TTL to CMOS Interfacing: Circuits and Applications

This page covers TTL to CMOS IC interfacing and CMOS to TTL IC interfacing. It also discusses how to interface LEDs and Lamps with TTL or CMOS ICs.

Figure 1 shows TTL to CMOS interfacing and CMOS to TTL interfacing circuits.

When a 5V supply is used for both TTL and CMOS ICs, the logic levels of TTL and CMOS differ. A single TTL IC can drive multiple CMOS ICs. However, the TTL output in a ‘high state’ produces approximately 2.4V, which is below the minimum voltage (3.5V) required by CMOS ICs.

TTL to CMOS Interfacing

For TTL to CMOS interfacing, a standard pull-up resistor is used, resolving the voltage level incompatibility. This is illustrated in Figure 1(a).

Figure 1(a): TTL to CMOS Interfacing

CMOS to TTL Interfacing

A CMOS IC can directly drive a low-power Schottky TTL IC. However, when interfacing with a standard TTL IC, a buffer is placed between the CMOS and TTL ICs. This configuration is shown in Figure 1(b).

Figure 1(b): CMOS to TTL Interfacing

Interfacing TTL and CMOS to LEDs and Lamps

Figure 2 illustrates how to interface TTL or CMOS ICs with LEDs.

Figure 2: TTL/CMOS Interfacing with LED

As shown in Figure 2(a), a high-state output from the TTL or CMOS IC will turn the LED ON. Figure 2(b) demonstrates the alternative configuration where a low-state output drives the LED ON.

Figure 3 shows how to interface TTL or CMOS ICs with Lamps.

Figure 3: TTL/CMOS Interfacing with Lamp

Lamps require higher current and are interfaced using transistors. A Darlington pair can be used to further amplify the current, as needed, which is shown in Figure 3(b). In both scenarios, the lamp turns ON when the output of the TTL or CMOS IC is in a ‘high’ logic state.