Nuts and Volts - January 2009

■ FIGURE 12

■ FIGURE 13

going to the off LED; it will become lit and the other one will go off. It will stay that way until you press the other pushbutton. The circuit is often called an SR Latch where S and R stand for SET and RESET. It’s also a flip-flop: one bit of memory.

Wrap-up

Build an LED Blinker

Just for fun, let's build a circuit that will blink the two LEDs in an alternate pattern; when one is on, the other will be off. We will use a CMOS digital IC: the CD4093. The IC contains four two input NAND gates as shown in Figure 12.

The signals in digital ICs are either high (+V) or low (0V). The output of a gate will switch between high and low, depending on whether its inputs are high or low. The input-output behavior of a gate is defined by its truth table. Table 1 is the truth table for a two-input NAND gate, where A and B are the inputs and X is the output. A 0 represents low (0V) while a 1 represents high (+V).

The CD4093 has what's called Schmitt trigger inputs. That means the value of the input voltage required for a 1 depends on whether the input is switching from low to high, or from high to low. It's meant to guarantee a “clean” transition on the output but, as we will see, it also allows us to build a simple square wave oscillator.

The circuit you will build is shown in Figure 13. Unlike Figure 11, this circuit is shown as a schematic. Part of your job will be to translate Figure 13 into something like Figure 11 before you start to build the circuit on your solderless breadboard. Q1 and Q2 are MOSFET transistors. Use a device like the 2N7000 or the BS170. Use a 1 µF capacitor for C1; either monolithic or tantalum. For R1, start out with a 1 megohm resistor which will give a slow blink. Try using smaller resistors to make it blink faster. When the LEDs blink fast enough, they will appear to be on all the time.

Now that you've got your Proto Buddy and an SBB, you can breadboard all sorts of circuits. You can find many circuits in issues of Nuts & Volts, and on the Internet. It's even more fun to dream up your own circuits and try them out. Instead of a battery, you can get a wall wart power module from many vendors; the surplus units are usually inexpensive. So, go have some fun with your new Buddy. NV

A complete kit for this project can be purchased from the Nuts & Volts Webstore @ www.nutsvolts.com orcallour order desk, 800 783-4624.

PARTS LIST

ITEM QTY DESCRIPTION

■ S1, S2 1 DIP Switch, Two SPST Switches ■ S3, S4 2 N.O. Momentary Push Button Switch ■ TB 1 Three Position Terminal Block ■ L1, L2 2 LED, T1 Bright ■ R1, R2 2 Resistor, 1K , 1/4W, 5% ■ R1 (EXP) 1 Resistor, 1M , 1/4W, 5% ■ C1 (EXP) 1 Capacitor, 1 µF ■ IC1 (EXP) 1 CD4093B ■ Q1, Q2 (EXP) 2 Transistor 2N7000 ■ N/A 15 Pin Jacks ■ N/A 10 Header Pins (Five Pair) ■ N/A 2 Jumper Block ■ N/A 2 HEX Standoff, M/F, 4-40, 1/4 in, 3/8 in long ■ N/A 2 4-40 Nut ■ N/A 2 #4 Lock Washer ■ N/A 1 Printed Circuit Board