CMOS 555 one shot timer not stable (LMC555), No issues with regular NE555 - Page 1
Forum section: electronics / projects

author: vis321
author_rank: Contributor
timestamp: March 18, 2020, 02:56:20 pm
content: Hello,I am trying to create a single shot timer circuit to enable to output of my single lipo battery protection circuit, every time you insert a new cell the output needs to be triggered enabled, for the mosfet to turn on.My idea was to implement a LMC 555 timer as it can operate at lower voltage, my lipo will very from 2.8v to 4.35v (LiHV).Testing the circuit with a regular NE555, there does not seem to have any issues, but when I switched over to a CMOS LMC555 timer, it would not work actually all the time. Sometimes it would pulse the output correctly the first two tries and other times it would continue to stay on or be erratic , I assume because of the CMOS logic, trigger and should not be left floating. I don't see another way?The circuit is supposed to pulse the output for the protection IC for 1 second when a new lipo cell is inserted (essentially when VCC is connected no buttons for trigger pin).I want to avoid adding a button to enable to output.The Lipo protection chip is a BQ29700, and the I am using is a dev board is BQ29700EVM-610, would need to trigger TP4Eval board datasheet see page 2 TP4 pulls up DOUT which enables the output when briefly pulled up:http://www.ti.com/lit/ug/sluuaz3/sluuaz3.pdfBQ29700 datasheet notes on page 16 to briefly pull up that pin to enable output... :http://www.ti.com/lit/ds/symlink/bq2970.pdfI will not be using a charger(will be using external).This is an extension/continuation to another post I made.Please advise.

author: Buriedcode
author_rank: Super Contributor
timestamp: March 18, 2020, 07:46:50 pm
content: It is good practice to not leave inputs floating, especially CMOS. According to the datasheet the input current can be 10pA so that could be triggered by looking at it funny. I assume you want the output to be active high for 1 second upon power up (new cell inserted). Why don't you do what most applications call for an connect the trigger and threshold pins together?According to your schematic you have threshold connected to VCC via a resistor, but the cap is connected across the power supply. I'm not sure if this is intentional or a mistake. You would really only need two passives to get an output pulse on power up - a capacitor from trigger and threshold to ground, and a resistor connected form trigger and threshold to VCC. Upon power up, the cap is uncharged, at 0V, which means trigger/threshold is at 0V - setting the output high. As the cap charges its voltage rises, eventually getting to 2/3VCC, which is the threshold voltage - making the output low. You'll have to calculate R and C for charging to 66% (the threshold voltage) after 1 second. I'm guessing 100k + 10uF?Edit: an alternative ultra low power version would be a single logic CMOS inverter - probably a schmitt trigger - and the same RC concept. But if you already have a CMOS 555 it works just as well, albeit with ~100uA draw.

author: vis321
author_rank: Contributor
timestamp: March 27, 2020, 07:10:15 pm
content: Thanks yes for pointing that out, once I connected the trigger and threshold pins together, and thats all I needed to solve my issue, I figured there was something off and did not realize to tie down both pins. many thanks.