2.

We have a noisemaker; now we need to trigger it for a fixed interval. This can be done with a second 555 wired in monostable mode, meaning that it emits only one pulse. Figure 4 shows it added to the circuit. S2 is a pushbutton, although you can

^{3. We have a noisemaker} that can be triggered for a fixed interval; now we need to measure an interval of time before the sound occurs. A third 555 timer can impose this wait period, if we adjust it with higher-

value resistors and a larger capacitor.

In Figure 5, C7 is charged through P1, a potentiometer (variable resistor). You can “tune” P1 to adjust the wait interval, and increase the value of C7 to make the interval even longer. At the end of the

555 CIRCUIT COMPLETE: The real-life version of the circuit in Figure 5. The top chip measures a time interval (using a fixed resistor that has been substituted for potentiometer P1). The red LED flashes at the same time that a sound is generated through a 1" loudspeaker (to the left of the chip).

66 Make: Volume 10

improvise just by touching 2 wires together. When this happens, IC2 emits a single pulse lasting about 1 second. This illuminates D2, an LED, to provide visual confirmation. The pulse also goes through D1, a signal diode, and activates IC1, which makes a sound as before, except that C4 prolongs it and causes it to diminish in frequency, creating a groaning effect.

Make sure this version of the circuit works before you continue.

interval, the output on Pin 3 goes negative. This connects with the trigger pin of IC2 and tells it to emit its brief pulse, which tells IC1 to make its sound.

Note that S2 has been moved so that it controls IC3. When you use the circuit to impose a time limit during a game, hit S2 at the beginning of each person’s turn.

So the circuit won’t make its rude noise if a player does move within the allowed time, a cancel-reset button, S3, has been added. You hit this button when a player makes a move. The “NC” beside it tells you that it is a normally closed pushbutton. You still need the power switch, S1, to disconnect your power supply when the gadget is not in use.

WHAT NEXT?

You can substitute other components instead of the timing resistors to make the 555 behave in interesting ways. In Figure 3, if you use a thermistor or a photoresistor instead of R2, you can control the audio frequency with heat or light. A photoresistor and the 555 in monostable mode can function as a motion detector. Search makezine.com for “555”. Also check out doctronics.co.uk/555.htm.

Hans Camenzind never imagined that his timer would become such a universal utility. He now thinks the internal design of the 555 isn’t particularly elegant and should have been given a makeover decades ago. Elegance in design can be a big deal for engineers, but for end users, utility is usually more important. The 555 is simple, accurate, and robust, tolerating a wide range of power supplies and able to drive not only LEDs and loudspeakers but also relays and even small motors.

For 25 cents, that’s more than enough.