This page describes a project that allows devices to be remotely activated remotely via two-tone paging.
I'll provide as much detail as possible here so that others can duplicate the project if desired. However, parameters specific to the system I designed (radio and tone frequencies) will be left out.
Also note that there are simpler ways to accomplish the same thing (i.e. using a commercial timer instead of a PIC microcontroller). This method allows for some future flexibility, since the PIC can be easily reprogrammed with different timing parameters, etc.
System Overview
The remote activation system is based on two-tone paging. A pager
and integrated custom controller are located at each siren. When the
pager receives the correct two-tone page, the controller, which is
built into the pager’s cradle, activates the device for a period of
three minutes. At the end of the three-minute period the controller
shuts the siren off. If it is necessary to shut the device off before
the three minutes are expired, the page can be resent and the
controller will toggle the device off. Other systems may have
different timing requirements, and those can be programmed into the PIC.
Technical Details
The remote activation system uses Motorola Minitor III pagers in conjunction with specially modified charging cradles and a PIC microcontroller running custom software. The charging cradles were modified in two ways. First, the normal charging circuitry was bypassed, and instead the output of a 7805 5 volt regulator is fed to the normal "charging" pins. This allows the cradle to power the pager without the need for batteries in the pager. Second, a third pin (pin 6) was added to the charging cradle to tap into the pager's alert output. See this page for a full pinout and to learn how to add a pin to the charging cradle.
A schematic of the activation circuit is shown below in Fig. 1. Voltage Regulator U1 provides 5 volts to both the pager and the microcontroller circuit. As mentioned above, the pager's charging cradle has been modified to bypass the normal battery charging circuit. The pager is powered directly from the 5V regulator. For this reason, it is important that the pager does not have batteries in it! If the pagers does have batteries in it, the batteries will be overcharged and damage the pager.
