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.
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.
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.