Make Electronics

Getting Somewhat More Serious 117

Experiment 14: A Pulsing Glow

Experiment 14: A Pulsing Glow

You will need:

• Breadboard


• 15-watt pencil-type soldering iron


• Thin solder (0.022 inches or similar)


• Wire strippers and cutters


• Plain perforated board (no copper etching necessary)


• Small vise or clamp to hold your perforated board


• Resistors, various


• Capacitors, electrolytic, 100 μF and 220 μF, one of each


• Red LED, 5 mm, rated for 2 volts approximately


• 2N6027 programmable unijunction transistor

Your first circuit using a PUT was a slow-speed oscillator that made an LED
flash about twice each second. The flashes looked very “electronic,” by which
I mean that the LED blinked on and off without a gradual transition between
each state. I’m wondering if we can modify this circuit to make the LED pulse
in a more gentle, interesting way, like the warning light on an Apple MacBook
when it’s in “sleep” mode. I’m thinking that something of this sort might be
wearable as an ornament, if it’s small enough and elegant enough.

I’m also thinking that this first soldering project will serve three purposes. It
will test and refine your skill at joining wires together, will teach you point-to-
point wiring with perfboard, and will give you some additional insight into the
way that capacitors can be used to adjust timing.

Look back at the original schematic in Experiment 11, on page 82. Refresh your
memory about the way it worked. The capacitor charges through a resistor un-
til it has enough voltage to overcome the internal resistance in the PUT. Then
the capacitor discharges through the PUT and flashes the LED.

If you drew a graph of the light coming out of the LED, it would be a thin, square-
shaped pulse, as shown in Figure 3-73. How can we fill it out to make it more like
the curve in Figure 3-74, so that the LED fades gently on and off, like a heartbeat?

One thing is obvious: the LED is going to be emitting a greater total amount of
light in each cycle. Therefore, it’s going to need more power. This means that
C1, in Figure 3-75, must be a larger capacitor.

When we have a larger capacitor, it takes longer to charge. To keep the flashes
reasonably frequent, we’ll need a lower-value resistor for R1 to charge the ca-
pacitor quickly enough. In addition, reducing the values of R2 and R3 will pro-
gram the PUT to allow a longer pulse.

Most important, I want to discharge the capacitor through a resistor to make
the onset of the pulse gradual instead of sudden. Remember, when you have
a resistor in series with a capacitor, the capacitor not only charges more slowly,
but discharges more slowly.

Time

Brightness

Figure 3-73

Time

Brightness

Figure 3-74. The original PUT oscillator

circuit in Experiment 11 made the LED

emit sharp, short flashes. The upper graph

shows what we might find if we measured

light output over time. The second graph

shows a gentler onset to each flash, fol-

lowed by a slow fade-out. Capacitors can

be used to create this effect.