Any help to make my switching network works

[Rinaishlene]

Hi I has designed a schmitt trigger switch which suppose ly to light up an LED when the input voltage increses to 8V and light off the LED when the input voltage decrease to 4V.The design req for the project is to have only a single power supply 12V,an uA471 op-amp and a npn BJT transistor.The opamp portion successfully acts as comparator but the problem is with the switching network where the LED just stays on and doesn't go off as required.
here I attached the cct for reference.

View attachment 37820

 

[audioguru2]

Hi Rin,
Welcome to our forum. ;D
Look at the datasheet for the 741 and you'll see that its output voltage can't go low enough to turn off the transistor. Add a resistor to ground at the base of the transistor to form a voltage divider of the output from the 741.
Since the transistor's base-emitter voltage is much lower than the voltage at the output of the 741 when it is high, then the small current in the added resistor won't affect the transistor when it is turned on. ;D

 

[Alun]

You might also need to use a lower value for R5 as the transistor might not saturate enough to turn on the LED fully, I'd go for 47k maximum and connect the base to 0V with 10k.

 

[Rinaishlene]

Well thanks for responding

But there is nothing I could do to improve my cct operation  because i read your both reply pretty late.....(by the time I read it,I had finished presenting and submitting report on the circuit design).


However ,I planned to try to implement your ideas on the circuit when I got back to campus after this semester break.

Thanks to both of you