Help to improve this filtering circuit

The following shows the diagram for a sensing and filtering input of a break wire alarm. The logic input is held low by the break wire and goes high once the wire is broken. The triggering voltage for the logic high is around 6V. R2 needs to be very high ( min 1M ) to minimise the current consumption, the R1 makes a low pass filter combined with C1. Once the wire is broken, the voltage on C1 needs to be as high as possible ( over VT+ 6v) to maximise reliability. Considering TVS has 1uA leakage, how would you choose R1 and R2?

IC is CD4093BE with Schmitt trigger. https://i.imgsafe.org/6ce38ca7fa.png

Kindest regards

by Baphomet12
March 13, 2017

I can't see that R1 is doing anything.When there is a break C1 charges through R1 and R2 in series. The 1uA leakage is going to drop 2 volts in R2. If the battery voltage has dropped to say 7 volts this will result in only 5 volts on C1. I think I would go for R2=1M and R2=0.

by mikerogerswsm
March 14, 2017

The R1 is there to make a resistive path from beak wire to logic gate. Please note that the long ( 60ft) wire acts as an antenna so there must be resistance as it's connected to CMOS gate. But as you said R2 should be lower. I think I change both R2 and R1 to 500K.

by Baphomet12
March 14, 2017

Understood.

by mikerogerswsm
March 14, 2017
Add comment...

Please sign in or create an account to comment.


No Answers

No answers yet. Contribute your answer below!


Your Answer

You must log in or create an account (free!) to answer a question.

Log in Create an account


Search Questions & Answers


Ask a Question

Anyone can ask a question.

Did you already search (see above) to see if a similar question has already been answered? If you can't find the answer, you may ask a question.


About This Site

CircuitLab's Q&A site is a FREE questions and answers forum for electronics and electrical engineering students, hobbyists, and professionals.

We encourage you to use our built-in schematic & simulation software to add more detail to your questions and answers.

Acceptable Questions:

  • Concept or theory questions
  • Practical engineering questions
  • “Homework” questions
  • Software/hardware intersection
  • Best practices
  • Design choices & component selection
  • Troubleshooting

Unacceptable Questions:

  • Non-English language content
  • Non-question discussion
  • Non-electronics questions
  • Vendor-specific topics
  • Pure software questions
  • CircuitLab software support

Please respect that there are both seasoned experts and total newbies here: please be nice, be constructive, and be specific!

About CircuitLab

CircuitLab is an in-browser schematic capture and circuit simulation software tool to help you rapidly design and analyze analog and digital electronics systems.