Security System Switcher

An audio signal can be used as a form of input to control any security system. For example, an automatic security camera can be configured to respond to a knock on the door. The circuit described here allows the security system to automatic in on state. It uses a transducer to detect intruders and a 5V regulated DC power supply provides power to the circuit.

As shown in Fig. 1, a condenser microphone is connected to the input of small signal Pre- amplifier built around transistor T1. Biasing resistor R1 determines to a large extent the microphone sensitivity. A microphone usually has an internal FET which requires a bias voltage to operate. The sound picked up by the microphone is amplified and fed to input pin 2 of IC1 (LMC555) wired in monostable configuration.

Fig. 1: Schematic Security system switcher Circuit diagram :

IC2 (CD4538B) is a dual, precision monostable multivibrator with independent trigger and reset controls. The output of IC1 is connected to the first trigger input pin 4 of IC2(A) through switch S1. If an intruder opens or breaks the door, IC1 is triggered by sound signals; the timer output pin 3 of IC1 goes high and enables first monostable multivibrator IC2(A). IC2(A) provides a time period of around 5 to 125 seconds, which is adjusted with preset VR1.

Another monostable multivibrator IC2(B) also provides a time period of around 25 to 600 seconds, which is adjusted with preset VR2. The output of IC2(B) is used to energise relay RL1. Indicator LED1 is provided to display the relay activity. Any AC/DC operated security gadget is activated or deactivated through a security switch. Thus, the security switch of the gadget is connected in the n/o contacts of the relay.You can also operate high power beacons, sirens or hooters in place of the security switch for any AC/DC operated security gadget.

Fig. 2: Proposed cabinet :

Assemble the circuit on a general-purpose PCB and enclose it in a cabinet as shown in Fig. 2 along with 5V adaptor for powering the circuit. Connect the security switch according to the circuit diagram and use appropriate AC/DC power supply required to operate the security gadget.

Warning! All relevant electrical safety precautions should be taken when connecting mains power supply to the relay contacts. With the help of single pole double throw (SPDT) switch S1, internal or external trigger input (active high signal) can be selected.

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This schema optimises the operation of a solar hot water system. When the water in the solar collector is hotter than the storage tank, the pump runs. The schema comprises two LM335Z temperature sensors, a comparator and Mosfet. Sensor 1 connects to the solar collector panel while Sensor 2 connects to the hot water panel. Each sensor includes a trimpot to allow adjustment of the output level. In practice, VR1 and VR2 are adjusted so that both Sensor 1 and Sensor 2 have the same output voltage when they are at the same temperature. The Sensor outputs are monitored using comparator IC1.

When Sensor 1 produces a higher voltage than Sensor 2, which means that sensor 1 is at a higher temperature, pin 1 of IC1 goes high and drives the gate of Mosfet Q1. This in turn drives the pump motor. IC1 includes hysteresis so that the output does not oscillate when both sensors are producing a similar voltage. Hysteresis comprises the 1MO feedback resistor between output pin 1 and non-inverting input pin 3 and the input 1kO resistor. This provides a nominal 12mV hysteresis so that voltage at Sensor 1 or Sensor 2 must differ by 12mV for changes in the comparator output to occur.

Pump Controller For Solar Hot Water System

Since the outputs of Sensor 1 and Sensor 2 change by about 10mV/°C, we could say that there is a degree of hysteresis in the comparator. Note that IC1 is a dual comparator with the second unit unused. Its inputs are tied to ground and pin 2 of IC1 respectively. This sets the pin 7 output high. Since the output is an open collector, it will be at a high impedance. Mosfet Q1 is rated at 60A and 60V and is suitable for driving inductive loads due to its avalanche suppression capability. This clamps any inductively induced voltages exceeding the voltage rating of the Mosfet.

The sensors are adjusted initially with both measuring the same temperature. This can be done at room temperature; adjust the trimpots so that the voltage between ground and the positive terminal reads the same for both sensors. If you wish, the sensors can be set to 10mV/°C change with the output referred to the Kelvin scale which is 273K at 0°C. So at 25°C, the sensor output should be set to (273 + 25 = 298) x 10mV or 2.98V.

Note:

The sensors will produce incorrect outputs if their leads are exposed to moisture and they should be protected with some neutral cure silicone sealant. The sensors can be mounted by clamping them directly to the outside surface of the solar collector and on an uninsulated section of the storage tank. The thermostat housing is usually a good position on the storage tank.

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This is a complete alarm system with 5 independent zones suitable for a small office or home environment. It uses just 3 CMOS ICs and features a timed entry / exit zone, 4 immediate zones and a panic button. There are indicators for each zone a "system armed" indicator. The schematic is as follows:

5 Zone Alarm System Circuit Diagram

Circuit Notes

Each zone uses a normally closed contact. These can be micro switches or standard alarm contacts (usually reed switches). Suitable switches can be bought from alarm shops and concealed in door frames, or window ledges.

Zone 1 is a timed zone which must be used as the entry and exit point of the building. Zones 2 - 5 are immediate zones, which will trigger the alarm with no delay. Some RF immunity is provided for long wiring runs by the input capacitors, C1-C5. C7 and R14 also form a transient suppressor. The key switch acts as the Set/Unset and Reset switch. For good security this should be the metal type with a key.

Operation
At switch on, C6 will charge via R11, this acts as the exit delay and is set to around 30 seconds. This can be altered by varying either C6 or R11. Once the timing period has elapsed, LED6 will light, meaning the system is armed. LED6 may be mounted externally (at the bell box for example) and provides visual indication that the system has set. Once set any contact that opens will trigger the alarm, including Zone 1. To prevent triggering the alarm on entry to the building, the concealed re-entry switch must be operated. This will discharge C6 and start the entry timer. The re-entry switch could be a concealed reed switch, located anywhere in a door frame, but invisible to the eye. The panic switch, when pressed, will trigger the alarm when set. Relay contacts RLA1 provide the latch, RLA2 operate the siren or buzzer.

Author : Andy Collinson

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