In most of the modern manufacturing and processing industries, there is complete industrial automation through sophisticated hardware and software like programmable logic controller (PLC), distributed control system (DCS), and supervisory control and data acquisition (SCADA). Microcontroller-based embedded systems play major role in industrial automation. One such widely used system is the programmable timer. The major applications of programmable timer are as follows:
1. Initiating the process after de-sired time
2. Switching on/off the process after predetermined time
3. Providing delay in between processes
4. Applying input to on/off type open-loop control system

Depending upon the requirement of process, one can set the time of programmable timer. As the time period expires, the timer will either trigger or shut the process. Earlier there were mechanical timers that used gear assembly (same as wall clock) and mechanical contacts. But the problem with these was that due to mechanical parts and movements, they were not durable. Electronic timers have become very popular as these have more functionalities and long operating life.

A simple electronic timer can be made using a single IC 555 in monostable mode that can switch on/off the process after desired time. Also, in chain process (where the end of first process starts second process and so on), one can use a number of such monostable blocks to make a sequential timer. But these circuits do not include additional features like digital display, system failure indication, remote operation and alarms. Since the precision and accuracy of these timer circuits depend upon the value of the resistor-capacitor components that may deviate, we might not get the exact and precise timing. To enhance the programmable timer for generating precise timing and additional features, microcontrollers (embedded controllers) are used with peripheral devices.

Fig. 1: Circuit diagram of programmable industrial on/off timer
Fig. 1: Circuit diagram of programmable industrial on/off timer

Some of the features of programmable industrial on/off timer presented here include:
1. Time set from 1 to 60 seconds (can be extended)
2. ‘On’ time and ‘off’ time can be programmed (from 1 to 60 seconds)
3. Repeat (continuous) and single operation
4. Fully remote-controlled within 100-metre range
5. User-friendly front-panel controls and display panel with LCD
6. Emergency stop buttons (on control panel as well as on remote)
7. Provision of potential-free relay contacts for connecting any 230VAC at 10A or 28V DC at 10A device/application

Fig. 2: An actual-size, single-side PCB for the industrial timer circuit
Fig. 2: An actual-size, single-side PCB for the industrial timer circuit
Fig. 3: Component layout for the PCB in Fig. 2
Fig. 3: Component layout for the PCB in Fig. 2

Circuit description
The complete hardware circuit is divided into two sections—programmable timer main circuit and remote control transmitter and receiver units. The main circuit is a fully-functional standalone unit with front-panel control switch and LCD display. Remote control transmitter and receiver are add-on units. The receiver outputs are connected with respective control input pins of main circuit. Receiver output facilitates user to program and control timer operation using remote within 100-metre range.

Main circuit. Fig. 1 shows the circuit diagram of industrial on/off timer. The circuit comprises microcontroller AT89C51 (IC1), LCD, transistors and a few discret components. The data pins D0 through D7 of LCD are connected with port pins P2.0 through P2.7 of microcontroller AT89C51. The control pins register select (RS), read/write (R/W) and enable (E) are connected with port P3 pins P3.0, P3.1 and P3.2, respectively. Preset VR1 is connected with pin 3 of LCD for contrast control. Switches S1 through S4 are connected to port pins P1.0 through P1.3 by 4PDT toggle switch S8. Two single-pole double-throw (SPDT) switches S6 and S7 are connected at P3.5 and P3.6. Switch S6 selects for either remote or keypad control, and switch S7 selects control for either repeat or single mode operation. Port pin P3.7 drives relay through transistors T1. Diode D5 acts as free-wheeling diode for relay RL1.

Power-on reset is provided by the combination of resistor R1 and capacitor C3. Switch S5 is used for manual reset or emergency stop function. A 12MHz quartz crystal along with two 33pF capacitors provides clock pulse to the microcontroller AT89C51.

The 230V, 50Hz AC mains is stepped down by transformer X1 to deliver a secondary output of 12V, 500mA. The transformer output is rectified by a full-wave rectifier comprising diodes D1 through D4, filtered by capacitor C4 and regulated by IC 7805 (IC4). Capacitor C5 bypasses the ripples present in the regulated supply. LED1 acts as the power indicator and R7 limits the current through LED1.


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