Wiring schema blogs

This is a complete alarm system with 5 independent zonessuitable for a small office or home environment. It uses just 3CM IC`s and features a timed entry / exit zone, 4 immediatezones and a panic button. There are indicators for each zone a“system armed” indicator. The schematic is as follows:

 

Each zone uses a normally closed contact. These can be microswitches or standard alarm contacts (usually reed switches).Suitable switches can be bought from alarm shops and concealed indoor frames, or window ledges.Zone 1 is a timed zone which must be used as the entry andexit point of the building. Zones 2 – 5 are immediate zones,which will trigger the alarm with no delay. Some RF immunity isprovided for long wiring runs by the input capacitors, C1-C5. C7and R14 also form a transient suppresser. The key switch acts asthe Set/Unset and Reset switch. For good security thisshould be the metal type with a key.

At switch on, C6 will charge via R11, this acts as the exitdelay and is set to around 30 seconds. This can be altered byvarying either C6 or R11. Once the timing period has elapsed,LED6 will light, meaning the system is armed. LED6 may be mountedexternally (at the bell box for example) and providesvisual indication that the system has set. Once set any contactthat opens will trigger the alarm, including Zone 1. To preventtriggering the alarm on entry to the building, the concealedre-entry switch must be operated. This will discharge C6 andstart the entry timer. The re-entry switch could be a concealedreed switch, located anywhere in a door frame, but invisibleto the eye. The panic switch, when pressed, will trigger thealarm when set. Relay contacts RLA1 provide the latch, RLA2operate the siren or buzzer.

An increasing number of appliances draw a very small current from the power supply. If you need to design a mains-powered device, you could generally choose between a linear and a switch-mode power supply. However, what if the appliance’s total power consumption is very small? Transformer-based power supplies are bulky, while the switchers are generally made to provide greater current output, with a significant increase in complexity, problems involving PCB layout and, inherently, reduced reliability.

Is it possible to create a simple, minimum part-count mains (230 VAC primary) power supply, without transformers or coils, capable of delivering about 100mA at, say, 5 V? A general approach could be to employ a highly inefficient stabilizer that would rectify AC and, utilizing a zener diode to provide a 5.1 V output, dissipate all the excess from 5.1 V to (230×√2) volts in a resistor. Even if the load would require only about 10mA, the loss would be approximately 3 watts, so a significant heat dissipation would occur even for such a small power consumption.

 At 100mA, the useless dissipation would go over 30 W, making this scheme completely unacceptable. Power conversion efficiency is not a major consideration here; instead, the basic problem is how to reduce heavy dissipation and protect the components from burning out. The circuit shown here is one of the simplest ways to achieve the above goals in practice. A JVR varistor is used for over-voltage/surge protection. Voltage divider R1-R2 follows the rectified 230 V and, when it is high enough, T1 turns on and T3 cannot conduct.

When the rectified voltage drops, T1 turns off and T3 starts to conduct current into the reservoir capacitor C1. The interception point (the moment when T1 turns off) is set by P1 (usually set to about 3k3), which controls the total output current capacity of the power supply: reducing P1 makes T1 react later, stopping T3 later, so more current is supplied, but with increased heat dissipation. Components T2, R3 and C2 form a typical ‘soft start’ circuit to reduce current spikes — this is necessary in order to limit C1’s charging current when the power supply is initially turned on. At a given setting of P1, the output current through R5 is constant.

Thus, load R4 takes as much current as it requires, while the rest goes through a zener diode, D5. Knowing the maximum current drawn by the load allows adjusting P1 to such a value as to provide a total current through R5 just 5 to 6mA over the maximum required by the load. In this way, unnecessary dissipation is much reduced, with zener stabilization function preserved. Zener diode D5 also protects C1 from over voltages, thus enabling te use of low-cost 16 V electrolytics. The current flow through R5 and D5, even when the load is disconnected, prevents T3’s gate-source voltage from rising too much and causing damage to device. In addition, T1 need not be a high-voltage transistor, but its current gain should exceed 120 (e.g. BC546B, or even BC547C can be used).