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Fire Station Alerting

How to turn on lamps, strobes, and alarms using an old Minitor pager

I've gotten so many emails from people asking how to turn on lamps or alarms when their fire department tones go off that I decided to create a page describing a few different techniques.  

First, let me say that it's not too hard to do if you have a little bit of electrical or electronics background.  However, if what you see on this page looks daunting, contact your local radio shop.  They'd be glad to take your money, and the end result should work with minimal headache for you.

Also, I'm only covering using Minitor pagers because so many fire departments have old ones laying around unused.  If you want an off-the shelf solution check out the Federal Signal Informer.  It can be purchased with relay outputs that can be used to switch loads, and it's computer programmable.  

You may want to also consider taking a look at my TwoToneDetect program, which might be useful if you don't have a pager to use.

And as a final disclaimer, this page is meant only as a guide.  Every installation is unique, so please don't think that what I have shown here will work for you.  Again, if you can't figure it out, your local radio shop would be happy to help. 

Minitor Pager Information

For a ton of useful background information about the Minitor pagers, see the Minitor page of the Batlabs site.   That site has a bunch of stuff I'm not going to bother listing here, so make sure to check it out.  You can also browse the other pages on this site...some of the info might be of use.

Minitor Alert Output

Minitor II, III, IV, and V pagers have an alert output.  If you look at the bottom of the pager, you'll notice a bunch of round metal contacts.  One of these contacts is used as an alert output.  The pinout of the II, III, and IV is the same and can be found on this page and on the Batlabs Minitor Page.   The alert output on the Minitor 5 is the same pin as the "Data TX to Programmer" pin as shown on this page.  Anyway...whenever the pager is beeping, there will be a voltage on that contact (usually about 2.5V if the pager is being powered from batteries).  If you want to go the super cheap but more complicated route, you can interface directly with this alert output contact by adding an additional pin to the factory charging cradle (an easier solution is to use the amplified charger...see further down the page).  For some ideas on how to add the additional pin, see this page.  Once you're tapped into the pin, you need to use that voltage to drive something like a light or alarm.  The small voltage and current that the pager supplies isn't enough to drive anything directly, so you'll need to use a transistor and a relay to switch a larger load.  See an example schematic below.


If you're not familiar with relays, a good tutorial on relays and how they work, can be found at this page.  A good source for purchasing inexpensive relays is All Electronics.  For another idea that uses this route, see the Solid State Relay section further down this page.

Minitor Amplified Charger

The Minitor amplified chargers have what's known as a relay contact closure output.  This means that there is a small relay in the amplified charger that closes when the pager alerts.  This closure can be set to latch on, or to turn back off after ten seconds.  See the Minitor II section of the Batlabs Minitor Page for more info on setting this up.

So what is a relay contact closure and how can you use it?  Basically, you can think of the two relay contact pins as two contacts of a switch.  When the relay is active, the two contacts are connected together.  The only thing you need to worry about is that these contacts aren't designed to handle large voltages or currents.  According to the Batlabs page, they can only handle 28V at about 1/2 amp or less.  If you want to switch more voltage or current than that (such as turning on lights in a bunkroom), you'll have to use those contacts to feed another, larger relay.  See the example schematic below.  Note that you'll need to have an external DC power source of some sort.  This could be as simple as  a "wall wart" DC transformer, or you could use the 12V from a base radio power supply.

Solid State Relays

Solid state relays are an attractive alternative for this application because they require almost no current and very low voltage to turn on.  In fact, you can probably find a solid state relay that would turn on from just the output of the pager with no external transistor needed.  All Electronics will sometimes have some nice solid state relays that would work well except that they often need at least 3V to activate, and the Minitors only put out about 2.5V on the alert output contact when they are run on batteries.  One possible solution is to run the pager off of 5V instead of the batteries.  When the pager is powered with 5V the output voltage is higher, and is probably enough to turn on the more common solid state relays.  This involves hacking a charging cradle to bypass the charging circuitry and adding a 7805 5V voltage regulator.  If you do this, make sure you don't have batteries in the pager when it's in the modified charging cradle!

Wiring solid state relays is very similar to conventional relays, except that the current required for the control signal is almost nothing.  You'll also notice that these solid state relays don't make a clicking or clunking noise when the activate, since there is no mechanical switching taking place.  The switching is all done in silicon, which makes them more reliable.  The downside is that they can generate a fair amount of heat when heavily loaded, so heat sinking will probably be required.

 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.