162 2 Industri I d d s i ia l l Rob o b bo tics i s Fu un ddamentals Copyright Goodheart-Willcox Co., Inc. Copyright Goodheart-Willcox Co., Inc. Procedure 1. Construct the experimental digital timer circuit of the following by first fi connecting the time base generator. Do not energize the SN7490 at this time. 2. Determine the charging time of the circuit by the t c = 0.693 (R A + R A B ) C 1 formula. Test the accuracy of the circuit. Record your fi ndings. fi 3. Determine the discharge time of the circuit by the t d = 0.693R B C 1 formula. Test the accuracy of the circuit. Record your fi ndings. fi 4. Turn off the power supply and complete the SN7490 counter IC. Connect it to the same power source used by the 555. Connect the second LED indicator to the D output of the 7490 IC. Connect the IC clock source to the output (pin 3) of the 555. + + – VCC C1 C2 5 V +VCC 5 V RA RB 8 200K 200K 1 7 SE/NE 555 SN7490 IC Time base generator Counter 2 6 3 5 4 14 1 13 2 12 3 11 4 10 5 9 6 8 A To clock source D B C 7 390 Ω Ω 390 Ω LED LED tester circuit Output (clock source) 0.01 μF 50 μF Goodheart-Willcox Publisher Digital timer circuit.