Tuesday, July 1, 2014

Experiment 5 (Chapter 5) -- Red Alert!

I really liked this project. Combining two 555 timer chips really helped cement how this little chip works. I had forgotten much about the pins and how the 555 functioned, so wiring up this circuit was very helpful. I highly recommend that as you wire up each 555 timer chip, you examine carefully the wires that connect to each pin and try to understand exactly what is happening at each pin.

You'll probably find as I did that there's a LOT going on in this circuit in terms of components -- resistors, capacitors, phototransistor, speaker, and the two 555s. Check and double-check your wiring because it's a mess. I must be getting better at checking myself because the first time I applied power to this circuit, it worked. That doesn't happen often!

I'm including two videos here; the only difference between the two is the substitution of a 1 microfarad capacitor for a 10 microfarad. All other components were left alone. I did make a mistake in the video by stating I had the 100kohm and 50kohm resistors in series for the 150kohm called for, but I actually had to put two 85kohm in series for a total of 170kohms. That probably did affect the frequency of the alarm a bit since this value affects the pulse length for the 2nd 555 chip (the one that feeds into the 555 connected to the speaker). The change was probably negligible, though. I did make certain all capacitor values were the ones specified in the schematic on page 28. Here's the first video with the 10 microfarad capacitor inserted first:

I'd be curious to see any videos you might have if you attempt any of the variations specified on pages 28-29. Charles includes a bunch of substitute chips for the 555 that could be tried out -- these include 7555, 4047B, 74HC221, and a bunch more. But his explanation of why the 555 is still favored makes sense... you begin to work with a chip so much you just know its pinouts and its quirks. That said, the 556 sounds interesting -- two 555s on one chip, although he says its becoming difficult to find. I might try to hunt one down and retry this circuit a bit later.

Here's the short second video where I substituted the 1 microfarad capacitor for the 10 microfarad:


  1. Great blog! Im just about to start going through Make:Electronics, and have found looking through your previous blog, and this one to be very informative, and inspiring! Keep it up!
    Luke (Essex, England)

  2. Hi, Luke.

    Glad you like the blog. And welcome to Make: Electronics... you're going to have a lot of fun and learn a LOT. I don't check in with the other blog that often, so feel free to post comments here, although it's been a while since I've looked at much of Make: Electronics, so I may only be able to offer up encouragement, not solid answers.