56 SERVO 12.2014
Diet Buddy some fairly aggressive
movement. Therefore, you should be
careful which food shelf you assign it
to. Prolonged light could cause Diet
Buddy to hop outta the fridge and
land on the floor. (Remember Humpty
Sharp-eyed robot builders will
recognize the Diet Buddy circuit as
similar to Randy Sargent’s venerable
BEAM bot, Herbie. His BEAM — or
Biology, Electronics, Aesthetics, and
“eyes,” along with a pair of wheels for
propelling Herbie in its search for
light. This light-attraction behavior is
the feature that makes your Diet
Buddy sound the alarm when the
refrigerator door is opened.
There are several circuit
differences between Herbie and the
1. The gain pins (pin 1 and pin 8)
on the LM386 are left open.
2. Only one LDR is attached to
the LM386 (pin 2), along with a
trimmer potentiometer connected to
the other op-amp input, pin 3.
3. One motor is used for
vibration. This motor is connected to
pin 5 and GND.
4. No wheels are used.
Seasoned builders know that the
light/dark resistance in an LDR is
extremely variable (no pun intended).
Therefore, a 50K or 100K trimming
potentiometer can be used for the pin
3 input. Likewise, the fixed resistor in
the circuit can vary between 1K W to
47K W. In the case of our circuit here,
1 LM386N audio op-amp
1 CdS LDR
1 Fixed 1K to 47K W resistor (NOTE: a 47K W resistor was used
in this circuit.)
1 Pancake-style vibrating motor
1 DB- 9 serial cable hood
1 CR2032 3V battery
1 CR2032 battery holder
1 50K or 100K trimming potentiometer (NOTE: This circuit used
a 100K pot.)
Diet Buddy schematic
Build the circuit freeform
inside the DB- 9 hood shell.
By soldering the LM386
pin 2 and GND terminal
to a pair of DB- 9 solder
tabs, the LDR can then
be plugged into the two