"Arfa Daily" bravely wrote to "All" (05 Sep 05 12:53:28) --- on the heady topic of "Re: Long time delay"
AD> From: "Arfa Daily" AD> Xref: core-easynews sci.electronics.repair:341392
AD> Provided you use a good quality low leakage cap, and high stability AD> high value resistors, the delay time should be predictable with AD> reasonable accuracy, using the standard 555 timing formula. Shouldn't AD> take more than 3 or 4 runs to get the time delay down to what you need AD> within a few seconds. It seemed to me that the OP needed a 'simple' AD> solution, which the 555 circuit is, requiring only one resistor and AD> one capacitor besides the device itself ( the CMOS version doesn't AD> need either the supply decoupling against crowbarring, or the control AD> pin decoupling ). AD> This is in contrast to the clock and divider solution, which I accept AD> is totally predictable, but requires a divide ratio of 162000 ie 18 AD> bits to arrive at 45 minutes,even using a low speed clock such as the AD> 60Hz line supply ( which will also need clamping and shaping to form a AD> suitable clock ). Also, some ANDing will need doing on the output of AD> the divider to get an output at the required time delay. Not as simple AD> as the timer chip solution, properly implemented.
AD> Arfa
The cmos version of the "555" timer is the "7555" and an excellent choice for stable delays. I've used one to get a very stable 42 minutes delay using a poly cap and high value resistor for the timing elements. Only problem with the "7555" is the output can't drive as much current.
The 4060 is a nice binary divider chain with 14 stages but it isn't the only one. The 4045 has 21 stages for example. However, the nicer device is the 4536 which has 24 stages and is programmable. In one mode is accepts BCD inputs to control 14 stages. For example, this goes nicely with decade trimmer switches which output BCD directly.
A*s*i*m*o*v
... That was a fascinating period of time for electronics