Light operated internal door latch

Previously, we have posted light controlled digital fan regulator, with which we can control the speed of the fan remotely from the bed with the help of a torchlight. Now, here is another light-operated project which can also operate remotely from the bed with a torchlight. The group electronics hobbyist at Best Engineering Projects 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.”

Check out other security-related projects posted on bestengineeringproject.com

1. Car Lock System using Arduino and GSM
2. Arduino Fingerprint Sensor Lock
3. Arduino and RFID based Door Access System
4. Electronic Code Lock Circuit
5. RFID Based Security System Using Microcontroller AT89C52

Circuit Description of Light Operated Internal Door Latch

The circuit of the light-operated internal door latch is built using a control unit and drive unit for the stepper motor. The light-dependent resistor is used here as a sensor, whose value is varied according to the intensity of light falling on it.

The control unit for the stepper motor is built 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 utilizes four Darlington pair transistor (T₁ and T_2, T₃ and T₄, T₅ and T₆, T₇and T₈) to increase the correct capability for operating the stepper motor.

IC₁ is here configured as an astable multivibrator for 1.5 seconds. IC₂ is configured as a monostable multivibrator and gets triggered from LDR₁ when the torchlight is focused on LDR₁. The sensitivity of LDR₁ is adjusted by potentiometer VR₁ to avoid the activation by ambient light LDR₁ is covered.

 

IC₂ is configured as a monostable multivibrator and is triggered when a torchlight is focused on LDR₁. The period of 15 seconds 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 the 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 pins 2 and 7 (shift-right and shift-left serial data input pin) of IC₄. Mode control inputs S₀ and S₁ are used for direct 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 to step the stepper motor. You can also stop the stepper motor manually by pushing switch SW₁ to the ‘on’ position.

The output of IC₂ is also given to clock pin 3 of IC₈. IC₈ is used herein trigger mode by connecting pins 2 and 6 directly. Switch SW₂ is used to reset IC₈. The driver circuit and winding circuit of the stepper motor are shown in figure 2. Connect Q₀ through Q₃ outputs of IC₄ in fig 1 to positive and ground power supply terminals of figure 2. The wave from the drive pattern of shift outputs of IC₄ is shown in table 1.

Figure 3 shows the locking arrangement operated by the stepper motor.

When you direct the torchlight on the LDR, the stepper motor runs in one direction to latch the door. If you again focus the torchlight on the LDR, the stepper motor runs in the reverse direction to open the latch.

BEP NOTE: During testing at BEP LAB, a stepper motor for reading/write head positioning in a 1.2 Mb Floppy disk drive unit, operating at 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.

PARTS LIST OF LIGHT OPERATED INTERNAL DOOR LATCH

Resistor (all ¼-watt, ± 5% Carbon)

R1 – R3, R5 – R9 = 1 KΩ R4 = 100 KΩ VR1 – VR3 = 47 KΩ VR4 = 200 KΩ

Capacitors

C1 = 1 µF, 16V (Electrolytic Capacitor) C2, C4 = 0.01 µF (Ceramic Disc) C3 = 200 µF, 16V (Electrolytic Capacitor)

Semiconductors

IC1, IC2 = NE555 (Timer IC) IC3 (N1) = 7400 (Quad Two input NAND Gate) IC4 = 74LS194 (4-bit bidirectional universal shift register) IC5(N2) = 7432 (Quad Two input OR Gate) IC6 (N3) = 7402 (Quad two input positive NOR Gate) IC7 (N4) = 7404 (HEX Inverter) IC8 = 7474 (Dual Positive Edge Trigger D Flip-Flop) T1, T3, T5, T7 = BC547 (General Purpose silicon NPN Transistor) T2, T4, T6, T8 = BD139 (medium power NPN Silicon Transistor) D1 – D4 = 1N4007 (Rectifier Diode)

Miscellaneous

SW1 = ON/OFF Switch SW2 = Push-to-on switch Stepper Motor LDR