Battery currents.

[Mint.]

I have built a circuit that utilises a 7812 and two 9V batteries to power a larger circuit with two relays and four 555 timers. The highest current consumption that the circuit needs is about 200mA, but it is only used in very short bursts of 15 seconds with high and low states, otherwise it is in standby mode which consumes 50mA. I also will probably keep the circuit fully off by a DPST switch when I am not using it. I have two 9V batteries in series which are old and probably are on their last leg, but I used them anyways as test batteries for the circuit and they seem to be giving off too little current to switch on my relays. How do I chose a battery that will be able to switch on the relays?

Here are some of the results:
Standby: 43.6mA
One relay activated: 67.1mA
Two relays activated: 90mA
Relay activated 555 multivibrator High: 120mA Low: 88.8mA
Both relays activated and with two 555 multivibrators running High: 194.2mA Low: 162.3mA
Frequency is around 1Hz

[Mint.]

Here is the circuit. The 7812 is not shown in this schematic. There are to identical copies of the circuit. One simply is connected to the other and they share the same voltage rails.

[Astroplio]

When you have the highest current draw, at what voltage level is the Vcc dropping?
I believe, since your batteries are old, it drops below or goes very near the required voltage for the regulator to work under spec.

From my experience, 9V batteries don't mix well with voltage regulators, unless you use them with LDO 3.3V perhaps, but that's an assumption.

I had used two 9V in parallel and a 6V regulator to get sufficient power output when I had built this DIY line follower robot:
http://myrobots.gr/robots.html   ("babycloud") Otherwise it wouldn't have worked as expected.

Of course now I know better, and I would have used a AA battery pack, which is also my recommendation for your project, since it will make it more reliable in the long run.

George :-)

[Mint.]

When you have the highest current draw, at what voltage level is the Vcc dropping?
I believe, since your batteries are old, it drops below or goes very near the required voltage for the regulator to work under spec.

From my experience, 9V batteries don't mix well with voltage regulators, unless you use them with LDO 3.3V perhaps, but that's an assumption.

I had used two 9V in parallel and a 6V regulator to get sufficient power output when I had built this DIY line follower robot:
http://myrobots.gr/robots.html   ("babycloud") Otherwise it wouldn't have worked as expected.

Of course now I know better, and I would have used a AA battery pack, which is also my recommendation for your project, since it will make it more reliable in the long run.

George :-)

Right! Thanks for suggesting to check the voltage. Its dropping down real bad, to 10v at first and then 8v in operation D:
What do I do?!

[Monkeh]

For the love of..

Here, have some compression.

[grumpydoc]

Right! Thanks for suggesting to check the voltage. Its dropping down real bad, to 10v at first and then 8v in operation D:
What do I do?!

Buy fresh batteries??

[michaelym]

The battery's series resistance increases with discharge. So even if the battery's measure 7v each and you have 14v in series, when you draw current the voltage drops. If the resistance is R and the current is I the remaining voltage is 14V-R*I

[TerminalJack505]

Here's an idea.  Buy one of those (for example) 8 AA battery holders that has a 9V battery snap connector. 

You can probably ditch the 7812 and just run your 12V rail directly from the battery pack.  You'll get a lot more juice from the AAs too.

[michaelym]

You can simply use an old charger. I have a whole box of them.