Hi Gengineer,
... I understand the open collector logic gates, like 74LS136, require a pull-up resister in its output terminal. ...
I suggest that you verify the current sinking capability of 74LS136 from the datasheet. I think the 74LS136 is only open-collector but it is not capable of sinking large current (0.05 A or 50 mA), 100 Ω might be a very low pull-up resistor. I am using a low-end smartphone which has a problem opening pdf files, I can't do the verification.
Now, my problem is that the logic gate isn't enabled to the extent that the reading still stays at 0.05A despite its two inputs being 0 (in which I know that isn't the case for a XOR logic).
Any advice?
If the input signals of two-input XOR gate are at dissimilar logic states, the output should be logic 1. The 74LS136 output is in "open state" but the ammeter provides a current path to ground. The current is approximately 5 V ÷ 100 Ω = 0.05 A = 50 mA.
If the input signals of two-input XOR gate are at similar logic states, the output should be logic 0. For 74LS136 the output voltage is very close to ground. One terminal of your ammeter is connected to this very low voltage while the other terminal is at ground. The voltage across the ammeter is very low, however, it can be read from the diagram that the ammeter's internal resistance is only 1 nΩ (1e-09Ohm). It might be possible that the voltage at the junction of 74LS136 output, pull-up resistor, and ammeter is about 50 pV. This results to 0.05 A = 50 mA so the ammeter display doesn't change.
You have to know or remember that an ammeter should be connected in series within the circuit where current will be measured. Delete the wire which connects the output of 74LS136 to pull-up resistor. Connect the positive terminal of the ammeter to pull-up resistor and the negative terminal of the ammeter to 74LS136 output.
Note also that whenever an input switch is thrown to logic high (5 V), a monitor LED is directly connected between Vcc and ground. You should insert a resistor in series with each LED, you can use a value of 220 Ω but it depends on the brightness you need.
Best regards,
G. Goodwin
