Transistor case styles.
Fig. 1 Transistor case styles.

Review Connecting Digital Logic and Transistors


There are two basic types of bipolar transistors - NPN and PNP. They do the same other than current flow is in opposite directions. Another variation is known as Darlington transistors. This configuration uses two transistors connected together to produce very high current gain. You will need to know these parts.

TTL inverter circuits SN74LS04 and SN74LS06
Fig. 2 TTL inverter circuits SN74LS04 and SN74LS06

TTL or transistor-transistor logic uses bipolar transistors. They operate at mostly 5-volts and are current operated devices. They use far more current than CMOS parts. Early versions of TTL have been around at least 50 years.

Fig.2 illustrates the internal diagrams for the SN74LS04 and the SN74LS06. Note the open collector output on the SN74LS06. This allows with pull up resister an interface to higher voltage logic such as the CD4000 series CMOS integrated circuits.



Connecting NAND and NOR gates to form inverters.
Fig. 3 Connecting NAND and NOR gates to form inverters.

Connecting NAND and NOR gates with inverter to create AND OR gates.
Fig. 4 Connecting NAND and NOR gates with inverter to create AND OR gates.

NPN transistor on NOR gate out creates open-collector OR gate.
Fig. 5 NPN transistor on NOR gate out creates open-collector OR gate driving a higher voltage load.

NPN transistor on NAND gate out creates open-collector AND gate driving a higher voltage load.
Fig. 6 NPN transistor on NAND gate out creates open-collector AND gate driving a higher voltage load.

Tri-State switch with NPN transistors for higher voltage output.
Fig. 7 Tri-State switch with NPN transistors for higher voltage output.

For more in Fig. 7 see Non-Inverting Tri-State Buffer-Switch Demo Circuit

Understanding basic digital circuits is key to understanding how to connect a computer or controller to operate a robot or read sensors in a greenhouse. Understand switching circuits, solid state relays, etc. means turning on a ventilation fan or a AC water pump.

Understanding basic chemistry allows one to monitor Ph or understand how a semiconductor device works. And knowledge of math and science makes understanding AC-DC motor operation possible and where to use them.



ULM2003 transistor array.
Fig. 8

Fig. 8 is the ULN2003 transistor array. It contains 7 complete NPN Darlington transistor drivers with open-collector output and suppressor diode for magnetic loads.

ULM2003 transistor array driver diagram.
Fig. 9

Fig. 9 is the diagram of each internal driver. The input resistor allows direct connection to digital logic. For more on the ULN2003:

See ULN2003A Darlington Transistor Array with Circuit Examples.

As a former adjunct instructor it became obvious when a student had even a basic chemistry or physics class in high school how fast they could grasp electricity or electronics. Many grow frustrated and quit because they lacked even basic science or math out of high school.

You must learn the basics. That is the goal of this series of lessons.

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