Testing a Mystery Circuit
The siren’s circuit contained 6 simple components: 3 resistors, 2 diodes, and a 220μF
capacitor. In addition there were 3 mystery
parts: one that looked like a coil (with no
markings), an 8-pin chip, and a 3-pin thingy
that was maybe an output transistor.
I guessed that the 8-pin chip was a microcontroller, because there weren’t enough
other components to create the two-tone
squealing sound. I couldn’t find its part
number when I searched online.
At this point I could have drawn the circuit
by following the copper traces underneath
the board, but so long as the 8-pin chip was a
mystery, a circuit diagram probably wouldn’t
help me much. So, I went back to first principles. I knew that circuits that oscillate are
often controlled by resistors and/or capacitors.
(How would you know this? Maybe by reading
a copy of my book, Make: Electronics!)
I selected a 100μF capacitor and touched
it across the 220μF capacitor while the siren
was running. I was disappointed to find that
this made no difference at all. (Incidentally,
the presence of a coil on the circuit board
suggests that this device may crank up the
internal voltage somewhat. If you work with
it while the battery is connected, you may
want to use insulated pliers.)
The capacitor experiment was a failure, but
what about the resistors? Their colored bands
told me they were rated at 1K, 470 ohms, and
200K. I guessed that 200K was the most
likely to control an audible frequency.
I added various standard-value resistors
to the solder pads under the 200K resistor,
using a 30-watt soldering iron (because the
kind of 15-watt iron that I normally use wasn’t
powerful enough to melt the pads). When
each resistor was attached, I tested the alarm
and found that the frequency changed. I got
precisely what I wanted when a 100K resistor
seemed to double the frequency (Figure G).
This was very encouraging, but for my
final mod, I needed something neater. I could
remove the original resistor and my add-on
resistor, and substitute a single resistor in
their place — but what should its value be?
Making the Mod
1 11 += R1 R2RT
In the siren, R1 was 200K and I had added
100K as R2. So,
+ = = approx K 200K 100K 200K 1 67
In other words, I could substitute a single 68K
resistor for the pair that I had tested.
But wait a minute. Would the lower resistance burn out the siren? Well, an alarm siren is
designed to run for very long periods, whereas
I was planning to use the modified version only
in short bursts, and at 9V instead of 12V. Under
these conditions, I guessed that a resistor of 1/3
the original value should be OK. The 9V battery
would probably fail before the siren did.
144 Make: makezine.com/26