555 only when needed

  • Thread starter oneamp
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  • #1
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Main Question or Discussion Point

I built a circuit using a 555 timer, and I want it to be powered from a 9VDC battery. Only, 555s use current even when their output is low. I thought about changing it for a monostable multivibrator, having the ground of the chip connected to the collector of an NPN transistor, the output also tied to the base, so that when the base was activated (via a momentary press button), the multivibrator would keep the transistor base high until the output was off, and not use any power when the output was low.

But, I'm not sure if that idea will work well. So I am asking if there is anything I can do to keep the 555 from drawing current when not in use. Control of the circuit is done only with the momentary push button, which activates the 555 trigger. No on/off switches or anything like that.

Thanks for any ideas.
 

Answers and Replies

  • #2
Averagesupernova
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If you switch the low side of the power supply then the output will float up even when you want it off. Do something similar and switch the + supply. You are on the right track. I have never looked to see what a 555 draws, maybe a CMOS version would be more battery friendly? Or maybe there is a better choice device to use?
 
  • #4
meBigGuy
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Even the cmos 555 versions have R1/R2/R3. Look at cmos monostable multivibrators like the 74HC123. There are a zillion different one shots/multivibrators. Just pick the one that suits you need. High voltage cmos, High speed cmos, re-triggerable, not retriggerable.

BTW, I HATE the 555. It's only usefull if you really need the ratio'ed references or you happen to have one handy.
 
  • #5
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Thanks. The CSS555C requires way too much effort for building a programmer. Further, I'm not sure that I could use separate capacitor and resistor pairs with it to change the timing, which I plan to do with a switch.

I see the LMC555 advertises a '1 mW typical power dissipation". Does this mean that it uses 1mW while idle (low output)? If this is the case, I can have a few weeks of battery life assuming ~550 mAh in a 9V battery. Better, but still draining fast when I'm not using it. What about the CMOS 555 with a transistor setup like in my idea? Will that work?

Thank you
 
  • #6
meBigGuy
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the 74HC123 has 4ua standby current.
 
  • #7
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the 74HC123 has 4ua standby current.
I was looking at that as an alternative as well. Thanks for directing me to that version of the 123
 
  • #8
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So, This 74HC123 with something like a 100k ohm resistor on the base of a transistor to allow current up to 75mA through the collector to a load for a period of time, variable and dictated by a switch that connects different capacitors to the IC. Does this sound appropriate?

Thank you
 
Last edited:
  • #9
meBigGuy
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pretty much. There are lots of application examples for the 123, 121 and other monostables. You can connect different capacitors or different resistors, or use a current dac for the resistor. The field is wide open.
 
  • #10
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thanks
 

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