112021

Another set of terms often used are high and low. High relates to logic 1 and low relates to logic 0. High and low is the most suitable terminology because the signal 0 and 1 is actually a voltage applied to an electronic device. One or two inputs, depending on the device, are applied to a component called a logic gate, there is a single output from the gate. We will look at some simple logic circuits and analyse the inputs/outputs in a truth table. A control module will have in the order of several million such devices within each processor. Please refer to the three diagrams in Fig.3. AND diagram: The common area between A and B indicates an output that is applicable to both A and B. OR diagram: This relates to an output that is applicable to A or (and) B. NOT diagram: This relates to an output that is not applicable to A or B. Switched circuit theory Prior to looking at a transistor operated device, we can use a mechanical switch to get an understanding of the voltage at different points. See Fig.4. The left-hand diagram shows an open switch, while the right-hand diagram shows a closed switch. Now we understand the theory behind logic gates, we can analyse an electronic version of each circuit. Please refer to Fig.5, which shows a AND gate. This circuit uses two transistors which are connected in series with one another. The emitter of the first transistor is directly connected to the collector of the second transistor. A light emitting diode is connected to the output as a visual reference. The emitter of the second transistor is connected to ground via a resistor. The output connection is on the ground side of the second transistor but upstream of the resistor. The applied voltage (5 volts) is connected to the collector of the first transistor. It can be seen from the illustration that even if (only) one of the transistors is switched on, by applying a voltage to the base/emitter circuit, current cannot flow from the supply to ground. It can be seen that there is no voltage applied to the light emitting diode so the LED remains switched off. Only when both transistors are switched on can current flow to ground via both collector/emitter circuits. Now please refer to Fig.6, which shows an OR gate. This circuit uses two transistors which are connected in parallel, both collectors have an independent 5-volt supply and both emitters can allow current to flow to ground separately. Again, a light emitting diode is used as a visual reference. 24 AFTERMARKET NOVEMBER 2021 TECHNICAL/SNAP-ON www.aftermarketonline.net Fig.1 Fig.3 Fig.4 Fig.5 Fig.2 Fig.6