runs through it, therefore also through the consumer. When a logical one emerges on the DI-8, the transistor goes to the saturation regime, the voltage between collector and emitter becomes very small (practically zero) and the transistor behaves as if the collector and emitter are short-circuited. In that way almost the entire battery voltage is available on the receiver power supply input. The PCB layout is shown on Pic.5.5-b: The component side is in the upper part, and the soldering side in lower part of the picture.Pic.5.5-c shows how a small transistor radio receiver, powered by a 9 V battery, is connected to the parallel port, over the interface from pic.5.5-a. The plate is connected with clamps via the cables A and B, and with the battery over C and D.