Friday, February 5, 2010

Chapter 3 - Exercise 15 Part 3

Today I wired up the alarm system portion of the breadboard... once again, I lacked two 2.2 microfarad resistors so I substituted a 4.7 microfarad for C3 (in Figure 2-113). When I wired up this noise maker using that combo, it worked... but probably didn't sound exactly like it should with two 2.2 microfarads...

I also modified the relay circuit to look like the one in Figure 3-95. When I break the magnetic switch, I hear the relay kick in and it locks, too - resetting the magnetic switch doesn't turn it off.

I hooked up the speaker... flipped the power... broke the magnetic switch... and nothing.


(Keep in mind that I'd already used my multimeter to test voltage/current settings - there was a charge of almost 12V built up on C1 capacitor... and about 11V on the two wires I used to connect to the speaker... so... faulty speaker?

Instead, I connected an LED in place... and it worked. The LED lit up just fine... but shouldn't it pulse? I rechecked all my wiring and components... everything where it should be and voltage across the LED... enough to light it at least. But not enough to get me any noise from the speaker. So I'm back to testing... probably won't get to it until tomorrow, but I'm anxious to build this device, so I'll do my best to figure it out... suggestions or tips welcome.

Movie of the LED lighting up below...


  1. The LED is pulsing, technically, but so fast that you can't see. Remember from an earlier exercise that sound is created from electricity pulsing very fast. The slowest you'd be likely to hear would be pulsing about 20 times per second (20 Hz), but I'm guessing the circuit is set to cycle at several thousand times a second, so there's no way you'd see that with an LED.

  2. Going off of what Nick said, this is actually a trick digital devices use to trick our eyes. By turning the LED on and off and a set rate, our Persistance of Vision makes it look as though the light is on at a particular brightness. The on/off cycle has to occur very quickly though, in terms of thousandths of a second...

  3. After loosely breadboarding the 'alarm trigger' half of this circuit, I transposed it to the bottom of the breadboard in the compressed layout recommended by the book, and, similar to your earlier problem with the locking relay, COULD ... NOT ... GET ... IT ... TO ... WORK! (Wouldn't wait for the magnetic switch signal to open)

    Stared at the circuit and the book drawings forever, reseated components, checked the online errata ... finally, it occurred to me to check the resistors. Sure enough, the new layout used one less resistor than the previous version, and I had grabbed the wrong resistor when I rebuilt the circuit.

    From now on, checking resistor values will be part of my troubleshooting path.