2 bit register

After doing some testing with through hole BJTs there was always lots of bad connections in the bread board, so I thought I could make some SMD logic gates, which would also give me some practice with SMD reflowing.

So ordered some PCBs with AND, OR, NAND, NOR, XOR, XNOR, NOT, and BUFFER gates using 0603 resistors/LEDs and SOT 23 NPN transistors.

When I was waiting for the PCBs I continued testing the through hole BJTs gates and found out that the AND, OR and BUFFERs do not work in logic circuits when using an NPN due the the voltage drop a cross the PN junction. So if you cascade a lot of these gates each subsequent transistor has 0.7v less at its base compared with it's predecessor until one has less then 0.7v in and in this case the transistor would not be able to turn on.

After the PCBs arrived I cut them up with a hack saw and sorted out the ones that I wanted to solder, then I got to work soldering them.

I designed the PCB with and optional switch and LED which would allow me to see the state of the Gate if the switch was switched on. But for bigger assemblies I did not need the switch and LED so I did not need to fit them.

Here are the two variants of the same gate, I marked with a red do the components I needed to install for the Non LED gate for easier assembly.

Now looking at the parts I have made on my other posts we can recreate them and make them look neater as well as adding an extra feature.

So here we have a (2 bit register) kind of it is two D latches (1 and 2) as well as a rising edge trigger which is compromised of 5 NOT gates (I used NOR basicly the same) a NAND gate and and final NOT gate, this works because there is a small delay between one input to the Nand gate and the other which has to go through all the NOT gates. So for that brief period of time the NAND gate is on which outputs low so that's what the NOT gate is for to invert the output, which then goes to the Enable pins of both D latches. (You may notice am extra NOT gate and the beginning of the rising edge trigger, that was there so I could test it as a falling edge trigger)

The D latches are the same as before except that they have buffers (2 NOT gates) driving the LEDs so that the LED does not affect the Latch it's self.