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PNP NPN combination switch for low water level alarm.

Discussion in 'General Electronics Discussion' started by Eguy, Jun 3, 2015.

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  1. Eguy


    Jun 3, 2015
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    Hi guys , I got a circute of water level indicator of over head tank from internet . It shows low middle and high level. There is also an alarm to prevent over flow. Overall I apreciate that circute as it is very simple but it lacks a low level alarm to inform the need to switch on the Pump. Over flow alarm and low level alarm are very necessary . So I tried to add something more to that circute to trigger the buzzer when water level is low . I tried many but nothing worked. I hope this will work. I would like to know your opinions about this PNP NPN combination switch.

    The first transistors is BC557 and the other one is BC547. 6 volt is aplaid to the base of PNP transister through water. Three volts drops due to the resistance of water. The 500 ohm resister drops 5 volts and gives 1 volt to the base. The 200 ohms resister connected to the emitter drops 4 volts and gives 2 volts to emitter. When water is above low level the current passes through the water keeps the base higher than the emitter. So the transister is OFF until the water comes to low level . The current flow stops through the water. So base gets only 1 volt . A 10K resister is connected to the ground to keep the base low. Now the base voltage is less than emitter voltage .Hence the transister is on . The out put voltage and current is low enough to trigger a 6 volt Buzzer. So I add one NPN transister. The output of first transister is fed to the base of second transister through a 1 k resister. When T1 switch on that trigger T2 .There is no resistance in the collector emitter path of T2 except the load 6volt Buzzer. So it can make a loud noise enough to make us alert.

    Attached Files:

  2. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    The 500 Ohm resistor is too low and will hold the transistor off. You will only have approx. 230 mV across the base and emitter terminals, this is not enough to fully turn on the transistor. Remove the 500R and see if that works.
    BobK likes this.
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