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I'm helping out a student with his homework, which includes building logic gates with transistors on a breadboard, and so far I've successfully built NOT, NOR, NAND, and NOR/NAND logic circuits.

However, the last question is to build this sequential logic circuit on a breadboard, and so far nothing is happening (the LED doesn't light up):

schematic

simulate this circuit – Schematic created using CircuitLab

For the above schematic, Node 1 and Node 2 are inputs (either 0 or 1 depending on whether a switch button is pressed) and Node 3 is the output, which is an LED (so presumably it should light up on some kind of combination of inputs).

My NOR gate (which works correctly) is wired up like this:

transistor 1 left: gnd

transistor 1 middle: switch > 10k resistor > 5v

transistor 1 right: connected to the right of transistor 2; connected to 1K resistor then 5V; connected to LED then gnd

transistor 2 left: gnd

transistor 2 middle: switch > 10k resistor > 5v

transistor 2 right: connected to the right of transistor 1

I then made two NOR gates which I need to modify to make the sequential logic diagram above. Right now I have it wired like this, but it's NOT working:

NOR gate 1:

transistor 1 left: gnd

transistor 1 middle: switch > 10k resistor > 5v

transistor 1 right: connected to the right of transistor 2; connected to 1K resistor then 5V; connected to LED then gnd; connected to NOR gate 2 transistor 2 in the middle

transistor 2 left: gnd

transistor 2 middle: connected to NOR gate 2, transistor 1 right

transistor 2 right: connected to the right of transistor 1

NOR gate 2:

transistor 1 left: gnd

transistor 1 middle: switch > 10k resistor > 5v

transistor 1 right: connected to the right of transistor 2; connected to 1K resistor then 5V; connected to NOR gate 1 transistor 2 in the middle

transistor 2 left: gnd

transistor 2 middle: connected to NOR gate 1, transistor 1 right

transistor 2 right: connected to the right of transistor 1

If none of my description makes any sense, can someone just tell me what the expected output for the LED should be? I have two switches, so I can put in 00, 01, 10, and 11 as inputs. What should happen to the LED in those cases?

Edit: Here's my best attempt at a schematic for the sequential logic circuit. Transistors may be flipped around, not sure, but the sides I picked should be consistent (I know which side is which on my breadboard.)

schematic

simulate this circuit

schematic

simulate this circuit

Tidy-up of schematic edited in by @transistor. I have assumed transistors C and E were reversed in each case.

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  • \$\begingroup\$ A schematic of what you actually built would help. \$\endgroup\$ – Brian Drummond Mar 18 '16 at 21:58
  • \$\begingroup\$ Added my best guess of what the schematic is (transistors may be flipped, not sure) \$\endgroup\$ – Ginny Mar 18 '16 at 22:25
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    \$\begingroup\$ Hmm, sorry but that schematic is a bit of a mess. If you google for "NOR gate schematic" and search for images. You will see some more readable examples of what such a schematic might look like. Having a readable schematic will also help in getting the circuit right. \$\endgroup\$ – Bimpelrekkie Mar 18 '16 at 22:31
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    \$\begingroup\$ If the down-pointing arrows indicate Ground/negative, then the transistors are backwards. The arrows in the transistor symbol should point towards the negative side of the power supply (which is usually considered "Ground/Zero volts"). Show where the unconnected ends of the voltage sources connect. \$\endgroup\$ – Peter Bennett Mar 18 '16 at 23:07
  • \$\begingroup\$ @Ginny: See my answer to Question about RS flip-flop asked a few hours ago. \$\endgroup\$ – Transistor Mar 19 '16 at 0:33
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Assuming the redrawn schematic is an accurate representation of what you actually built, then the problem is that you have the LED connected directly in parallel with the B-E junction of Q2.

The circuit is working, but Q2 prevents the voltage across the LED from rising high enough to light it up.

The fix would be to make the connection between the base of Q2 and the LED using another 10 kΩ resistor. This will limit the current into Q2's base and allow the LED to light up.

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