Experiment 20: A Powerful Combination part 1

Experiment 20 of Make:Electronics describes a hardware device that protects a computer (or any other electrical device) from being used unless a specific numeric code is entered on a keypad. The experiment is built around two 7400 logic chips (74HC08 and 74HC04). These chips process the input from the keyboard and trigger a 555 chip if the three digit input is correct. The 555 chip in turn switches a (latching) relay.

Initially I had a problem finding a numeric keypad suitable for this experiment. The Velleman 12 keys Keypad KP-12 wasn’t available at my local supplier. Luckily I found an alternative that is fulfilled the requirements (and looks suspiciously similar to the Velleman).

Breadboarding was done according to figure 4-84 of the book. I double checked all the connections. At this stage I left the relay out and tested the circuit. After punching 1-4-7 the LED on the output the 555 timer lit up however with further testing I discovered a problem. Every numeral code that includes 1 and then 7 lit up the LED (e.g. 1-9-7). After comparing the circuit on my breadboard with figure 4-84 I discovered that I mistakenly had pin 4 and pin 5 of the 74HC08 logic chip connected (see image below). Also digit 4 of the keypad was connected to pin 5 instead of 4. After I corrected the mistakes everything worked fine. Next I’ll try to connect the relay to the circuit.

Overview of the circuit on the breadboard. Relay is disconnected from the rest of the circuit.

The mistake that I made visible in the ‘fish eye’

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