Hi all,

I have been looking for days not so eventaully have given up and came here for advice.

I have a power line (3.3v 3ma max) that I want to turn off/on but I also need it to be latching so, my MCU toggles it and it turn on then toggles it and it turn off. I have looked at the logic familes but get a little confused trying to remember what a MUX vs SPST vs Analog switch vs J-K Flip flop can do and which is best so I thought I'd ask here.

The MCU wil be in low power in between events and so does not want to be driving a load / pin, also the solutions must be very LOW power ideally <1ua

Basically I'm looking for a very low power, MCU controllable latching SPST switch.


Thx

Steve

 

Hi Steve,
You need half a CD4013 D-flip-flop divider to drive a CD4066 bilateral switch. The CD4066 has 4 switches that can be paralleled for a lower voltage drop. The ICs are Cmos that draw no supply current.
The clock input of the CD4013 needs a positive pulse from the MCU.
If the output of the MCU goes open-circuit while it is sleeping then the clock input of the CD4013 needs a high-value resistor to ground.

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Uncle $crooge

 

I agree with Audio, but please note that the unused inputs to CMOS devices need to be connected to either Vcc or Gnd in order to prevent an unwanted drain on the supply line.

As Audio wrote, CMOS has a very low quescent supply current, but that assumes that all inputs are either high or low. If you leave one or more inputs open, there may be a significant supply drain.

In the case of the 4013, you need to connect the set and reset inputs to gnd. And to make it toggle, connect D to Q'. (Q' = Q bar)

All inputs of the other section, can be connected to Gnd or to Vcc, whichever is the most convenient.

If you have any problems, post your circuit so we can assist more.

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Len

 

Hi guys,

First thanks for the quick response, however I probably should have shown you the sensor schematic from the start (see link below) , but I do like to try to figure it out myself before I come here and pester. Anyway now understanding a little more maybe there is a different way?

http://www.discovercircuits.com/PDF-...tionalarm2.pdf

Could I not use a 74LVC1G79 (cute little single flip flop) as a toggle power supply? simply connect pin 2 of LMC7215 to the D pin of the 74LVC1G79 and use MCU to turn it off / on?

Probalbly a very good reason why not and I'm sure I will be told very soon

Thanks again.

 

I looked at the data for the 74LVC1G79 and it is very fast. The clock waveform must have rise and fall times of < 2.5 ns!

So I expect that you would not be able to make it work, particularily given your limited knowledge.

The 4013 would be a much safer choice. I forgot to mention that you will need a 100nF ceramic capacitor across the Vss Vdd pins.

What does MCU mean?

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Len

 

Hi,

MCU - Micro Controllor Unit (PIC / AVR) like an ATTiny13v, the one I'm using.

Isn't the 2.5ns a Min not Max, Can't clocks pulses can be held high indefinitly, isn't all that happens is that the device waits for the low?

Anyway, even if it is the MAX time I would like to know is the principle is right? if it is I'll look for something slower or maybe see if my MCU can handle it


Thx

Steve

 

Take a look at NC7SP74 from Fairchild. This is good old 74HC logic, and you should be able to power your comparator directly from one of the outputs. Connect the beeper directly to the power supply, not to the output of the flip-flop.
ZVN4306 does not appear to be a good choice for the buzzer switch if you are using a 3.3V supply. The Vgs(th) is too high (3V max). I would look at something like Fairchild's FDN327N, or some other low-threshold MOSFET. I'm a little baffled by this, because Dave Johnson is an excellent engineer.

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Ron

 

Hi Ron,

Thanks for the advice, I'm not using the buzzer I'm feeding it into a ATtiny13v so I can use it as a vibration sensor for my low power alarm project.

I'm keeping everything up to and including the 0.22uf cap, with this extra info does this anything change? apart from not having the ZVN4306 etc.

 

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Len

 

I believe the diode, resistor, and cap are just there to delay the turn-off of the beeper (hold it on for a couple of seconds after the vibration stops). I would eliminate them, take the output from the comparator, stretch the pulse in software if you need that function. You will get really crappy logic levels from the diode-R-C combination.

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Ron

 

Hi again everyone,

OK removed the cap, diode and resistor, get a very sharp pulse now (as the Attiny13v was looking for a change not pulse width I don't think it makes any difference to the functionality but I like less components, so thanks for that Ron)

I cannot find any NC7SP74, looked on RS, Farnell and Digikey, anyone know where I can get some in the UK ideally? or a comparable one for the job?

Thx everyone almost there (with this part anyway :shock: )

 

Does the latch have to be tiny? How tiny?

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Ron

 

Hi Ron,

I like small, I like small a lot but technicaly it does not have to be. I just like plenty of board space and keeping things small (have I said that enought yet!!).

What is the criteria I should look for as I can probably find a flip flop in a small package, is it just the current source for the LMC?

Thanks

 

Can you get 74HC74 in the UK? It is made in a variety of 14 pin SMT packages. As I said, you should be able to drive the comparator circuitry directly from one of the outputs.
Be sure you connect all unused inputs to GND, including those on the unused flip-flop (74HC74 is a dual). Note that set and reset are negative true.

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Ron