Safety Guard

Simple unit, Protects home appliances from voltage spikes

Protect your home appliances from voltage spikes with this simple time delay circuit. Whenever power to the appliances is switched on or resumes after mains failure, the oscillator starts oscillating and D5 blinks. This continues for three minutes. After that, Q14 output of IC CD4060 goes high to trigger the gate of the SCR through D4. At this moment, the voltage is available at the cathode of the SCR, which energizes the relay coil to activate the appliance and D6 glows. Switch SW1 is used for quick start without waiting for delay.

Safety Guard Circuit Diagram

Parts	Description
R1	1M
R2	470R
R3	820R
R4	56K
R5	470R
R6	1K
R7	10K
C1	1kuF-25V
C2	100nF-63V
C3	0.02uF-63V
C4	10uF-25V
C5	10uF-25V
D1	1N4007
D2	1N4007
D3	1N4007
D4	1N4148
D5	Red LEDs
D6	Red LEDs
RL1	12V Relay
IC1	AN7809
IC2	CD4060
SW1	Switch
T1	24V-AC Centre Tapped Transformer

Circuit Operation:

At the heart of the circuit is IC CD4060, which consists of two inverter gates for clock generation and a 14-bit binary ripple counter. Here the clock oscillations are governed by resistor R1 and capacitor C1. In this circuit, only two outputs of the IC (Q5 and Q14) have been used. Q5 is connected to an LED (D5) and Q14 is used to trigger the gate of the SCR through D4 as well as reset the counter. The anode of the SCR is connected to +9V and the cathode is connected to the relay coil. The other pin of the relay coil is connected to the negative supply, while its contacts are used for switching on the appliances.

Source : http://www.ecircuitslab.com/2011/05/safety-guard.html

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Little Door Guard

If some intruder tries to open the door of your personal home, this circuit sounds an alarm to warn you against the tried intrusion. The circuit (Fig. 1) uses with no trouble on hand, reasonably feed components. For compactness, an alkaline 12V battery is used for powering the unit. Input DC provide is additional regulated to a steady DC voltage of 5V with the support of three-pin regulator IC 7805 (IC2).

Fig. 1: Circuit of the door guard

Assemble the unit on a general-purpose PCB as shown in Fig. four and mount the identical on the door as proven in Fig. 3. Now mount a section of replicate on the door body such that it is exactly aligned with the unit. Pin configurations of IC UM3561 and transistors 2N5777 and BC547 are proven in Fig. 2. 

Fig. 2: Pin configurations of UM3561 and transistors 2N5777 and BC547

Initially, when the door is closed, the infrared (IR) beam transmitted by means of IR LED1 is mirrored (by the replicate) back to phototransistor 2N5777 (T1). The IR beam falling on phototransistor T1 reverse biases npn transistor T2 and IC1 does now not get positive provide at its pin 5. As a consequence, no tone is produced at its output pin 3 and the loudspeaker remains silent. Resistor R1 limits the operating current for the IR LED.

When the door isopened, the absence of IR rays at phototransistor T1 ahead biases npn transistor T2, which gives supply to  sureIC1. Now 3-sirensound generator IC UM3561 (IC1) gets power via resistor R5. The output of IC1 at pin three is amplified by means of Darlington-pair transistors T3 and T4 to supply the alert tone via the loudspeaker. 

Fig. three: Back view of the door assembly

Rotary switch S2 is used to choose the three preprogrammed tones of IC1. IC1 produces hearth engine, police and ambulance siren sounds when its pin 6 is attached to point F, P or A, respectively.

Fig. four: Suggested enclosure with major elements layout

Author : T.K. Hareendran

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