This article discusses the Mailbox Program with Counter and Light Indicator functions. The Mailbox system has a hole for inserting letters and a door which is used to take letters. This mailbox is equipped with sensors to detect incoming mail and taken mail. The mailbox will count the number of incoming letters and it will Reset the data when the letters are taken. Indicator lights (Green, Yellow, Red) are used to indicate the status of the mailbox.
Program Objective
Process:
When the incoming mail sensor SENS_IN (0.02) is Active, then the program will:
- Increase the counter COUNT_MAIL (D0) value for the number of letters.
- Check the COUNT_MAIL (D0) counter value.
- If the COUNT_MAIL (D0) counter value is Not Equal to “0”, the GREEN_LAMP (100.00) indicator lamp will be ON.
- If the COUNT_MAIL (D0) counter value is Greater Than Or Equal to “5”, the YELLOW_LAMP (100.01) indicator lamp will turn ON and the GREEN_LAMP (100.00) indicator lamp will turn OFF.
- If the COUNT_MAIL (D0) counter value is Equal To “10”, the RED_LAMP (100.02) indicator lamp will turn ON and the YELLOW_LAMP (100.01) indicator lamp will turn OFF.
When the mail retrieval door sensor SENS_OUT (0.03) is Active, then the program will do the following things:
- Reset the COUNT_MAIL (D0) counter value to “0”.
- Turn OFF all indicator lamps.
PLC Inputs and Outputs Addressing
| Comment | Input (I) | Output (Q) | Memory Word | Memory Bits | Timer |
| PB_START | 0.00 | ||||
| PB_STOP | 0.01 | ||||
| SENS_IN | 0.02 | ||||
| SENS_OUT | 0.03 | ||||
| GREEN_LAMP | 100.00 | ||||
| YELLOW_LAMP | 100.01 | ||||
| RED_LAMP | 100.02 | ||||
| COUNT_MAIL | D0 | ||||
| SYSTEM_ON | W0.00 | ||||
| IR_YELLOW_LAMP | W0.01 | ||||
| IR_RED_LAMP | W0.02 |
PLC Program for Mailbox
RUNG 0 (START SYSTEM)
In this Rung, when the PB_START (0.00) button is pressed, the memory bit SYSTEM_ON (W0.00) will be in the HIGH state. Because the KEEP(011) instruction is used, the memory bit SYSTEM_ON (W0.00) remains in the HIGH state even though the PB_START (0.00) button has been Released.
The memory bit SYSTEM_ON (W0.00) will become a LOW state if the PB_STOP (0.01) button is Pressed.
RUNG 1 (COUNT MAIL)
In this Rung, when the NO contact of memory bit SYSTEM_ON (W0.00) and Sensor SENS_IN (0.02) in the HIGH state, the value in memory word COUNT_MAIL (D0) will increase (+1), because it uses the Binary Increment @++(590) Instruction.
RUNG 2 (RESET COUNTER)
In this Rung, when the NO contact of memory bit SYSTEM_ON (W0.00) and the Sensor SENS_OUT (0.03) in HIGH state, the value in memory word COUNT_MAIL (D0) will be Reset to zero “0”. Because the MOV(021) instruction moves the zero value “0” to the memory word COUNT_MAIL (D0).
RUNG 3 (GREEN LAMP ALARM)
In this Rung, when the NO contact of memory bit SYSTEM_ON (W0.00) and the value in memory word COUNT_MAIL (D0) are Not Equal to “0”, then the Output GREEN_LAMP (100.00) will be ON.
The GREEN_LAMP (100.00) output will be OFF if the NC contacts of memory bit IR_YELLOW_LAMP (W0.01) and IR_RED_LAMP (W0.02) are in the HIGH state.
RUNG 4 (YELLOW LAMP ALARM)
In this Rung, when the NO contact of memory bit SYSTEM_ON (W0.00) and the value in memory word COUNT_MAIL (D0) is Greater Than Or Equal to “5”, then the Output YELLOW_LAMP (100.01) will be ON and the memory bit IR_YELLOW_LAMP (W0.01 ) will be in the HIGH state.
Output YELLOW_LAMP (100.01) will be ON blinking with an interval of 1 second, because the NO contact of P_1s (1.0 second clock pulse bit).
The YELLOW_LAMP (100.01) output will be OFF if the NC contact of memory bit IR_RED_LAMP (W0.02) in the HIGH state.
RUNG 5 (RED LAMP ALARM)
In this Rung, when the NO contact of memory bit SYSTEM_ON (W0.00) and the value in memory word COUNT_MAIL (D0) is Equal to “10”, then the Output RED_LAMP (100.02) will be ON and the memory bit IR_RED_LAMP (W0.02) will be in HIGH state.
The RED_LAMP (100.02) output will be ON blinking with an interval of 1 second because of the NO contact of P_1s (1.0 second clock pulse bit).
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