last of the hard stuff & loose ends…
In general, counting chips only require a pulse on the clock line. More specifically a rising or falling edge of a pulse but that might be a little too much detail. To be simple and direct, the trigger is what generates this pulse. But the buzzer is different because it does not care about the pulse edge but the length of the pulse itself.
The alarm is difficult because what is desired is not a buzz or a beep as long as the switch is closed but a short beep just long enough to be heard by the speaker but no more. But the question is how long should the buzz be? This is harder than it might seem. I don’t know what the studies of human perception have shown about what the minimum duration of a sound is to be heard. And how does that differ from person to person? Can I make the sound so that the speaker hears it and knows it happened but so that the audience does not notice? (And I did try to find information via a Google Search, too much and too technical.)
The buzzer itself is something I have used my self several times over the last 20 years. It comes from Radio Shack, of all places, and is real easy. Apply power and it screams. Turn off power and it shuts up. Works over a voltage range of 5 to 12 so is good most anywhere.
The best think I can try as a discrete chip is a “Monostable Multivibrator”. The particular chip is a 74121 which is supposed to be triggered by the rising edge of a pulse then stay high for a length of time that is set by two other components, a resistor and a capacitor. But what values of those should I use? As a first start I chose a 1.0uF capacitor and a 10.0k Ohm resistor. That should give me a pulse of six or seven milliseconds or 0.006 to 0.007 seconds. Yes, I could see that on the oscilloscope, but could I hear it?
Alarm circuit based on 74121
So this is an experiment. Can I hear a 7 millisecond sound? Or will I have to try several different values of capacitor and resistor to get what I want? That will remain to be seen.
One thing I did get from talking to potential customers about this idea. Some people want the beep, some don’t. So I added a switch to the circuit to stop the triggering pulse to the beep.
At this point I have the schematic captured. Although there is something to note, namely the miscellaneous stuff. These are two types. First are holes for legs or “stand-offs” for the board. These keep the electrical parts from touching and shorting out with the metal box that this board will, most likely, be packaged in. Four of them, one for each corner of the board.
The other type of thing is a “fiducial”. These are little markers so that the automated assembly equipment can map what it has been told about the board to an actual piece of fiberglass. On the schematic neither the stand offs nor the fiducials are an active part but they will be needed on the board layout so they must be included here. Fiducials are three per side, assuming that there will be parts on the front and the back of the board. Having parts on the back of the board does increase assembly cost but often it is either good to do electrically or is the only way to get all the parts into the amount of board allowed. (Like 5 pounds of parts on a 2 pound board perhaps?) And at this point I can’t make the decision yet to NOT to put parts on the back side. On the other hand this is a prototype that I will hand assemble, should I care?
That will depend on the next step…
Miscellaneous Parts Necessary for the Layout and Final assembly
Next time: There is a fork in the road for the design path, which way will I go?
Also, is this useful to you?
