Make: Volume 93

PROJECTS: Electronics Fun & Fundamentals

represent the pin functions. In each of the pinout

diagrams, you’ll see that some of pin labels have

a bar above them. This notation is commonly

used to tell you that the input or output is active-

low. The schematic of the circuit is in Figure

K

.

For the latch, I used another 7555 timer, wired

in bistable mode. (You may be familiar with this

concept if you’ve read my book Make: Electronics.)

Using a 7555 in this way is more convenient, in

this circuit, than using a component specifically

designed as a latch. Figure

L

shows how it

works.

When testing the circuit, use a 10µF capacitor

for C2, to slow the ...