Need help in a simple seemingly simple RC Circuit  - Page 1
Forum section: electronics / beginners

author: Sultanpepper123
author_rank: Regular Contributor
timestamp: September 16, 2022, 03:33:03 pm
content: Hello everyone,It's me again, I've been struggling in getting this circuit working in my work and again I'm really embarrassed to say I spent a considerable amount of time trying to get it working. My objective to try to use low power (3.3V/μA), low cost and jellybean components. The idea of this circuit is we have six latching buttons for tamper detection connected to one single dedicated pin while in a battery operated backup mode , In a tamper situation the button toggles the inputs state from O/C to ground which generates a single sharp drop of capacitor charge down to zero, the reason for using this RC circuit is that I need to be able to manipulate my 1 interrupt pin (dedicated) to get an interrupt from all six switches without having a single switch to latch the pin to ground indefinitely, making it impossible to detect any further tamper switching events .Now this circuit works if implemented with one switch and the output works like shown below without problems.Blue trace = Switch (pressed = 0V)Green trace = RC Circuit outputThe problem starts to happen when making another 5 similar circuits and joining them at the same output, the voltage drop now does not drop to zero , drops down from 3.2V maybe to around 2.44V(low) which is far the region to be recognized by the microcontroller as a zero (low). I've tried using series diodes at the output of each circuit but it did not help ( tried using MMBD4148 and 1N4007) .I would really appreciate it if someone would share his thoughts on how to make this work and what may be the culprit ? . If anyone needs more information, please let me know, and thanks to everyone for taking the time to read this thread.BRSP

author: pcprogrammer
author_rank: Super Contributor
timestamp: September 16, 2022, 03:53:19 pm
content: You might give this a try.I did not do calculations on the resistors nor capacitors. The diode is to suppress the positive spikes.Edit: it might need a single pull up resistor instead of the pull down resistor after the capacitors.

author: Sultanpepper123
author_rank: Regular Contributor
timestamp: September 16, 2022, 04:41:50 pm
content: Thank you sir for sharing the circuit and for taking the time to share your thoughts.From the looks of it this circuit seems to be quite similar to what I'm working with but the last resistor is shared at the output , correct ?Best regardsSP

author: pcprogrammer
author_rank: Super Contributor
timestamp: September 16, 2022, 05:17:30 pm
content: Yes the resistor is shared, but it might have a problem with the pulse only being positive. This kind of circuit has been used in early day's drum machines and electric pianos to make the attack/decay envelope.

author: eugene
author_rank: Frequent Contributor
timestamp: September 16, 2022, 05:23:25 pm
content: Why not use a single circuit with multiple switches in parallel?

author: Sultanpepper123
author_rank: Regular Contributor
timestamp: September 16, 2022, 05:55:02 pm
content: Quote from: eugene on September 16, 2022, 05:23:25 pmThank you sir for the suggestion. Unfortunately as soon as one switch is triggered(released since it is a tamper switch) it will stay latching the line to ground, I think any further triggering from other switches will not be possible .The device is intended to have several lines of tamper switches so the first accessible location when tripped it will stay tripped till the device gets assembled back together again.Best regards