I am building a circuit, part of which is giving problems (see attachment). I designed an AND gate using two transistors (Q2 and Q3) in series. The inputs to both transistors come from (A) square wave signal generator and (B) 555 timer in monostable mode. The output from the AND gate is to be an input to the active low input of another circuit say X. The idea is that the circuit X is to be enabled(active low) periodically from the low signal of the the signal generator (A) or an arbitrary low pulse from the 555 (B). Since the output pulse of 555 when triggered is high, the output needs to be inverted hence the transistor Q1. The problem I'm facing is: When Q3 is turned off by low signal of signal generator, the output taken above R7 goes low as intended, however when Q1 receives a pulse from 555 and inverts it to turn off Q2, Q2 does not turn off. Output at R7 just drops by fractions of a volt say from 4.8V to 4.5V. Please any help or pointers will be greatly appreciated. What am I doing wrong? I have tweaked resistor values and positions without any success. Please help!!!

 

R10 needs to be a much higher value, perhaps 10k.

 

If the signal generator is high, it provides a path for R7 through the base of Q3 and R10 preventing the signal from going low.

 

Remember the base-emitter junction looks like a forward-biased diode so, with only a 330Ω resistor in series with the base, the positive voltage from the generator goes through the resistor and raises the 1kΩ emitter resistor voltage, independent of any voltage on the transistor collector.

 

For future reference you can get tiny gate logic, single gates,
in a package.

https://www.google.com/url?sa=t&rct...inyLogic.pdf&usg=AOvVaw0Rj9hRvDpJTihGaIeQ4YrC


Regards, Dana.

 

I would try some MOSFETs or Opto-Isolators, and ditch the inverting Q if raising the value of R10 doesn't work.

 

Hi

Pull down the base of Q1 and Q3 with a ~22k resistor to gnd so they don't float.

eT

 

@AlbertHall Thanks a lot! The 10k resistor worked perfectly! @ElectricSpidey and @crutschow Thank you very much for the explanation. I never thought of it that way. Learning everyday I guess. To all those who contributed thanks a lot for your quick responses

 

Change the output PNP transistor then you can get the output voltage close to 5V.