Delayed Turn On Alarm (version II) (ART201E)

Loads up to hundreds of watts can be controlled with this version of the Delayed Turn-On Alarm. This circuit drives a relay that controls heavy-duty loads such as sirens, horns, and small appliances powered from batteries or the ac power line.

Operation is the same as described in the previous project, and a turn-on delay of about 20 seconds is obtained with the components shown in the schematic diagram. You can alter the time delay by changing both C1 and R1. Cl should be between 10 and 1,000 µF, and R1 between 10 k ohm and 1 M ohm.

The sensors are magnetic types or wires, normally closed, and several of them can be wired in series to protect large areas, as explained in the first version (ART002E).

Power comes from AA cells, a battery, or ac-to-dc converters rated to currents of 250 mA or more. As the current drain is very low when the relay is off, batteries can be used.

A schematic diagram is shown in Fig. 1.

Figure 1 – Schematic diagram of the alarm

K1 depends on the power supply voltage and current drained by the load. A mini DPDT, 1 A relay (Radio Shack 27 5-249) can control loads up to 100 W from the ac power line. This relay uses a 12 V power supply in this project.

If you want to control heavy-duty loads, you can replace this relay with a 10 A mini SPST relay (Radio Shack 275-248), but you have to note the difference in the terminal layout.

To operate the unit, press S1 before leaving home. After the programmed time delay, the alarm will turn on automatically.