Make Electronics

Experiment 14: A Pulsing Glow

118 Chapter 3

Figure 3-75 shows these features. Compare it with Figure 2-103 on page 85. R1

is now 33K instead of 470K. R2 and R3 are reduced to 1K. R4 also is 1K, to so

that the capacitor takes longer to discharge through it. And C1 is now 100 μF

instead of 2.2 μF.

Assemble this circuit on a breadboard, and compare the results when you in-

clude R4 or bypass it with a plain piece of jumper wire. It softens the pulse a

bit, but we can work on it some more. On the output side of the PUT, we can

add another capacitor. This will charge itself when the pulse comes out of the

PUT, and then discharge itself gradually through another resistor, so that the

light from the LED dies away more slowly.

Figure 3-76 shows the setup. C2 is large—220 μF—so it sucks up the pulse that

comes out of the PUT, and then gradually releases it through 330Ω resistor R5

and the LED. You’ll see that the LED behaves differently now, fading out inside

of blinking off. But the resistances that I’ve added have dimmed the LED, and

to brighten it, you should increase the power supply from 6 volts to 9 volts.

Remember that a capacitor imposes a smoothing effect only if one side of it

is grounded to the negative side of the power supply. The presence of the

negative charge on that side of the capacitor attracts the positive pulse to the

other side.

I like the look of this heartbeat effect. I can imagine a piece of wearable elec-

tronic jewelry that pulses in this sensual way, very different from the hard-

edged, sharp-on-and-off of a simple oscillator circuit. The only question is

whether we can squeeze the components into a package that is small enough

to wear.

Figure 3-76. The second step toward a
gentler flashing effect is to add another
capacitor, C2, which charges quickly with
each pulse and then discharges slowly
through R5 and the LED below it.

Same components as before, plus:
R5: 330Ω
C2: 220 μF electrolytic
Power supply increased to 9 volts

R1

9V

DC

C1

C2

R2 R3

R4

R5

Q1

Figure 3-77. On a dark night in a rural area, the

heartbeat flasher may be attractive in unexpected

ways.

R1

C1

R2

Q1

R3

R4

Figure 3-75. The first step toward creating
a gentler flashing effect is to use a larger
capacitor for C1 and discharge it through
a resistor, R4. Lower-value resistors are
necessary to charge the capacitor rapidly
enough.

R1: 33K
R2: 1K
R3: 1K
R4: 1K
C1: 100 μF electrolytic
Q1: 2N6027