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Introduction
The basic bistable multivibrator we examined in the last experiment has a basic shortcoming: there are no external inputs to this circuit. After all, we can't very well use grounding wires in a digital circuit to get it to change states at specific times. We need to apply external input signals in a standard manner.
Since we are basically using RTL inverters to create the multivibrator, the obvious approach is to convert the inverters into NOR gates, and apply the input signals to the additional inputs. This would cause this circuit to behave just like the RS NOR latch demonstrated in one of the interactive pages on this site.
In this experiment, we will ad the components necessary to turn the basic bistable multivibrator into an RS NOR latch circuit.
Schematic Diagram
The circuit for the NOR version of the bistable multivibrator requires only that we add a transistor and input resistor to each of the two inverters already present, thus transforming them into 2-input NOR gates.
Note that we have now labeled the inputs and outputs using the standard latch nomenclature. It doesn't really matter which transistor you designate as the Q output, so long as you remain consistent with all input and output assignments. Thus, the designations here are strictly arbitrary.
Parts List
To construct and test the RS NOR latch circuit on your breadboard, you will need the bistable multivibrator circuit you constructed in the previous experiment, plus the following experimental parts:
- (2) 15K, ¼-watt resistors (brown-green-orange).
- (2) 2N3904 or 2N4124 NPN silicon switching transistors.
- Orange hookup wire.
- Black hookup wire.
- White hookup wire.
