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https://makezine.com/projects/laser-tripwire-alarm/

I need help determining the pull up resistor needed for this circuit.

It uses a 555 Timer IC.

The resistor R1 acts as a pull-up resistor for pin 2. It helps to prevent false triggering from static electricity. But in many cases it can be left off without causing any problems.

enter image description here

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    \$\begingroup\$ Please add the schematic into your question. That way we don't have to follow a link to understand your question and it will still be relevant if the link dies. \$\endgroup\$ – Transistor May 4 '19 at 20:15
  • \$\begingroup\$ What measurements have you made for light interruption on each pin? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 4 '19 at 20:53
  • \$\begingroup\$ Looking at the photos on the page you linked is quite clear that R1 has a value of 1k and R2 is 100 Ohm \$\endgroup\$ – Elmesito May 4 '19 at 20:58
  • \$\begingroup\$ Will include schematic next time. @Transistor \$\endgroup\$ – fixit7 May 4 '19 at 22:56
  • \$\begingroup\$ @Elmesito Only reference to R1 is this. "The resistor R1 acts as a pull-up resistor for pin 2." \$\endgroup\$ – fixit7 May 4 '19 at 22:58
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enter image description here

This does not look like a great design missing so many details such as the light beam interruption and how stray light is blocked is very important.

You have an obligation to show what values you used and what layout and voltage vs light measurements. Then actually explain your problem with a question!

So I decided to simulate here on Falstad

enter image description here

On this, if you simulate a slight loss of light by sliding the pot to the right the LED goes ON ( Alarm ) and latches with Trig Pulled up on pin 2 . Almost any value works for Trig pullup, 1k to 1M

Then you can click on my SPDT switch to reset the alarm. Then slide pot back to left to simulate light detected and repeat.

The R2 value determines your threshold of detection, with the Control Reference exactly 2/3 of Vcc.

Edit 5:06 pm
Now seeing your values, looks like R2 is far too low and requires a lot of light to trigger. Increase it until you trigger then back off a bit just to simulate loss of light beam.

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  • \$\begingroup\$ Resistance of photocell is 165 ohm when laser is hitting it. \$\endgroup\$ – fixit7 May 4 '19 at 22:47
  • \$\begingroup\$ What is the "Threshold" voltage for each state? and Vcc \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 4 '19 at 22:47
  • \$\begingroup\$ Resistance of photo cell is 165 ohms. \$\endgroup\$ – fixit7 May 4 '19 at 22:48
  • \$\begingroup\$ Resistance is 24.6 from the 200K scale. I am confused as to what that value that is. @Sunnyskyguy EE75 \$\endgroup\$ – fixit7 May 4 '19 at 22:53
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    \$\begingroup\$ Datasheet shows everything. dot for pin 1 \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 6 '19 at 20:52
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I used a 2 inch piece of 3/4 inch pvc pipe to shroud the photo cell.

It reduced the amount of background light enough to where the circuit works.

enter image description here

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