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

author: Sultanpepper123
author_rank: Regular Contributor
timestamp: September 16, 2022, 03:03:20 pm
content: Hello everyone,I've been struggling in getting this circuit working in my work and I'm really embarrassed to say I spent around 3 days 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 which goes into a hex Schmitt trigger inverter outputs this glitch to a positive pulse, 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 .Blue trace = Switch (pressed = 0V)Green trace = Inverter outputNow the circuit works when the pullup and the resistor in the RC circuit at the input of inverter do not exceed 22K ohms as the Schmitt trigger inverter struggles to work with weaker pullups ( 680 K ohm for example ), my intention is to have the lowest possible current consumption, The Inverter I'm using anM74HC14as this what is available in my local area. I've scoured the internet for quite some time but I've not found any datasheet stating anything of help. Are there any type of inverter/Schmitt trigger that works without problems with big resistors at their inputs ?.I've thought of maybe using inverting buffers which have Schmitt trigger inputs , but they are going to be ordered internationally so I would really appreciate it if someone shed some light on what to look for if they tolerate the high resistor pullup situation. If anyone needs more information, please let me know, and thanks to everyone for taking the time to read this thread.BRSP

author: ledtester
author_rank: Super Contributor
timestamp: September 16, 2022, 04:56:51 pm
content: Please attach your pics. It's easier to view them and will also preserve them on the site.QuoteWhen the switch is pressed (and thus latched) current will stop flowing through R2 when the cap fills up leaving only current through R1. So as far as current consumption goes you shouldn't have to worry about the size of R2.

author: pcprogrammer
author_rank: Super Contributor
timestamp: September 16, 2022, 05:22:25 pm
content: On another note, you did not have to start a new thread for more or less the same question as in your other thread herehttps://www.eevblog.com/forum/beginners/need-help-in-a-simple-seemingly-simple-rc-circuit/The schmitt trigger can shape the pulse for you, but won't solve your problem of mixing it with the other switches, unless you use an open collector/drain device. Only then can you tie the outputs together.

author: Sultanpepper123
author_rank: Regular Contributor
timestamp: September 16, 2022, 05:56:51 pm
content: Quote from: ledtester on September 16, 2022, 04:56:51 pmThank you sir for the advice, I've attached the picture for preservation.Best regards

author: Sultanpepper123
author_rank: Regular Contributor
timestamp: September 16, 2022, 06:00:46 pm
content: Quote from: pcprogrammer on September 16, 2022, 05:22:25 pmThank you sir for the suggestion, For this approach I was thinking of using a hex inverter for each channel/circuit and use a Schottky diode in series at each output before tying them together , would you approve of this approach ?Best regards

author: pcprogrammer
author_rank: Super Contributor
timestamp: September 16, 2022, 06:04:11 pm
content: Instead of R2 to VCC try it to GND and see if the ouput of the schmitt trigger inverts to constant high, and only a low pulse when the switch is closed. Make sure to use a clamping diode to suppress the voltage below ground, as shown in the schematic I posted in your other thread.If that works look for a schmitt trigger with open collector or open drain. A 74HCT05 for instance. Then you can connect the six outputs together and it should work. Don't forget a pull up on the output of these open collector inverters.

author: pcprogrammer
author_rank: Super Contributor
timestamp: September 16, 2022, 06:05:22 pm
content: Quote from: Sultanpepper123 on September 16, 2022, 06:00:46 pmA series diode can work, but as I wrote in my other post an open collector inverter is easier.Edit:https://www.ti.com/lit/ds/symlink/sn74hcs05.pdf?ts=1663351530053&ref_url=https%253A%252F%252Fwww.google.com%252F