Make Electronics

Experiment 20: A Powerful Combination

198 Chapter 4

• Latching relay, 5 volt, DPST or DPDT, “2 form C” package, Panasonic DS2E-
  SL2-DC5V or similar. Must have two separate coils (one to latch, one to
  unlatch) with separate inputs. Quantity: 1.


• LEDs, 5mm generic, your choice of colors. Quantity: 3.


• Ribbon cable, with six conductors minimum, if you want to do a really
  neat job. You can use a cable of the type sold for hard drives, and split off
  the six conductors that you need, or shop around on eBay.


• Tools to open your computer, drill four holes, and make saw cuts between
  the holes, to create a rectangular opening for the keypad (if you want to
  take this project to its conclusion). Also, four small bolts to attach the key-
  pad to the computer cabinet after you create the opening for it.

The Schematic

This time I’d like you to study the schematic before building anything. Let’s

start with the simplified version, shown in Figure 4-83.

1 8

555

timer

0 1 2 3 4 5 6 7 8 9

Computer

on switch

Logic gate

power input pins

5VDC

Regulated

Figure 4-83. A simplified schematic showing the basic structure of the combination lock

circuit.