Circuit shown below is used to count objects by sensing light blocked by obstacle which is detected by LDR(Light Emitting Resistor). Fig: Circuit Schematic Using Eagle. Components Used: LDR Resistors(R16-100K, R17-330 ohm, R11-47k, R12-100K, R10-330 ohm, Resistors used for 7 segment display are al 330 ohm) Capacitors(C1-10uF,C2-10uF,R15-0.01uF) LED 555 Timer ICs IC 7490 IC 7447 7 Segment display IC 555 Timer: First 555 IC is configured in monostable mode.Monostable circuit is dark sensor circuit,whose output is given to second 555 IC which is configured in astable mode.here pin 3 of monistable is connected to the pin 4 of astable circuit.When output from monostable is given to astable ,astable starts giving pulses to the counter 7490.Frequency for counter module is set up using R4,R3 and C2. IC 7490 Decade Counter: 7490 decade counter will give 4-bit output in binary at pins-12,9,8,11. Pins 12,9,8,11 are...