Automotive 12v timer help

[bbortnick]

Hi;

I would like to build a 12v timer with the following specs: Provide 12 volts to a .35amp (4w) device while the car ignition is on, and then for an additional 12 hours approximately after the ignition is turned off. Once off, current draw must be very low so as not to kill the battery.
Probably 555 timer based?

Thanks for any advice.
Barry

 

[crutschow]

12 hours is too long for a 555. You could use a CD4060 based timer as shown here.

You can drive the load by a 1A or larger P-MOSFET with the gate controlled by the appropriate CD4060 output (pin 3). The P-MOSFET source is connected to +12V and the P-MOSFET drain goes to the grounded load.

You also need to use the output of the CD4060 going high to inhibit the CD4060 from continuing to oscillate, otherwise it will turn back on after another 12 hours. The collector of small NPN transistor, such as a 2N2222 connected to pin 11 can perform that function (emitter to ground). Connect the transistor base to the CD4060 pin 3 output through a 20kΩ resistor.

And you will need to reset the counter when the ignition is on (ignition accessory power to pin 12 through a 20kΩ resistor). This will turn on the load. The counter will then start counting when the ignition goes off. The resistor and capacitor shown in the referenced schematic should be removed.

The circuit will draw about 0.6mA when OFF which is negligible for a car battery.

 

[IanP]

Hi;

I would like to build a 12v timer with the following specs: Provide 12 volts to a .35amp (4w) device while the car ignition is on, and then for an additional 12 hours approximately after the ignition is turned off. Once off, current draw must be very low so as not to kill the battery.
Probably 555 timer based?

Thanks for any advice.
Barry

It can be done in several ways (incl. 555 timer, dividers etc) but the simplest, and I mean THE SIMPLEST, will be based on a small microcontroller, and I always advocate for the use of the PICAXE (the tiniest in the family is the PICAXE-08).

Here is an example:
https://www.edaboard.com/threads/128088/

All what you need to do is to modify tje loops so they will count for 12hours, or whatever you need ..

:wink:
IanP

 

[bbortnick]

Thank you so much for the help; as I have no capability to program a uController, I would need to use the other option.
If you could be so kind as to review my schematic and parts list, that would be very kind:
https://obrazki.elektroda.pl/5742213300_1363146268.jpg
I have only a basic understanding of semi-conductor components and circuitry, so your comment about removing the resistor and capacitor, I assume are the timing adjustment components; does that make this circuit now default to 12 hours without them?
Thanks!
Barry

 

[crutschow]

The removed resistor and capacitor are for a power-on-reset function that is replaced by the direct connection to the ignition switch.

You are missing the resistors and capacitor for the oscillator. If you look near the bottom of my referenced article you will see those are connected to pins 9, 10, and 11. You select their values according to the table for the time you want. For 12 hours it would be pin 9 Cap = 0.22µF, pin 10 resistor = 10MΩ, and pin 11 resistor = 1MΩ.

If you want a flashing indicator that the circuit is working you can add the resistor and LED shown at pin 7 (which will blink ON and OFF for about 44 seconds for your circuit values (88 second period). You can increase the value of the resistor to 500Ω or more to reduce the current drain from that if you like.

To minimize the effects of any voltage transients add a 1 ohm resistor in series with the 12V to the circuit and also a 470uF, 20V (or greater) capacitor from the output (circuit side) of the resistor to ground.

Edit: The parts you selected look fine.

Edit 2: In the automobile it should work fine since there are other loads that will pull the output low when the switch is off but if you want to test the circuit in the lab you will need to add a 20k ohm resistor from the ignition switch output to ground.

 

[bbortnick]

Thanks for the help Zapper; please find an updated schematic and parts list:
https://obrazki.elektroda.pl/1395241400_1363199164.jpg
If you could please review, that would be great!
Thanks,
Barry

 

[crutschow]

Thanks for the help Zapper; please find an updated schematic and parts list:
https://obrazki.elektroda.pl/1395241400_1363199164.jpg
If you could please review, that would be great!

The 470uF cap needs to go from the right side of the 1 ohm resistor to ground.

Why are you using such high wattage resistors? They can all be quarter watt.

Otherwise, looks OK.

 

[bbortnick]

Thanks Zapper, I misread the instructions for the 470 cap.
Attached is the updated schematic:
https://obrazki.elektroda.pl/2860678400_1363208774.jpg
Does the mosfet dissipate much heat/does it need to be heat sinked?
Barry

 

[crutschow]

From the data sheet , the MOSFET has a maximum ON resistor of 0.2 ohms so the dissipation when ON will be .35² x 0.2 = 25mW. This will cause a barely detectable temperature rise with a bare transistor, so a heatsink is certainly not needed.

 

[bbortnick]

Undergoing bench testing See pic.
The timings are not quite what are expected, but hopefully, some combo will do the job.
Thanks for the help.