Please check my attached schematic.

This is more of a learning circuit for me so I'm trying to figure out a few things.

I've got the LM393 comparator part of the circuit working great thanks to many members of this forum. Now I'm trying to switch on an LED only when both LDRs detect darkness at the same time. I thought I would use one of the TTL NOR GATE circuits supplied by a kind member of this forum but there's a small hitch.

The output at the LM393 pins 1 and 7 (o/p) drops from 7.53V when in the light to 373mV when in the dark. However, when I connect the base of Q7 and Q8 to pins 1 and 7, Led's D4 and D5 light up slightly. The NOR GATE works fine but I'm trying to avoid led's D5 and D4 from lighting up.

I stepped up R4 and R5 to 1K, did not work. I also tried swapping around the position of R4 and R5 with their connected leds but, did not help?

Can somebody please tell me why this is happening and how can I avoid it.
Thanks a lot -JDR04

 

Your Q3 and Q4 (NOR gate) is directly sitting across +9V and 0V lines. If one of them conducts, it'll be like shorting the power lines with a low resistance.

The resistance of R9 and R10 are not consistent too. Why one of them is 10K while the other is 1K. You want D3 to be lighted when both inputs of NOR is low, right?

Allen

 

I have made a slight modification on your circuit by adding another transistor Q5. When both inputs of Q3 and Q4 are LOW, their collectors would be High and thus Q5 would conduct and light the LED D3

Allen

 

Thanks absf, R9 and R10 are meant to be 10K. The objective is to light up D3 when both LM393 outputs go low. Thanks again JDR04

 

Thanks again absf, I'm a bit concerned about the short you mentioned in your previous post, from looking at your schematic it appears it will still be there?

 

No, there is a 1K resistor between +9V and both collectors of Q3 & Q4 so it should be safe. If you look at your circuit again, your collectors are connected directly to the +9V line.

Allen

 

Thanks for your help and time on this one absf. I appreciate it JDR04