Previously, we have posted light controlled digital fan regulator, with which we can control speed of fan remotely from the bed with the help of torch light. Now, here is another light operated project which can also operate remotely from the bed with touch light. The group electronics hobbyist at Dreamlover technology designed and verified a very unique electronic circuit “light-operated internal door latch with which one can close or open the door of your room remotely.”
Circuit Description of Light Operated Internal Door Latch
The circuit of light operated internal door latch is build using control unit and driver unit for the stepper motor. Light-dependent resistor is used here as sensor, whose value is varied according to intensity of light fall on it.
The control unit for stepper motor is build around two timer ICs (IC1 and IC2), NAND gat IC (IC3), 4-bit bi-directional universal shift resistor (IC4), OR gate IC (IC5), NOR gate IC(IC6) hex inverter IC(IC7) and dual D-type positive-edge triggered flip-flop IC (IC8) where driver unit utilize four darlingeton pair transistor (T1 and T2,T3 and T4,T5 and T6,T7and T8) in order to increase the correct capability for operating the stepper motor.
IC1 is here configured as astable multivibrator for time period of 1.5 second. IC2 is configured as monostable vibrator and get trigger from LDR1 when torch light is focused on LDR1.sensitivity of LDR1 is adjusted by potentiometer VR1 to avoid the activation by ambient light LDR1 is covered.
IC2 is configured as monostable multivibrater and is triggered when a torch light is focused on LDR1. The time period 15 second is adjusted by potentiometer VR4. Both the output (output from IC1 and IC2) is fed to NAND gate IC (IC3). The output of NAND gate IC (IC3) is given to clock pin of IC4. The Q0 and Q1 outputs of shift register IC4 are ORed by using OR gate IC (IC5) and its output is fed to NOR GATE IC (IC6) The Q2 output forms the second input for NOR gate IC (IC6). The output of NOR is given to pin 2 and 7 (shift-right and shift-left serial data input pin) of IC4. Mode control inputs S0 and S1 are used for direction charging of shift register IC4. The Q1 output of dual D-type flip-flop IC8 is fed to S0 directly and S1 after inversion by N4 (IC7). The output of IC2 from pin3 is connected to reset pin1 of IC4 through resistor R 4 to reset IC4 in order to steps the stepper motor. You can also stop the stepper motor manually by pushing switch SW1 to ‘on’ position.
The output of IC2 is also given to clock pin 3 of IC8. IC8 is used here in trigger mode by connecting pin 2 and 6 directly. Switch SW2 is used to reset IC8. The driver circuit and winding circuit of stepper motor shown in figure 2. Connect Q0 through Q3 outputs of IC4 in the fig 1 to positive and ground power supply terminals of the figure 2. The wave from drive pattern of shift outputs of IC4 is shown in the table 1.
Shift Register Output Waveform
Figure 3 shows the locking arranged operated by the stepper motor.
When you direct torchlight on the LDR, the stepper motor runs in one direction to latch the door. It you again focus torch light on the LDR, the stepper motor runs in reverses direction to open the latch.
BEP NOTE: During testing at BEP LAB, a stepper motor for read/write head positioning in a 1.2 Mb Flopy disk drive unit, operating of 12V with 3.6 degree revolution per step, was used. Connect the colored terminal wires of the motor to the driver unit as shown in figure 2.
Resistor (all ¼-watt, ± 5% Carbon)
R1 – R3, R5 – R9 = 1 KΩ
R4 = 100 KΩ
VR1 – VR3 = 47 KΩ
VR4 = 200 KΩ
C1 = 1 µF/16V
C2, C4 = 0.01 µF
C3 = 200 µF/16V
IC1, IC2 = NE555
IC3 (N1) = 7400
IC4 = 74LS194
IC5(N2) = 7432
IC6 (N3) = 7402
IC7 (N4) = 7404
IC8 = 7474
T1, T3, T5, T7 = BC547
T2, T4, T6, T8 = BD139
D1 – D4 = 1N4007
SW1 = ON/OFF Switch
SW2 = Push-to-on switch