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Subject: Re: one shot

Date: 10/26/01 at 7:32 AM
Posted by: David Ashby
E-mail: dashby@quin.co.uk
Message Posted:

In Reply to: Re: one shot posted by John Dunn - Consultant on 10/24/01 at 9:34 PM:

Hello Teddy,

John's suggestion is the best approach. The Cmos CD4020 is a 14-bit counter, so it's Q13 output goes high 8192 counts after the last master reset (MR) input. If 60Hz is available, you can use this via another flip-flop (to divide it by 2) to clock the 4020, giving roughly 5-minute pulses from the Q13 output.

Alternatively, the 4020 can be clocked by a CMOS 555 configured in astable mode to run @27Hz to generate a 5 minute pulse on this pin. However, while the 4020 static supply current is @ 1uA max (if my memory serves me correctly), a CMOS 555 runs @ 60uA typ, 200uA max. The current taken to charge/discharge the timing capacitor will be in addition to this, so to keep the supply current down, use smaller C, larger R.

This might be too much for your power budget. Another possibility is the CMOS4060, which combines an oscillator and 14-bit counter in one package. Quiescent supply for this device will not be as low as the 4020, but should be lower than the 555+4020. The trade-off is, the 4060's oscillator will not be as accurate as the 555. Again, you'll need to factor in the current for the timing capacitor used with the 4060.

The oscillator, rather than the counter, accounts for the power drain, so to obtain a much lower power solution needs another oscillator - perhaps a discrete (transistor or FET) design.

Yet another possibility is if you can find a real time clock IC which uses a 32kHz watch crystal. The power will be very low, but use of the device will probably be too complicated - I haven't looked into this.

Hope this gives you some ideas.

David Ashby.

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