12v check circuit

Look up comparators.

Bill

-- Ferme le Bush

R2

+15 ---+------------[33R]-------------+------+ +--- Vout | | | | | |k | o R1 | +-------[R3]-------+ [D2] [Rly] / [330R] | 1Meg | | | o o no | | | +------+ | | | | 10K | \ | | | | +----+---[R4]---|+ \ | /c | Gnd | | >---+---[3.3K]---| NPN | 12v [ZD1] Vin ---+---|- / \e | | | | / | | | | LM324 | | Gnd | Gnd | | | +--------------------------------+

How it works: ZD1 holds Vref at 12 volts. When Vin goes above 12 V, the comparator output goes negative, the transistor turns off, and the relay de-energizes, connecting Vin to Vout. When Vin goes below

12 V, the comparator output goes +, the transistor turns on, the relay energizes, and Vout is connected to ground.

ZD1 is a 12 volt zener. D2 is a 1N400x diode. The value of R2 depends on the current drawn by the relay. As shown, it is chosen to drop about 3 volts with a 12 volt relay that draws about 100 mA. If a different relay is chosen, change the value of R2 drop the voltage to the relay rating at the current the relay draws. RLY-451 from

$1.35) would work fine.

The supply connection to the LM324 is not shown. Pin 4 of the 324 goes to + and pin 11 goes to ground.

Ed

If you really don't care too much what the trigger level is, you could just use a relay.

Vin ---+--\ /------- Vout | | | ) O NO | ) | )