Batteries and Voltage Regulators PLEASE HELP!

[Mint.]

I have a few questions on batteries and voltage regulators, so I would appreciate any kind of help.
1. Is it a good idea to hook up two 9v batteries to a 7812 to drop the 18 volts? My circuit will be using 170mA maximum for a short period of 10 seconds. Otherwise in standby it will use 20mA and most of the time it will be turned off by a switch.
2. How long will my batteries last and should I get somekind of special 9v? I will have access to the battery compartment to change them, but I'd prefer they would last as long as possible.
3. How do I connect my switch, should I use a DPST to cut off both terminals, or just use a normal SPST?
4. The 12v powers a relay, will the batteries current be enough to turn on the relay?
5. Any other tips would be much appreciated
Cheers!

[T4P]

170mA? ... Good luck being able to draw that much ... or if possible even last 2 hours ...
Use a DPST switch yes, the batteries? Again it will dissapear in a short while

[Mint.]

170mA? ... Good luck being able to draw that much ... or if possible even last 2 hours ...
Use a DPST switch yes, the batteries? Again it will dissapear in a short while

Thats why I asked!
Well its only going to be used a max of 5 or 6 times a day.
Just did some calculations and here is what I got:
40 mA relay with 555 timer, circuit from relay is an astable multivibrator, 35mA in low mode, 75 mA in high mode.
2 circuits with 40mA + 2 circuits drawing 75 mA during high = 230mA max
Suggestions would be much appreciated.

[Mint.]

Also forgot to ask, I'll be running the circuit power from the relay through 20 meters of wire, will the voltage drop be significant to impact the operation of my circuit at the other end?

[M. András]

why waste that amount of power to drop 6 volts of voltage?. use aa or d cells to get few volts above to use that voltage regulator, and you have much longer battery life, 9 volts one are good volt nothing

[mariush]

Why don't you generate the 12v locally from a boost converter? Shouldn't be that hard to generate 12v @ 200mA  when needed.

Then you could use 4 x 1.5v Ni-Mh batteries giving you 5.5-6.5v at high mAh, and you can use a low voltage drop to get stable 5v for your components... something like this springs to mind :  http://www.micrel.com/_PDF/mic2940.pdf  (up to 0.35v @ 1A)

For the 12v you can probably use something like this : http://www.ti.com/lit/ds/symlink/mc33063a.pdf  ... just not sure if it can do 200mA @ 12v from 5v. Search for other ICs if that's the case, I'm sure there are out there.

[Mint.]

why waste that amount of power to drop 6 volts of voltage?. use aa or d cells to get few volts above to use that voltage regulator, and you have much longer battery life, 9 volts one are good volt nothing

Circuit will be running for 20secs max at a time, I do not think that's enough to make the thing hot. Also the 555 has a max rating of 16 volts, therefore I cannot use the 18v from batteries.

[Mint.]

Bumping, really need some advice on this.

[T4P]

why waste that amount of power to drop 6 volts of voltage?. use aa or d cells to get few volts above to use that voltage regulator, and you have much longer battery life, 9 volts one are good volt nothing

Circuit will be running for 20secs max at a time, I do not think that's enough to make the thing hot. Also the 555 has a max rating of 16 volts, therefore I cannot use the 18v from batteries.

No. energy is a precious commodity with batteries, heat we can deal with but extra draw is pointless

[Things]

Is the only reason you're aiming for 12V is because of the relay coil?

If so, they will often latch at a significantly lower voltage. I've had 12V relays even latch down to 6V.

Keep in mind that running them under voltage means they will pull less current and thus may have issues latching, however this will more than certainly be more "efficient" than using a 7812 and 2 9Vs in series.

A 555 will run fine from 9V, and a 12V relay probably will too, so you can parallel the 2 9V's and ditch the 12Vreg.

[David_AVD]

Is the only reason you're aiming for 12V is because of the relay coil?

If so, they will often latch at a significantly lower voltage. I've had 12V relays even latch down to 6V.

Keep in mind that running them under voltage means they will pull more current, however this will more than certainly be more "efficient" than using a 7812 and 2 9Vs in series.

What the???  A relay coil has a fixed resistance value.  If you run it under voltage it will draw less current.

[Things]

Oops, yes, that's what happens to your brain after no sleep for a few days

[Mint.]

Ok here is how it is set up currently on the perfboard.
We got 12v input rails, the rails also determine the relay voltage output.
The voltage rails power the 555 timer.
I am really lost now, what should I do?

[Things]

What do you mean the rails determine the relay voltage output? Relays are simple on-off switches, they have no voltage output.

Maybe if you post a schematic of your circuit people could give more relevant replies.

Dan

[Mint.]

What do you mean the rails determine the relay voltage output? Relays are simple on-off switches, they have no voltage output.

Maybe if you post a schematic of your circuit people could give more relevant replies.

Dan

Ahh right, sorry. I've attached the schematic now.

[qno]

You can use a mosfet or a transistor to switch the second 555 on or off.

[Mint.]

Ahh screw it, I'm just going to do this. If this is a mistake, at least I'll learn from it