FLOODING ALARM

INTRODUCTION:

Since I live in an area with a high water table, I have to deal with the occasional flooded basement that occurs when the power goes off and I am either not there to take care of the problem, or when I am asleep and do not know that there is a problem. Normally I have a "sump pump" that is sunk into a pit which is located in one corner of my basement that turns itself on/off as needed during the course of the day. However, if the pump does not operate on schedule, I have problems very quickly. What I am designing and fabricating, is a water alarm which would wake me up during the night when the power goes off and my basement begins to flood. The alarm that I am proposing to build would be battery powered and would be contained in a water-tight case which would sit on my basement floor very close to my existing sump pump pit. Extending into the existing pump well, would be a limit type float switch. When the float is not in contact with the surface of the water, the contacts of the switch would not be touching and the circuit would remain in an open (off) state. Once the level of the water got high enough so that the float contacts the surface of the water and began to rise, this would close the switch contacts turning on the circuit and sounding an alarm.

DESCRIPTION:

At the heart of the circuit would be a 555 timer which would be configured to run in an astable mode of operation. The on/off output of the timer would be used to control the pulsing of the alarm. The output of the timer would be a square wave voltage pulse which would vary between approximately zero and 5 volts DC. This square wave output would be applied to the base of an NPN transistor. When the "high" portion of the square wave pulse is applied to the base of the transistor, the transistor would "turn-on" and allow collector-emitter current to flow. By connecting one side of the alarm to the voltage source, and the other to the collector of the transistor, the output of the transistor would in turn be used as a switch to pulse the alarm on and off alerting me to a problem. The timing (duration and/or duty cycle) of the on/off pulses would be achieved through an RC timing circuit.

When the 555 timer portion of the circuit is configured so that it runs in an astable mode of operation by connecting pins 2 and 6 together as shown, the timer will trigger itself and run free as a multivibrator. When the water level gets high enough so that it closes the contacts on the switch, the external timing capacitor will then charge through the series resistor combination of Ra + Rb. Once charged, the capacitor will then discharge through Rb. The duty cycle of the circuit can be precisely set by the ratio of these two resistors. While configured to run in the astable mode of operation, the external capacitor charges to approximately 2/3 Vcc and then discharges to approximately 1/3 Vcc. The charge and discharge times, and therefore the operating frequency are independent of the supply voltage.

555 TIMER CIRCUITRY:

The internal circuitry of the 555 timer consists of two linear comparators with a series resistor arrangement consisting of three resistors which are used to set the reference levels of the two comparators at 1/3 Vcc and 2/3 Vcc respectively. The outputs of these two comparators are then used to set and reset a flip-flop unit. The output of the flip-flop is then brought out through an amplifier stage as the output. Besides just producing the chip output, the output from the flip-flop is also used to drive the collector of an internal transistor low to discharge the external timing capacitor.

When configured to run in an astable mode of operation, the external timing capacitor charges toward Vcc through the external resistor combination of Ra and Rb. The capacitor voltage rises until it goes above 2/3 Vcc, which is the threshold voltage at pin 6. At this point, the output from one of the internal comparators triggers the flip-flop so that the chip output at pin 3 goes low. At the same time, the internal discharge transistor is turned on causing the output at pin 7 to discharge the external timing capacitor through Rb. The capacitor voltage then decreases until it goes below the trigger level which is set at 1/3 Vcc. At this point, the output from the other comparator triggers the flip-flop again causing the chip output to go high, and also turn off the discharge transistor so that the capacitor can begin charging once again. This cycle will continue as long as power is applied to the chip providing that there are no other circuit problems.

PERTINENT VALUES:

The output of the 555 timer will be a square wave that will be high during the capacitor charge time, and low during the discharge time.

The charge time is determined as follows:

t1 = 0.693(Ra + Rb)C1

The discharge time is determined as follows:

t2 = 0.693(Rb)C1

It then follows that the total period, T, can be determined as follows:

T = t1 + t2 = 0.693(Ra + 2Rb)C1

From this the frequency of operation, F, can be determined as follows:

F = 1/T = 1.44/(Ra + 2Rb)C1

Finally, the duty cycle of the circuit, D, is determined by:

D = Rb/(Ra + 2Rb)

COMPONENT LIST:

Rin = 220

Ra = 47K

Rb = 100K

Rl = 1K

R2 = 10K

C1 = 10uF

C2 = 0.01uF

switching transistor = 2N4401

D1 = 1N4005

Radio Shack piezoelectric alarm

National Semiconductor 555 timer

12 volts supplied by two 6 volt RAYOVAC heavy duty lantern batteries connected in series

unit housing provided by Rubber Maid

Emerson Pump Float Switch

CALCULATED VALUES:

t1 = 1.02 seconds

t2 = 693 x 10-3 seconds

T = 1.713 seconds

F = 583.77x10-3 Hz

D = 40.48 %