Make Electronics

Switching Basics and More 85

Experiment 11: A Modular Project

Step 1: Slow-Speed Oscillation

Figure 2-103 is a schematic version of the previous PUT breadboard circuit
shown in Figure 2-98, drawn so that the layout looks as much like the bread-
board as possible.

15K

470K

27K

2N6027

2.2uF

6V

DC

Figure 2-103. This makes it easier to see what’s happening in the breadboard version.

The 15K resistor and 27K resistor establish the voltage at the gate. The 470K
resistor supplies the anode of the PUT, but the PUT begins in its “off” condition,
blocking the voltage. So the voltage starts to charge the 2.2 μF capacitor.

You may remember that a resistor slows the rate at which a capacitor accu-
mulates voltage. The bigger the resistor and/or the larger the capacitor, the
longer the capacitor takes to reach a full charge. In this circuit, the capacitor
takes about half a second to get close to 6 volts.

But notice that the PUT is connected directly with the capacitor. Therefore,
whatever voltage accumulates on the capacitor is also experienced by the
PUT. As the voltage gradually increases, finally it reaches the threshold, which
flips the PUT into its “on” state. The capacitor immediately discharges itself
through the PUT, through the LED (which flashes), and from there to the nega-
tive side of the power supply.

The surge depletes the capacitor. The voltage drops back down, and the PUT
returns to its original state. Now the capacitor has to recharge itself all over
again, until the whole process repeats itself.

If you substitute a 22 μF capacitor, the charge/discharge cycle should take
about 10 times as long, which will give you time to measure it. Set your me-
ter to measure volts DC and place its probes on either side of the capacitor.
You can actually watch the charge increasing until it reaches the threshold, at
which point the capacitor discharges and the voltage drops back down again.

So now we have an oscillator. What’s next?