Fig. 1

Basic TTL Tri-State Buffer Circuit Examples

Here I have several examples of Tri-State driver circuits based on MOSFET outputs.

The output has three states of HIGH (Vcc), LOW (GND), and Hi-Z. Q1 is switched on for HIGH, Q2 for LOW, both Q1/Q2 switched off for Hi-Z.

Fig. 1 is an example tri-state buffer circuit. The output is turned on-off based on the logic level on the enable pin. HIGH is on.

I found this on the web and it is nearly useless for most practical reasons. Being 5-volt TTL logic my output voltage is limited to 5-volts. That could create problems turning on some MOSFET transistors.

Basic TTL Tri-State Buffer Circuit Example 2

Fig. 2

Fig. 2 is functionally identical to Fig. 1. It suffers the same drawback of TTL logic and low voltage.

Fig. 3 Tri-State switch used single CD4011 with MOSFET output.

Fig.3 uses a single CD4011 quad, 2-input NAND gate integrated circuit. IC1B is used as an inverter to replace the 7404 from Fig. 2. IC1C and IC1D form an AND gate replacing the 7408.

Because this is CMOS my operating voltage can range from 3 to 15 volts. In most of these projects I use 12 volts.

By using 12 volts that assures turn on of most MOSFETs and IGBTs.

The drawbacks are +Vcc is limited to 15 volts. Unless the CD4011 Vcc is 3 or 5 volts an interface circuits must be used between the CD4011 and the microcontroller.

That is discussed at the following:

Fig. 4 Tri-State switch non-inverting truth table.

Fig. 5 Tri-State switch inverting truth table.

Referring back to Fig. 3 if IC1B is connected at point X the output will be inverting. This is shown in the truth table in Fig. 5

Fig. 6 Connecting NAND - NOR gates to form AND - OR gates.

Fig. 7 H-bridge motor control based on tri-state switches.