+----/ o-----+-------------------- +12 output
| |
+ ---+---o o-----+-----RELAY-----+
PB | |
R1 \ -----
/ / \ TL431
|<------------/ \ or
/ /_____\ LM431
R2 \ |
| |
- ----------------+---------------+
You need 2 resistors, a relay and a TL431, plus a momentary
pushbutton switch. MPB - 1B for the switch, and RLY-428 for
the relay are good choices, available from Allelectronics
http://www.allelectronics.com/
The specified relay will handle 8 amps and can be driven
directly from the TL431. If you use a different relay,
you need to be sure that the TL431 can drive it directly.
Make sure that the relay's coil resistance is over 120
ohms. The higher the resistance of the relay coil, the
better. It will be tough to find a better relay choice than
the one specified. R1 is 2200 ohms. R2 is 352 ohms and can
be made with a 330 ohm and a 22 ohm resistor in series.
When you press the pushbutton, the relay will energize,
provided the battery voltage is over about 9.1 volts.
The relay contact, shown above the PB (pushbutton)
symbol will close conducting the 12 volts to the output
and to the TL431 (or LM431 - same device) through the
relay coil. The 431 will conduct as long as the voltage
from the battery is over about 9.1 volts.
The circuit shown will draw about .025 amps from your
battery pack. When the voltage drops to close to below
about 9.1 volts the relay will de-energize, and no
current will flow either to the load or to the circuit.
Ed