1. Figure 1 shows a 555 Using a 0.1 microfarad ( F) capacitor for C1, a chip seen from above, 120 kilohm (k ) resistor for R1, and a 1 megohm
with its pins identi- (M ) resistor for R2, the LED flashes about 5 times
fied. The circular mark each second. (The other components in the circuit
stamped in its body is have no effect on timing: R3 protects the LED from
adjacent to Pin 1. excessive current, while C2 protects the 555 timer
Figure 2 shows a basic from random electronic noise.)
light-flashing circuit using the astable mode of the Suppose you use a 1 F capacitor instead of the
555, meaning that the output on Pin 3 flips to and 0.1 F capacitor as C1. Now each cycle lasts 10 times
fro between positive and negative for as long as the as long. Conversely, if you use a 0.01 F capacitor for
power is switched on. The cycle time is determined C1, the cycles are 1/10 as long. You can also change
by a capacitor and two resistors. A capacitor has the timing by adjusting the resistor values. The
electrical storage capacity (hence its name), while value of R1+R2 affects the “on” cycle, while R2
resistors reduce the flow of electricity. If you put a alone determines the “off” cycle.
resistor in sequence with a capacitor, the resistor With high resistance and a small capacitor, the
slows the charge and discharge times of the capaci- 555 will cycle very fast indeed — easily fast enough
tor, thus offering a simple way to use electricity to for its pulses to make musical noises through a
measure time. loudspeaker.
When you close switch S1 in the circuit, current Figure 3 shows a modified version of the circuit.
flows through R1 and R2 and gradually starts The LED and its series resistor have been replaced
charging capacitor C1. IC1 (the 555 timer) monitors with a different resistor, capacitor C3, and L1, a
this process. When C1 acquires /³ of the positive 1" RadioShack miniature loudspeaker. (Note: You
voltage powering the circuit, the 555 reverses its cannot drive a full-size loudspeaker with a 555
output on Pin 3 from positive to negative and forces timer unless you add an amplifier.) Make sure you
C1 to discharge itself through R2. When the charge update the values of R1, R2, and C1, which have
on C1 diminishes from /³ to ¹/³, the chip flips back been changed to make the 555 run faster. Now
to its original state, resets its output from negative when you connect power, you should hear to positive, and repeats the cycle. a low-pitched drone. >>
555 CIRCUIT BEGINNING: This is how the LED-flasher circuit in Figure 2 should look,
using components plugged into a breadboard.
64 Make: Volume 10
