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TLC555-Q1: Delayed power on switch

Adam Doyle
Prodigy 20 points
Part Number: TLC555-Q1

I need to turn on an integrated load switch that is triggered by a single 5V logic high when a mechanical contact is closed for 10ms (+-1ms). If the contact is open for any amount of time the delay count is immediatly reset to the beginning. Meaning if the contact is closed for 9ms then open for 0.1ms the contact needs to be held closed for 10ms before turning on the load switch.  It seems like a 555 timer is the best IC for this function. This application is battery powered so the 555 timer will be powered by the output of the mechanical switch when this switch is closed. The battery voltage can vary between 8V-3V but this cannot affect the behavior of the 555 timer including 10ms delay. The switch has 50 Ohms of contact resistance when closed. Below is a 555 circuit that seems to have the behavior I am shooting for. Replace "5V" with the switch output, LED to load switch control input, and the R and C values will be different. Will this circuit perform the overall function I described or am I missing something? Also, this circuit needs to be extremely reliable with little tolerance allowed over a long life span. Is this kind of reliability possible using such a circuit? I'm sure there are many other ways to realize the behavior I need so any advice will be much appreciated!

Thanks in advance!

  • 0
    TI__Guru*** 149776 points
    Adam,

    This will work because pin 6 is high at start ensures output low after startup. After the allotted time pin 2 will be low to make output high.
    The missing part is to discharge the capacitor properly if the pulse is short to reset to capacitor charge. The internal diodes on pins 2 & 6 will provide some discharge to the timing capacitor. An external diode would be better.

    How long does the output have to be high after the 10ms delay? Once the contact pulse stops the 5V for the timer goes away.
  • 0 in reply to Ron Michallick
    Prodigy 20 points

    Ron,

    After testing on a breadboard it does function properly. Great catch with the diode. without the diode I was seeing a quick negative spike at the end of a switch event. Also I plan on putting a ~10nF capacitor across the Vcc pin and ground to stabilize the power rail during turn on, turn off, and output transition periods.

    The output should turn off immediately after switch open so this will work.

    Would you happen to have any reliability or failure analysis data on this part? The only thing I am able to find online is a very general failure study with little detail.