I'm developing a small intruder alarm that is triggered using a magnet and reed-switch (see
this thread for the main circuit design) that needs a 15 second delay. I considered a 555, but I've read so much about them being terribly innacurate that I decided against it.
The MC14541B was suggested over at
this site but I'm not sure how to set it up. From the looks of it, I could use a 4340Hz clock with a 2^16 divider (A,B set to 0,1) to bring it down to 1/15Hz. According to the datasheet, f=1/(2.3*Rtc*Ctc), but I can't work out what would be good values for the timing resistors and capacitor.
Would 47nF and 2.2k ohms be ok? My math is as follows:
1/(2.3 * 2200 * (47/1000000000)) = 4205Hz
1/(4205 / 65536) = 15.585s
Rtc = 2.2k ohms
Ctc = 47nF
Rs = 4.4k ohms
How would I link this in with my existing circuit (see other thread)?
Also, can I use a 7474 from the output pin of the MC14541B to latch the output high and drive a siren oscillator circuit? The idea would be that the output pin would go to the clock of the 7474, and the D and RST pins would go to the key switch like the ones in the original circuit do. I could then use a transistor (2N2222, BC108, etc) to switch the oscillator on and off.