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What is a Real Time Clock (RTC) in a PLC?

Real-time clocks (RTC) are not clocks like the one on your nightstand that automatically changes time as the season changes from standard time to daylight savings. Those work with a radio receiver inside the casing tuned to the national time observatory that measures light frequency from a very stable and accurate source to keep us all on schedule. RTCs measure elapsed time by setting the start and end times and interim setpoints. Elapsed time is stored in an internal data storage device. An oscillator circuit inside the device keeps an accurate accounting of the hours, minutes, and seconds from start to finish. Interim setpoints cause the RTC to trigger a control signal that tells another device component to act in a specific manner. Programmable logic controllers (PLC) are such a control device. They have an internal RTC that controls other devices connected to it in a network that must take action at specific interim setpoints of the RTC, such as a house automated control system.

A Simple RTC Application

A simple high school example of the application of an RTC will reveal the simplicity and multifunctional nature of the circuit. This section is paraphrased from this article.

I was asked to take on a small project for an inter-school competition. I had three days to get it done. My teacher asked me to make an automated school bell. I had an idea of how to do it.  But three days to make everything work like a charm was a challenge. The school already had similar controls for lights in the school garden and electric heaters. How hard could it be? The circuit needed was another extension of the real-time clock (RTC) that controlled the lights and heaters.

The school had some kind of PLC or equivalent to control other devices on the school network. This new task is a part of the multifunctional project. You can control any device with voltage and current that your relay can handle! You set start and end time, and when the current time reaches set time, the relay for the school bell is triggered and will stay on until it reaches the end time you set previously. These are the parts that were required to get the job done:

  1. DS1307 RTC chip
  2. 32,768 kHz crystal
  3. 8 pin IC socket
  4. Capacitor 100nF
  5. 3 metal film resistors of 10kΩ
  6. Socket for a CR2032 battery
  7. CR2032 battery to count time when Arduino is out of power

Automation and RTC Operate Hand in Hand

It’s incredible how a simple concept like RTC can push industrial automation to new heights. However, once you get the world of industrial automation and the SCADA control system where the PLCs operate, the level of sophistication and complexity rises by orders of magnitude.

It’s the real-time monitoring and control (RTC component) for industrial automation that brings the whole process to life. We’d have little need for artificial intelligence and machine learning if not for the terabytes of information created by real-time monitoring and control every second. PLCs and their embedded RTC modules are built to withstand the harsh environments of industrial venues. Agriculture hydroponics is one of those venues.

Cultivable land is decreasing rapidly due to rapid urbanization and industrialization. The shortage of water has dramatically affected agriculture[1] because of the rising problem of drought. Thus, there has been a significant rise in the practice of hydroponics to grow food. Programmable logic controllers (PLCs) and their RTC components play an essential role in automating the process to increase yield and keep the proper climate and nutrient mix of the growth mediums. Hydroponics involves growing plants in water with a mineral nutrient solution mixed with soilless culture with a continuous supply of oxygen at room temperature with indirect light. The system uses less water and fertilizer as compared to the soil system. The measured and monitored parameters are pH, humidity, electrical conductivity of water, and concentration of nutrients in the water. These are useful for designing and developing the PLC to manage the hydroponic automation systems to maintain optimum plant growth. Let’s see how it’s done.

PLC Management of a Hydroponic Operation

A simplified view of the PLC operation is shown above. A central unit houses the PLC board and its RTC components. The power supply is located within this central enclosure. Data is fed to the PLC from the hydroponic sensors. The information is then used to determine the RTC setpoints for maintaining the nutrient water’s proper ph. The acidic limit of the water is the trigger for adding nutrients. The other PLC control actions are to drain and refill the water reservoir when condition setpoints determine it needs to be changed. The temperature sensor feeds data to the PLC as to when to turn on the indirect light and operate the fan to maintain the proper environment in the hydroponic enclosure.

Time is the basis for when specific actions need to be taken by the PLC to automate the entire operation. The real-time clock (RTC) triggers the correct actions to be taken through its control relay system. Control of the nutrient pump is the most critical process. Nutrients must be sent directly to the roots in specific time intervals. The RTC timers are set daily in this setup and act to initiate the nutrient pump’s on/off sequence.


Real-Time Clocks are a simple electronic circuit used to trigger a device to take a specific action on a time-based basis. Remember that it is not a clock that keeps the time of day. It is a clock that measures elapsed time from a beginning set point to an ending setpoint. You must manually set the clock up and include any interim set points when a relay must be triggered to control an action of the device the PLC is controlling.

[1] Agriculture-Drought-Report-2019.pdf (rpo.co.za)

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This entry was posted on November 27th, 2020 and is filed under Automation, Education, Electrical, General, PLC. Both comments and pings are currently closed.

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