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How do I make a dc circuit auto-off after 60 minutes


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I am designing a circuit that detects the presents of an object using an IR emiter and transistor detector. The circuit is powered by a 9v battery and sounds a buzzer when the IR light beam is interrupted.
I would like to add a feature to auto OFF the circuit when no objects interupt the IR beam within 60 minutes, to conserve batteries in the event I forget to switch it off. The power switch will be cycled to activate the circuit again.

I suspect a 555 timer circuit embedded in the above circuit is part of the solution. Is the 555 timer the way to go or is there a better way?
what would this circuit look like?

Is it possible for the circuit to automatically reset the 60 minute timer after each interrupted event and power down after 60 minutes of inactivity?

Thanks for you help,
Jeff in Florida

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Uman,
A very low-power CD4060 oscillator/counter chip can give a short pulse after 60 minutes if its oscillator is set to 273Hz. This pulse can trip a latch that controls a transistor that feeds power to your circuit. A simple 2-diode OR gate can be connected to the CD4060's reset pin, one diode for power-up reset and the other diode for reset after an event. The data-sheet for the CD4060 is here:
http://www.fairchildsemi.com/ds/CD/CD4060BC.pdf
The number 2240 sticks in my mind, it is a chip that combines a 555 with a counter/timer but I couldn't find it. Maybe you can find it.

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Try my timer it works well on 9vdc too!
Use a Larger capacitor like 2200uF to 4700uf and experiment with it.
It is a delayed off timer. When you push the switch it turns on the load and turns it off after the timming cycle. off. If the switch is pushed again durring the timing cycle it resets the time limit.

3_min_timer.jpg

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Uman,
The 555 will time to 1 hour or more but you will probably have difficulty finding a very-low-leakage capacitor. It can be reset at any time with its reset pin.
The CMOS version, ICM7555, due to its very low supply current, is recommended for your battery operation. Its data sheet is here:
http://pdfserv.maxim-ic.com/en/ds/ICM7555-ICM7556.pdf

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According to the schematic posted, the capacitor will not maintain the current necessary to keep the transistor conducting. The current is C dvdt. The the dvdt is 0, so the voltage will go to 11.3 and stay at 11.3. It will not discharge like it would with a resistor because the PN requires current to operate, where a resistor does not.

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Kevin,
That's correct, when the capacitor voltage reaches about 11.3V, then the timing period is over and the transistor turns off.
The switch resets the circuit by discharging the capacitor (and turning-on the transistor). The transistor stays-on during the timing period when base current flows through the resistor to charge the capacitor. When the capacitor charges to about 11.3V, then there will no longer be base current and the transistor turns-off.

But this simple circuit relies on having very low leakage currents in the capacitor and transistor, otherwise the transistor will never turn-off.
I believe that ALL transistors should have a resistor between the base and emitter, to bypass leakage currents.

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Hey audioguru,
I am very interested your proposed circuit. What is a "4060"? I search Fairchild and could not locate the data sheet. Please help.

Also,

I would like the shut off circuit to reset its timer and start over, if a signal goes high from a second circuit (feed back). If no signal is detected, than the shut off circuit would power off the circuit after a predetermined time.

Is this possible? My mulitmeter powers off if no signal is detected for 10 minutes.

Thanks,
Jeff in Florida

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Uman,
I posted a link to the data-sheet, then later I posted a link to a whole 4060 timer circuit. If your computer can't open a PDF file, then download an Acrobat Reader from many manufacturers' sites.
As I said earlier, the 4060 can be reset with a simple OR gate, made with diodes. One diode in series with the power-up-reset capacitor, and another diode between the reset pin and your (active-high) reset control.
The posted project is for a 24 hour timer, using an unstable electolytic capacitor in its oscillator. To make it a 1 hour timer, I would use a much smaller value and stable capacitor then select appropriate divider pins for the set-up LEDs, with the output taken from the divide-by-14 pin.

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Hey Audioguru
Thanks for your help and everyone else too. I ordered the CD4060 oscillator/counter chip from Fairchild semi and will try to integrate it into the circuit and make it work. Sorry about my confusion on the 4060, I ment to ask about the 2240 chip with the 555 timer you mentioned. How is it different than the 4060? It is an oscillator/counter right?

Thanks again,
Jeff in Florida

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Uman,
The 555 is designed to be powerful, with a 200mA output. But it uses quite a lot of power-supply current even when not driving anything, which causes a small battery to run down quickly. There is a CMOS version called the L555 or ICM7555 which uses hardly any supply-current. It has a powerful 100mA output.
The 4060 has a CMOS oscillator and long divider. It uses hardly any supply-current and has only about a 10mA output. Because of its long divider it can use a small and stable cap in its oscillator and still give long times.
The 2240 ate batteries.

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