Experiment 18: Reaction timer part 2

Last Friday I’ve built the circuit as displayed in figure 4-37 (pg. 174) of the book Make:Electronics. The circuit consists of (in my case) three Kingbright 7 segmented digits. Each digit was connected to a 4026 decade counter. The decade counters are coupled by connecting the output of pin 5 (carry) to the clock input (pin 1) of the next counter. These are the blue jumper wires in the middle of the images below. As you can see a lot of jumper wires were needed. Every pin of the decade counters needs to be connected either to the digits or to the positive or negative voltage.

When I applied voltage to the circuit for the first time a problem occurred. The display immediately showed random numbers although I did not touch the tactile switches. A closer examination of the circuit showed me the cause of this problem. The pull-down resistor of the push button clock input of the first counter was not properly connected. This problem was easily fixed and the circuit worked then flawlessly however with one mistake. I had connected the first decade counter to the leftmost digit instead of the rightmost.

Although this experiment is a bit tedious with all the connections it was worth the trouble once I saw that the display worked properly and I was able to count to 999.  

Overview of the circuit corresponding to figure 4-37 of the book.  
Top down image of the circuit where the  tactile switch to the left is connected to pin 1 (clock input)
and the switch to the right is connected to pin 2 (disable clock) of the first counter.

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