Experiment 11: A Modular Project
90 Chapter 2
Step 4: Pulsed Output
If you wanted to use this audio signal as some kind of an alarm, a steady dron-
ing noise is not very satisfactory. A pulsing output would be a much better
attention-getter.
Well, the first section of the circuit that you assembled created a pulsing signal
about twice per second. You used it to flash an LED. Maybe we can get rid of
the LED and feed the output from the first section to the second section. The
lower block diagram in Figure 2-112 explains this concept.
Can it really be that simple? Well, yes and no. The trick is to make the output
from the first section compatible with the input to the second section. If you
simply connect a wire from the cathode of the first PUT to the anode of the
second PUT, that’s not going to work, because the second PUT is already oscil-
lating nicely between low and high voltage, about 1,000 times each second.
Add more voltage, and you will disrupt the balance that enables oscillation.
However, remember that the voltage on the gate of a PUT affects its threshold
for conducting electricity. Maybe if we connect the output from Q1 to the gate
of Q2, we’ll be able to adjust that threshold automatically. The voltage still has
to be in a range that the PUT finds acceptable, though. We can try various re-
sistors to see which one works well.
This sounds like trial and error—and that’s exactly what it is. Doing the math
to predict the behavior of a circuit like this is far too complicated—for me, any-
way. I just looked at the manufacturer’s data sheet, saw the range of resistor
values that the PUT would tolerate, and chose one that seemed as if it should
work.
If you remove the LED and substitute R10 as shown in the breadboard diagram
in Figure 2-113, you’ll find that the fluctuating output from Q1 makes Q2 emit
a two-tone signal. This is more interesting, but still not what I want. I’m think-
ing that if I make the pulses out of Q1 less abrupt, the result could be better,
and the way to smooth a pulsing output is to hook up another capacitor that
will charge at the beginning of each pulse and then release its charge at the
end of each pulse. This is the function of C3 in Figure 2-114, and it completes
the circuit so that it makes a whooping sound almost like a “real” alarm.
If you don’t get any audio output, check your wiring very carefully. It’s easy to
make a wrong connection on the breadboard, especially between the three
legs of each transistor. Use your meter, set to DC volts, to check that each sec-
tion of the circuit has a positive voltage relative to the negative side of the
power supply.
Figure 2-115 shows how your circuit should actually look on the breadboard.Power
Fast Oscillator
Amplifier
Loudspeaker
Slow Oscillator
Fast Oscillator
Amplifier
Loudspeaker
Power
Figure 2-112. Top: The basic functions of
the noisemaking oscillator circuit shown
as a block diagram. Bottom: The same
functions with a slow oscillator added to
control the fast oscillator.