I have a lovely Atmel touch sensor whose output is high only as long as it's touched. I want both off-on and on-off edges of its output to initiate two 3 sec pulses (and the time between edges is > 3 sec). I route the output through a dual inverter (delay), which is XORed with the original to create a brief positive pulse at each edge. The brief pulses each trigger in succession the 555 which outputs the two 3 sec pulses. If I just wire the inverter, XOR and 555 to the V supply (3.3V), then everything works great.

Complicating the situation is that I want to run off battery power and of course want to consume as little current as possible for long battery life. As such, I want the touch sensor to sense continuously. It is a low quiescent current sensor, so it doesn't load the battery too much. But I want to energize the inverter, XOR and 555 only while the touch sensor is touched and for an additional 3 sec after it is no longer touched in order to do some start and stop functions elsewhere in the circuit (controlled by a PULSED_33V signal).

It's easy to power the components with the sensor touched/on in that I just send the sensor output to the base of an NPN tied high, which draws down a PMOS gate, which supplies 3.3V. My challenge is finding a 'bootstrapping' strategy to keep power on for a moment just after the output goes off. I sought to use two NPN's as an OR gate to the PMOS, the 555's pulse being fed to the base of the second NPN, and putting a nice fat 10uF capacitor between the 3.3V supply and the components' power inputs to energize them briefly until the hi-to-low 3 sec pulse arrived to take over keeping the PMOS on. But it's not going as well as I'd hoped.

In the attached pic, notice the dual inverter (IC5) on the left. I actually had to include an additional capacitor, since the interver's delay wasn't quite enough to provide a sufficiently delayed version to the XOR (IC2). And I had to include an additional NPN inverter because the 555's (IC1's) TRiGger input expected a high to low transition to trigger it. Continuing to the right is the RC for timer, an LED socket and resistor as a troubleshooting indicator, and finally the second NPN of the OR to the PMOS. On the far right is the fat capacitor that I hoped would be sufficient to maintain power until the on-off edge percolated through to the 555.

Can anyone suggest a better strategy for keeping those components powered until the 555 can take over? Or show me what I might tweak to get it to work? Or just show me how my circuit is hopelessly flawed? Thank you so much.

When the components are hard wired to 3.3V (removing R115 and shorting C9), touching the sensor causes the LED to light up for 3 sec and when the sensor is no longer touched, the LED lights up for 3 sec again. But when trying to bootstrap with R115 and C9 present in-curcuit, the LED is bright for 3 sec, then is less bright until the sensor is not touched at which point the LED just goes off and stays off.

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  • \$\begingroup\$ This is just a canned comment to let you know that what you're trying to build from discrete analog components (possibly incorporating Opamps and/or NE555) is a digital control problem and thus can easily and with lower parts count be solved with a microcontroller with really minimal firmware to write. \$\endgroup\$ Nov 18, 2019 at 20:37
  • \$\begingroup\$ Ah, at least you're using the MIC1555 which can work down to a supply voltage of only 2.7 V, and "only" uses a quiescent current of about 240 µA; that's only 100 times as much as say a typical 32 bit ARM microprocessor (e.g. stm32f030f4) in stop or standby (which can be woken up by the sensor without any ado). \$\endgroup\$ Nov 18, 2019 at 20:48
  • \$\begingroup\$ Ok, Marcus. I'll try it. Will go ahead and buy a $50 devel board and a $1.36 chip from Mouser. I've got <10 DIOs, one AI and it's a low speed application, so even slower clocks would do, requires nonvolatile memory. I'm currently using a Bluetooth Bee Standalone so it can talk to my Android phone, so if you know of a similarly low power stand by microcontroller with an RF coms option, I might go for that instead. I have only intermediate level expertise in modern microcontrollers. Many thanks. \$\endgroup\$ Nov 18, 2019 at 23:21
  • \$\begingroup\$ "I want to energize the inverter, XOR and 555 only while the touch sensor is touched and for an additional 3 sec..." - why? \$\endgroup\$ Nov 21, 2019 at 0:53


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