From: Winfield Hill
Subject: Re: Low current "hibernation" circuit
Date: 16 Nov 2002 14:00:08 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.10
> Does anyone have a low power, low current drain circuit that will
> turn another circuit "off" after a certain period of time, until
> triggered again. I want to have the ability to put another circuit
> into "hibernation" after a period of time, until triggered to wake
> up. I would like it to have as low a current drain as possible,
> as it must be battery operated, and have as low a parts count as
Hi Kim. For long-term low-power timers it's customary to use
cmos counters with an moderate-frequency RC oscillator. For
example, check out the CD4060 or 74HC4060 type of logic ICs.
The CD4060 can be used with 3 to 16-volt power supplies, while
the more modern 74HC4060 part is for 2 to 6-volt supplies.
Both '4060s are 14-bit counters with an included oscillator.
The reference to 14 bits means it can count up to 2^14 or 16384
pulses before wrapping around.
You can use it with a logic flip-flop element to perform timing
operations, or in certain cases you can use it all alone. The
circuit below is an "hour of power" and will provide an output
for 1 hour after the reset switch is pressed.
. x | Vdd |------+-----+--- +V
. ,-|>o-+--|>o--+---|> CK Vss |--, | |
. | | | |______________| gnd R4 |
. | R1 | reset | Q14 | | E Tr1
. | | C | | +-- R3 --+-- B 2n4403
. +- R -+--||---' | 1n4148 | C
. '-------------------- | --|<|--' |____ out
. f = 1/RC start | D1 hour of power
. R1 = 10R _|_ |
. V+ ---o o------+-- R5 -- gnd R3 to R5 = 10k
After one hour bit Q14 toggles and goes HI, which turns off the
drive to output transistor Tr1, and also disables the oscillator
via D1, freezing the circuit in the output-off state.
This timer circuit uses the last bit of the 14-bit 4060 counter
as an on/off flip-flop, so this means it only has 8192 cycles
available for timing, which means the oscillator will have to
run at 8192/3600 = 2.27 Hz. The data sheet says the frequency
is about f = 1/2.2RC, so if C is a 0.22uF film capacitor, then
R = 1.09 megohm. You could use a 1M 5% resistor with a series
200k adjustment pot.
Naturally you wouldn't like to wait an hour to check each trim
adjustment! Fortunately there are several good spots to look,
either at point x with an oscilloscope, or at one of the slower
outputs using a watch for timing, etc. For example, the Q4
output should cycle on/off every 7.048 seconds, Q5 every 14.1
If you want to use a more sophisticated timer chip, check out
the 16-bit MC14541 chip. It includes other functionality such
as auto-reset and Q/Q* selection.