Here is a circuit that starts counting when a letter is inserted in the mailbox of your home or office. It is designed to save your time going to the mailbox to check for letters inside. The number of letters present in the table is indicated by a seven-segment display. The block diagram of the circuit is shown in figure 1.
Fig. 2 shows the circuit diagram of the electronic mailbox with letter counting device. It is built around a white LED (LED1) and a LDR (LDR1), a popular timer NE555 (IC1) in monostable mode, a counter, a seven segment controller CD4033 (IC2) and some other components. LED1 and LDR1 together function as a sensor.
The resistance of LDR1 changes according to the intensity of the light incident on it. When the LED1 light falls on LDR1, its resistance is low.
The voltage at pin 2 of IC1 depends on the light falling on LDR1. In the dark, the voltage at pin 2 is low, and vice versa.
When a letter is inserted in the mailbox, it passes between LED1 and LDR1. This interrupts the light falling from LED1 to LDR1. As a result, the resistance of LDR1 increases. This resistance change provides a trigger pulse to pin 2 of IC1, generating a short duration square wave pulse on its output pin 3. This pulse acts as a clock input for the display controller CD4033 (IC2). The IC2 output pins are connected to several segments a, b, c, d, e, f and g seven-segment display pins (DIS1) as shown in the circuit. Its common pin 3 is connected to ground through the current limiting resistor R6. Alternatively, it may also provide a strength each for each segment after eliminating the resistance R6. Seven-segment display DIS1 shows the number of letters present in the frame up to nine.
When a letter is delivered to the mailbox, LED2 turns on momentarily, indicating that a letter is received and DIS1 increments the display on an account. That is, for each pulse received at the input pin 1 of IC2, its output is advanced by a count, which is reflected in the seven-segment display. When the counter reaches nine, it is automatically reset to zero and the cycle repeats. Switch S1 is used to reset the count.
After picking up the letter from the mailbox, always reset the counter with switch S1.
Construction and testing
Figure 3 shows a real-size single-sided PCB for the mailbox circuit and its arrangement of components in Figure 4. Lock the PCB in a suitable box so that the LED1 light falls on LDR1. Make sure the wiring is correct to avoid any errors. For troubleshooting, check the voltages at various test points as shown in the table.