I am trying to design a circuit involving an LMC555 CMOS timer operating as a monostable multivibrator. Essentially, I want a momentary switch to turn on a set of LEDs for ~20 seconds and then turn off. Details about the circuit can be found at http://www.electronics-tutorials.ws/waveforms/555_timer.html.
Based on my requirements, I am running this circuit off of a 9v battery. Ideally, I would like this circuit to draw as little current as possible when it is inactive (hopefully ~20uA or less) so that the battery might last a year or so (the circuit will only be "active" about 0.1% of the time). After building the circuit, I measured the current from the battery to be roughly 200uA, which is well inside the spec of the part. However, I wanted to see if I could make this much lower.
In order to limit the current drawn when the timer is off, I tried to design a latching circuit that would stop current from flowing to Vcc when the output is 0v (and the switch hasn't been pressed). The design is heavily influenced by Kevin Darrah's video "Low Power Arduino! Kill Power Circuit Tutorial" on YouTube.
The actual circuit I built looks something like this.
When I implement this circuit on a breadboard, I get weird results. When connecting pin 2 (trigger) to the NMOS/PMOS, the pin is pulled to ground and the output stays high. Doing a little more investigation, about 6 mA is flowing into the trigger when the 555 timer is completely off (which seems incredibly odd). It appears the circuit misbehaves whenever the trigger is connected to the MOSFETS. I have gotten this circuit working by manually connecting this wire to 9v/Gnd but find that the circuit draws 3mA.
Anyways, can anyone help explain why this circuit doesn't work in practice? In addition, if you could recommending a new method of deceasing the current to the chip that would be awesome. If you need more information or other measurements, I can provide those as well. Unfortunately I'm stuck with using the LMC555 so I can't perform any drastic changes to my design in that respect.
Best Answer
If you're "stuck" with a 555, this should work for you:
I don't have an LMC555 model, so I worked it out with a bipolar 555 and got less than 20nA of quiescent current between pulses.
Also, I picked the MOSFETs semi-randomly from the LTspice library, so you may want to pick something else which may be more appropriate for your needs.
HOW IT WORKS:
S1 is a Normally Open momentary pushbutton switch, and when it's open, Q2 (an N channel enhancement mode MOSFET) gate is pulled to ground through R1, turning Q2 OFF and disconnecting Q2 drain from ground. this causes Q1 (a P channel enhancement mode MOSFET) gate to be connected to the positive supply rail (V1+)through R2, turning Q2 OFF, which will disconnect Q1 drain from the positive supply rail. Under these conditions, U1-8 will be disconnected from V1+ and U1 will therefore draw no current from the supply.
When S1 is made, Q2 gate will be connected to V1+ through R3, turning Q2 ON and connecting Q2 drain to ground. This will pull Q1 gate down to ground through R4 and Q2, turning Q1 ON, which will connect V1+ to U1-8, the chip's power input pin.
At the same time, C2 will start charging up to V1+ through R6, which will bring U1 out of RESET, and a negative spike will be generated at U1-2 by C1 being abruptly connected to ground through Q2. The negative spike at U1-2 will trigger the timer and cause U1-3 to go high for 1.1 RtCt seconds, and this pulse will be output from the circuit and used externally.
The pulse is also used internally to keep Q2 gate high for the duration of the pulse by connecting U1-3 to Q2 gate through D1. This high will persist for the duration of U1's output pulse and serves to keep Q1 and q2 latched ON after S1 is released, which will keep U1-8 connected to V1+ until the pulse times out.
When that happens, Q2 will turn OFF, which will turn Q1 OFF, which will in turn disconnect U1-8 from V1+, turning U1 OFF and causing U1 to draw essentially zero quiescent current from V1 until the next time S1 is pressed, starting the cycle anew.
Here's the LTspice circuit list just in case you want to play with the circuit. Enjoy! :)