# will this circuit work?

Status
Not open for further replies.

#### another guy

##### New Member
Hey guys

i posted a thread last week with the same topic..i wanted a circuit wen a top sensor is reached a 7 segment display must display F for full and when it reaches the bottom sensor it must display E for empty but didnt get the help..well i have tried to draw a drawing and just wanted your opinion.will the circuit work if no..what must i do to make it work and how can i do it?Basically im gonna use a non latching hall effect sensor to sense the top and bottom position and use a float that will go with the level of the water.

Thanx

#### Attachments

• 11.2 KB Views: 185

#### MikeMl

##### Well-Known Member
Works in theory, but:

The flip flop will not sink enough current to turn all the segments to form an E or F. You will need a switching transisor between the Flop and the display.

The dual opamp is running open-loop as a comparitor with one input grounded, which means that it will switch very close to 0V at the other input.

Depending on the sensor, you will likely need to bias the reference input of the two opamps up at some positive voltage, like +1V so the sensor can trip it.

Better, use a schmitt-trigger hex inverter to condition the sensor signals to convert them to logic levels.

Another suggestion: it seems to me that your application really needs a third state, i.e. display a 0 to indicate that neither end point sensor is tripped. That would represent the state where the tank is either draining or filling, but has not yet reached the end points. To do that, don't use a flip-flop, just the logic levels from the sensors, like this truth table:

Code:
Top Bot  Display
1     0       F
0     0       0
0     1       E
1     1       *
* never can happen, don't-care term

Two or three Nand or Nor gates would encode the two sensor signals into the three signals required to drive the seven segements.

#### another guy

##### New Member
hey...thanx for the reply..and i understand fully what u saying but theres just a thing that im unsure of is that how will i bias the opamp so that it will trip the sensor.can anyone help me with that thanx.i have attached the datasheet of the sensor

thanx

#### Attachments

• 234.3 KB Views: 202

#### MikeMl

##### Well-Known Member
These already produce a logic level output. You do not need to use the opamps at all.
If you want a logic output, just connect a pull-up resistor between the sensor output and the logic Vdd. When the sensor is off, the resistor will pull the output to a logic high. When the sensor is on, it is capable of sinking 20mA.

With 20mA, you could drive the segments of the display directly.