Time out circuit

[coolcat14]

I'm new to electronics, but I want to build a circuit to time out my dome light.  Say after 15 min. it turns off.  (this is for a dome buggy).  I have a toggle switch installed, and so I want the light to turn off after 15 min. has elapsed.  I also want, if I turn the toggle switch off, the light to turn off.
I have an off delay circuit already laid out from a book, however it requires a push button or momentary switch to activate the circuit however I have a lock in toggle switch.  So I was thinking to make my toggle switch turn on a time out circuit which will give my off delay circuit the pulse needed to make it active.
any ideas?
thanks,
chris

[Shahriar]

Dear coolcat14

There is a project in project section of this site named: "Courtesy Light "
it is very similar to what U are looking for, but U should  make somechanges...

HTH - Shahriar

[prateeksikka]

HI THERE!
USE AN RC CIRCUIT IN SUCH A WAY THAT VOLTAGE ACROSS C RISES EXPONENTIALLY,
FEED THE CAP VOLTAGE TO THE BASE OF A TRANSISTOR IN CUT OFF.CHOOSE R AND C SUCH THAT AFTER 15 MINS VOLTAGE ACROSS C REACHES 0.7V,THE TRANSISTOR WILL  BE DRIVEN INTO SATURATION AND CAN BE MADE TO SWITCH ON AN LED OR A RELAY IN COLLECTOR CIRCUIT.ISNT IT SIMPLE! 8)
PRATEEK

[audioguru]

Hi Prateek,
For a 15 minute time period, the capacitor's value and its leakage current will be huge. Maybe you could use forty-five 20M resistors in series and a 1uF low-leakage poly cap. Or one 20M resistor charging forty-five 1uF poly caps in parallel, driving a Mosfet.
It is the same problem with a 555 timer IC, so I would use a  CD4541 oscillator-timer or a CD4060 oscillator-divider IC with a reasonable resistor and capacitor value. ;D

[prateeksikka]

hi audioguru!
i know that the limits would be crossed and it will be quite difficult to chose RC combination for the same.but what would be the problem with a 555?
what are these CD4541 AND 4060?
I havnt heard bout them earlier. :o
prateek

[audioguru]

hi Prateek,
The same problem of an extremely high resistor value (900M!) and a low-leakage capacitor, plus more current from the 555's input messing-up the timing. The max recommended value of the timing resistor for a 555 is only 20M. A 20M timing resistor would need a 45uF extremely low-leakage timing capacitor (forty-five paralleled 1uF poly ones!). ;D

Go to www.datasheetarchive.com and look at the Cmos oscillator/divider/timers. They make long time durations a piece of cake. Google has many timer projects for them.;D

[Guest Alun]

Use a ICM7555 or even a TS555 with a 10M resistor and a 100uf tantalum capacitor, this will give about 18 miniutes 20 seconds but when you take into account for the leakage current it will be about 15 miniutes.