Two Digit Counter Circuit

Two Digit Counter Circuit is used as model racetracks. As compared to mechanical lap counters, which suffers from poor contacts or the mechanism easily jam, this electronic lap counter is based on the principle characteristics of phototransistor. Hence, it has very high reliability and efficiency. The counter can count maximum 99 laps, which can be preset to individual requirements. The display is shown on two 7-segment displays.
About the Circuit :
The circuit uses six ICs and two 7-segment displays. The triggering circuit is built around IC1, which consists of four NAND gates and phototransistor T1. IC2 and IC3 are used as a counter ICs, IC4, and IC5 are used to drive two 7-segment displays.
When any object passes the “checkpoint”, it interrupts the light falling T1. T1 has a feature that in presence of light it has very low resistance but when light falls on it interrupted its resistance increases to a very high value. Hence the flip-flop designed around IC1 gets set, which provides clock input to IC2. Now IC2 starts counting. After counting from 0 to 9 it gives clock input to IC3 and now IC3 starts counting. Thus, we can count maximum 99 laps. The output of IC2 and IC3 are in BCD form. To convert this BCD logic into 7-segment display, IC4 and IC5 are used, which are BCD to 7-segment display driver. The display is reset by pressing switch S1.
Power can be restored and the counter reset by pressing the Reset switch RS. The phototransistor T1 should be mounted in a cylindrical tube to screen it from ambient light, which might otherwise keep the phototransistor resistance low and block the circuit. The sensitivity may be adjusted by P1.
Power Supply :
The circuit requires a DC power supply of 5V, which is designed using regulator IC6. Diodes D1 and D2 are used as a rectifier diode and capacitor C1 are used as a filter capacitor.