Make:Electronics Experiment 16 (Emitting a Pulse)



Hi there, I have a question regarding Experiment 16 in Make:Electronics. I managed to get the circuit to work by following the book exactly. The circuit diagram is attached for illustration.

However, I have a question regarding what is mentioned on pg. 156 of the book, "R5 and R6 are known as "pull-up resistors" because they pull the voltage up"..."As long as the reset voltage is high, the timer is able to emit a pulse. It shuts down when the reset voltage drops".

I am a little confused about the function of R6. Basically, I tried two things, and measured the voltage on pin 4 of the timer:

1) removing R6, but retaining the connection from the positive terminal by replacing R6 with a jumper wire
--> the voltage on pin 4 is originally 9V. The LED lights up when I press S1, and pressing S2 brings the voltage on pin 4 to 0V, shutting off the LED. The circuit works the same as before.

2) removing both R6 and thus the connection from the positive terminal
--> the voltage on pin 4 is originally about .5V. Pressing S1, the voltage on pin 4 goes to 1V, and the LED still manages to light up. Pressing S2 brings the voltage to 0V and shuts off the LED. The circuit again, works exactly as before.

It seems to me whether I have R6 or not, or even just doing away with the connection from the positive terminal, doesn't make a difference. Am I doing something wrong here?

Thank you!
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