low voltage timer

[justDIY]

looking at the datasheet for National LMC555, it seems to indicate it works down to 1.5v ... am I reading this correctly?

I'd like to build a one-shot timer to control lights in an art piece ... push a button, lights come on for N seconds, and then off until the next viewing.

the lights are powered with two AA 1.5v batteries, using a switchmode boost converter (outputting around 22v@25ma). when the batteries are new, the voltage is a little over 3v, however, as time goes by, the switcher sucks the batteries dry, and eventually stops working properly around 1.5v (minimum input for the switcher is 1.8v)

my plan is to use the 555 to control the switchers enable pin, rather than bothering with the added bulk of a switching transistor of some sorts.

is the C555 the best chip for this job, is there something better / smaller / simpler?

oh, how do I figure out what the stand-by current of the 555 is? The switcher draws 1uA while in shutdown, which is probably lower than the self-discharge current of the batteries.

 

[Hero999]

looking at the datasheet for National LMC555, it seems to indicate it works down to 1.5v ... am I reading this correctly?

Yes.

oh, how do I figure out what the stand-by current of the 555 is?

The datasheet says 250:mu:A@5V, if you're worried then use a small p-MOSFET to turn the 555 off in standby mode.
**broken link removed**

 

[eng1]

A small PIC (PIC12F629 for example)?

From the datasheet:

Low power consumption:
- < 1.0 mA @ 5.5V, 4.0 MHz
- 20µA typical @ 2.0V, 32 kHz
- < 1.0µA typical standby current @ 2.0V

 

[justDIY]

oh come on ... tempting me with microcontrollers!

talk about an atombomb of a solution

on the other hand ... PWM sounds nice ... fade in/fade out

trying to time out 300 sec with the 555 takes a whopper of a capacitor (47u) and huge value resistor (5.8M) ... maybe the PIC is a simpler solution, especially since this is basically a one-off

 

[ljcox]

is the C555 the best chip for this job, is there something better / smaller / simpler?

A monostable IC such as the 4047 would require less components. From memory, I think they work at 3 Volt. Check the data sheet.

 

[Hero999]

You're better of using a propper CMOS timer IC like the CD4060, CD4521, CD4536 or CD4541, are there any others I've missed?

I don't know if any of the above are available in HC flavours which will work down to 2V.

 

[justDIY]

none of my favored suppliers even carry much of the 4000 series any more, the 555 only needs 1 resistor and 1 capacitor, that's not many parts ... I like eng1's idea, even thought it's a lot more expensive.

 

[justDIY]

well, the order is placed, and the most expensive solution wins

to up the cost even more, I'll be using a qpox QT100 touch sensor instead of a mechanical switch

thanks for the input everyone!

 

[Hero999]

none of my favored suppliers even carry much of the 4000 series any more,

Which is a shame because TTL and CMOS logic often provides good cheap designs, especially if you don't have a programmer. PICs and programmable logic are good but in away it's a shame that some suppliers have stopped supporting the more traditional alternatives.