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Remote Device Activation

Remote Device Activation Using Minitor III Pagers and a PIC Microcontroller 

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. 

U2 is a Microchip PIC12F510 microcontroller.  It is programmed to do nothing until pin 7 (whic is connected to pin 6 of the pager) goes high, and then turn on the output for three minutes or until pin 7 goes high again, in which case it turns the output back off.  The assembly language code for this project is located here.  Pin 7 of U2 is connected to Pin 6 of the Minitor pager.  This pin is logic high when the pager is alerting (beeping).  The output of U2 is connected to a transistor that turns on a relay, which is used to switch power to the remote device.

 Possible Failure Mode

If a pager is inserted into the cradle and the ground pin does not make proper contact with the pager but the power and alert pins do, there will be a positive voltage present on the alert pin relative to the cradle ground.  This can result in false alert indications in two scenarios.  One, when the pager is first inserted into the cradle the ground contact may be the last pin to make contact, resulting in a very short duration positive voltage present on the alert pin.  The other scenario is if the ground contact corrodes or otherwise loses contact over time.  In this scenario the pager itself is not powered and operating, but there will still be a positive voltage present on the alert pin.