P1 is of experimental value. Start with 220 Ohms or so and modify to suit your
needs. The transistor is a general purpose kind and is not critical, almost any
pnp type will work. L1 is a bell-transformer which is usually already present
in the house. If you wish, you could use a battery instead of the bell
transformer. Just hookup a 9-volt battery to points 'A' and 'B' (A=+) the diode
(D1) is to protect the circuit from accidental polarity reversal and is optional,
but required for use with the bell transformer.
T1 is a General Purpose PNP transistor and probably anything will work.
L2 comes out of an old am transistor radio. They look like miniature transformers
and are usually colored red or green. You have to fiddle with different
transformers as the sound can vary depending on the value. The loudspeaker is
a 8 Ohm type and must be larger than 200milli-Watt. I used a 2Watt type, but
anything over 0.2W will do. It really sounds like a bird and when you release
the doorbell button the sound slowly fades away. I have used this circuit in
my house for over 20 years and even build the "Birdie" for others. Although an
old circuit, the experimentation and the final results still give a punch.
Remember to Have fun!
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Copyright © 1993 - Tony van Roon