This handy circuit does the job for you. It converts your coffee table into one big button: if you rap on the table, the TV sound is muted right away. When the film resumes, you can restore the sound by rapping on the table again. The circuit uses a piezoelectric transducer fitted in the bottom of an enclosure. The transducer acts as a shock sensor. The transducer is fitted at the rear of the enclosure touching a screw that rests on the table, and two feet are fitted at the front to improve the operation of the transducer. A yoke made from a length of solid electrical wire provides a mount for an IR transmitter that sends the mute command to the television set. This gives the IR transmitter a clear view of the television set, even if cups and the like are standing on the table.
Just a PIC The circuit is very simple and essentially consists of an 8-pin PIC microcontroller. The piezoelectric transducer is connected to the GP0 input, and zener diode D1 limits the maximum voltage applied to the input. The GP5 output drives an IR LED via transistor T1 to send IR commands to the television set. Pushbutton S1 allows the circuit to learn the right mute command.
If it is pressed and held for a while, indicator LED D2 lights up and the mute code transmitted by the original remote button cell or two 1.5 V cells in a battery holder. The circuit draws less than 100 nA in the quiescent state (ultra-low sleep mode), so a power switch is not necessary.
The PCB for the advert killer has been kept very small by using predominately SMDs. Note: the prototype version shown in the photo does not have a programming connector, but this connector is present in the final version.
Construction The circuit board and a pair of 1.5 V cells can easily be fitted in a standard box measuring 5 × 2.5 × 7 cm (2 × 1 × 2 3/4 inch). Of course, a much smaller box can be used with a button cell. Remember to program the PIC before closing the box (the source and hex code are available at ). Drill a hole the bottom of the box at the rear so that the middle of the piezoelectric transducer is accessible from outside when the box is resting on its base. Then secure the piezoelectric transducer in the bottom of the box with hot-melt glue or silicone adhesive. Thread a screw into the hole until it touches the transducer. Glue the screw firmly in place (see the accompanying construction drawing: (1) screw, (2) piezoelectric transducer, (3) PCB). At the front of the box, fit two feet to enhance the operation of the vibration sensor. Fit a bracket (made from 2.5 mm² / AWG13 electrical wire, for example) on the box for mounting the IR LED. This gives it a free view of the television set. With a bit of imagination, you can transform the box into something that looks nice on the table. For instance, the author ‘dressed up’ the prototype with a toy dragon.
Use Start by learning the right mute code. Press and hold the button until the indicator lamp (D2) lights up (longer than 3 seconds). Hold the original remote control a few centimeters in front of the IR receiver and press the Mute button twice. If the code is received properly, it will be saved and the circuit will return to normal mode. If no code is received within 10 seconds or a bad code is received 6 times, the program automatically returns to normal mode. The learning function works with virtually every remote control unit.
Now you can place the advert killer on your coffee table or the like. Tap the table or the box when you want to switch the TV sound on or off. The sensitivity can be optimized by choosing the right location on the table. The advert killer also responds to cups set down hard on the table, but everyone quickly learns not to do this, and it has the advantage that the table is less likely to be damaged.
Internet Link: www.elektor.com/120277