24V interface for microcontroller

       Industrial control applications often use PLCs (Programmable Logic Controllers) that operate at 24V logic levels. This voltage creates a challenge for the safe use of microcontrollers. Such a design requires a physical barrier between the microcontroller and the 24V signal to avoid damage to the microcontroller in the event of a fault or short circuit.

　　A simple and inexpensive way to transform 24V to a microcontroller is to use the ULN2003 or ULN2803 transistor drivers, which have seven and eight outputs, respectively. These ICs can provide 500 mA for a light bulb or solenoid valve. The damping diodes in these ICs eliminate the need for multiple passive components, especially for designs using coils.

　　Because they are digital inputs, passive components require multiple devices, complicating assembly, increasing cost, and increasing the requirements for troubleshooting and maintenance. Few simple ICs can withstand voltages above 24V.

　　For this voltage, you can use an interface IC, such as the MC1489 or SN75189 converter, to act as an RS-232 line receiver. These ICs can accept digital signals up to ±30 V. They also have the added benefit of some hysteresis in the level shifting, so they can discern some electrical noise in the signal (Figure 1).

　　These devices can be connected directly to the microcontroller. If mounted in a DIP socket, they can be easily replaced if damaged. The circuit uses a 78L05 linear regulator to reduce the power supply voltage to 5V. The 1N4007 is used as circuit protection between the 24V DC and the 5V LM7805 regulator to prevent possible power reverse connection. This situation must be considered when programming the microcontroller because the signal will be supplemented to the input terminal.