Wanna Make A Garage Door Open Alarm
This is a continuation of my previous blog. I could use two 555 timer chips in the astable mode to generate a European type siren with the high-low ‘warble’ tones, but I decided I would do it with a couple transistors. Actually I coul+d use a single 556 dual timer chip, but that’s not commonly available. I’m trying to make an open garage door indicator warning siren, optionally with a light. What I will have is a reed switch with a circuit and a few batteries to run it. This will be located in the rafters in the garage ceiling so that when the door is open the magnet on the door will close the reed switch. The reed switch will then connect the batteries to the circuit. The circuit will then transmit a radio frequency signal on about 88 MHz, consisting of the RF carrier, modulated with the high-low warble tones. The radio in the house will be tuned to this frequency, and when the door is closed, it will be silent. When the door is opened, the radio will then sound the alarm sound, alerting me that the garage door is open.
I don’t have to use a reed switch, but I have a lot of them that I salvaged from old phones. They were used to detect when the handset was in the cradle, so basically they were the same as the switch hook. In all the years that I worked on the phones, I never saw a reed switch go bad. More than a dozen times there were complaints that the phone would not hang up. But in all cases, the problem was that the user had dropped the handset on the floor or table, and the magnet, being heavy, came loose and moved away from its proper position, causing the reed switch to stop closing. Replacing the handset always took care of the problem.
I thought about using a Microswitch instead of the reed switch. The Microswitch is like the ones for the mouse buttons but bigger. They are small and have holes for mounting to something solid. Some even have a metal tab that sticks out to make contact with a door or whatever. I could also use a magnetic alarm switch for a window. It’s basically the same thing, a reed switch and a magnet.
Back to the G. D. Open sensor
I observed the garage door, which is a roll-up type, as it traveled in its tracks. One problem I see is that it wobbles a lot, and stops at a place that may vary by a good fraction of an inch, maybe as much as a half inch (12.5mm). It also may vary its position in the track, so the combination of the two variables means that if I mount a magnet on the door, and a reed switch on the structure, it must work with the magnet as close as almost touching, or as far as an inch away. I see a problem with this, as the reed switch may not close when it’s so far away. I came up with a couple alternatives that might be better.
One is to mount a mercury tilt switch on a pendulum that gets moved several inches when the door is open. The position of the pendulum would not matter as long as it was past a certain point. But the mercury switches are very difficult to obtain because mercury is considered a hazardous material. And it could be very expensive.
Another solution involves the use of a LED and photocell to make a light beam sensor. One way would be to put the LED on one side of the door and the photocell on the other side. When the door is open, the beam would be interrupted and the alarm would go off. Another way would be to mount both the LED and photocell on one wide of the door, and put a reflector on the door or use a shiny metal spot on the door to reflect the light. When the door is opened, the reflection would turn on the photocell.
One problem I see with this sytem is that there is a continuous demand for current to light the LED, so using batteries is not economically possible and since the stuff is located up in the garage ceiling, it would mean a climb on a ladder to replace the batteries every few weeks. I’d like to avoid that if possible.
Another use
Another useful tool is the wire tracer. I have one of these the little box that clips on to the pair of telephone wires and sends the “deedle-deedle-deedle” tone down the wire, so it can be traced by the probe. I could put this circuit into a small box with a pair of clip leads and m,ake one just like the commercial models that sell for $30 or more. I really don’t need to. because I picked up a genuine “Bell System 139B Test Set” at a swap meet, and I took it apart and fixed the intermittent contact problem with the battery holder. That’s one thing that will really make a person angry. I use the tone tracer box at work to find a telephone line among the hundreds or thousands of others. One end may be located a thousand feet away on the other end of campus. If I put the tone tracer box on the phone line and walk all the way down to the other end of campus and can’t find the line because the box stopped working, and I have to walk all the way back to fix it, then walk all the way down again, it can get really frustrating.