Experiment 17: Set Your ToneChapter 4162Experiment 17: Set Your ToneI’m going to show you two other ways in which the 555 timer can be used. You will need the same items as 168,in Experiment 16, plus:• Additional 555 timer chip. Quantity: 1.• Miniature loudspeaker. Quantity: 1.• 100K linear potentiometer. Quantity: 1.ProcedureLeave the components from 168,Experiment 168,16 where they are on the bread-board, and add the next section below them, as shown in Figures 4-21 and 4-22. Resistor R2 is inserted between pins 6 and 7, instead of the jumper wire that shorted the pins together in the previous circuit, and there’s no external input to pin 2 anymore. Instead, pin 2 is connected via a jumper wire to pin 6. The easiest way to do this is by running the wire across the top of the chip.I have omitted the smoothing capacitor from the schematic in Figure 4-22, because I’m assuming that you’re running this circuit on the same breadboard as the first, where the previous smoothing capacitor is still active.A loudspeaker in series with a 100Ω resistor (R3) has been substituted for the LED to show the output from the chip. Pin 4, the reset, is disabled by connect-ing it to the positive voltage supply, as I’m not expecting to use the reset func-tion in this circuit.Now what happens when you apply power? Immediately, you should hear noise through the loudspeaker. If you don’t hear anything, you almost cer-tainly made a wiring error.Notice that you don’t have to trigger the chip with a pushbutton anymore. The reason is that when C1 charges and discharges, its fluctuating voltage is connected via a jumper wire across the top of the chip to pin 2, the trigger. In this way, the 555 timer now triggers itself. I’ll describe this in more detail in the next section “Theory: Inside the 555 timer: astable mode,” if you want to see exactly what is going on.In this mode, the chip is “astable,” meaning that it is not stable, because it flips to and fro endlessly, sending a stream of pulses for as long as the power is connected. The pulses are so rapid that the loudspeaker reproduces them as noise. In fact, with the component values that I specified for R1, R2, and C1, the 555 chip is emitting about 1,500 pulses per second. In other words, it creates a 1.5 KHz tone. Check the table 181,on page 166 to see how different values for R2 and C1 can create different pulse frequencies with the chip in this astable mode. Note that the table assumes a fixed value of 1K for R1!