Mighty Mule Output Hack


Senior Member
I have a long driveway, and it bugs me to have a car drive in when I'm not expecting it. I have used PIR (pasive infra red) devices to alarm in the house when we have visitors, but PIR systems have many false alarms from squirrels, deer, bobcats, coyotes, and skunks. I love the wildlife, but the false alarm to real human ratio is probably 10:1.

So I bought a "Mighty Mule" magnetometer based system that only detects moving magnetic masses. Works great. Only problem is, it doesn"t have relay contacts to allow anything to be connected to it. My system has four annumciators and a counter, so project was to get the Mighty Mule to spill his guts via a dry contact closure.

The MM lights a "visitor" LED and beeps when an event happens. While the beep only goes a few seconds, the light stays on until a "reset" button is pressed. For my system I need a short pulse. It seemed like I could trigger on the LED signal, then pulse the reset button to get what I want. Built the circuit around a 08M2, and it works great.

The MM has a twelve volt wall wort power input, but an internal regulator drops voltage to 5V. Perfect. I have a few surplus 5 volt relays for dry contact output. At first I thought that cmos one shot or 555 timer might be the way to go, but with a little thought, the flexibility of the PicAxe seemed perfect. Probing around the MM board, I could see the reset button simply connected an MPU lead to ground. Easy to do with a PA. The green "visitor" LED has the negative lead grounded, a resistor in series with the MPU output lead and the high lead of the LED. YeeHah. So the PicAxe code is very simple:

#rem mighty mule reset circuit.  when signal comes in, normally green visitor lamp stays lit until manually reset.  This will use a relay to signal aditional devices, for 2 seconds, then reset MPU.  C.1 is input, looking for high when green led is on.  C.2 is output, going low to complete circuit for relay energise.

#picaxe 08m2

high c.2                'short beep to say hi
pause 1000
low c.2                  'relay coil
high c.1                 'reset button


if pinc.3=1 then    'visitor green led on

    high c.2              'energise relay / external beeper/counter
    pause 2000        'beep length 2 seconds
    low c.2               'turn off relay / external beeper/counter

    low c.1                'reset mule
    pause 500           'not sure how long
    high c.1                'ready for next round


pause 1000                 'let stuff settle, not sure how long
goto begin

For wiring I bent up all the PicAxe leads and hot melt glued it piggy back on the RF chip on the MM board. Wires were soldered to whatever place on the MM board was easy to get to. The little output relay was glued to a blank space on the MMboard and the output NO contacts were soldered to extention wires to go to my existing alarm circuit.

Before I started this project, I searched all over the net to see if someone else had done it first. Could only find a lot of folks looking for dry contact output, but no solution. I think the PicAxe was the perfect solution. Here's the schematic. Hardly worth the pencil lead and paper, it is so simple: