Experiment 18: Reaction timer part 4

Previously I added 555 timer in astable mode to generate pulses for the counter chips. In the next step a second 555 chip is added. This timer runs in bistable mode (see pg. 176 of Make:Electronics). The purpose of this timer is to freeze the counting when the tactile switch is pressed (figure 4-40 of the book). The output (pin 3) of the timer is connected to pin 2 (disable pin) of the first 4026 decade counter. Adding this timer to the breadboard is straightforward. The only problem that I face is that the breadboard is becoming very crowded. The good news is that the circuit is almost finished. In the next step I will only have to add one more 555 chip.

Overview of the circuit which is becoming very crowded. 

Detail of the circuit with the two added 555 chips in the middle and the tactile switches left.