Very Simple Code Practice Oscillator

The other day I was throwing out some junk, one of the items was a Philips Memory Phone.
In case you're curious, it was a model D-9039.

I couldn't resist to have a look inside before dumping it, and look what I found ...
Like most electronic phones, it contained a "ringer" integrated circuit. The IC I found was a L3240, in an 8-pin DIP package. If you find another type of IC (e.g. BA8206, MC34012, HK621, etc), just look up the datasheet on Google.

These IC's are meant to work on the 'ringing voltage' that is on the line if someone calls you. This ringing voltage is a rather high level AC, so the IC contains a bridge rectifier and some zener diodes to provide the supply for the ringer circuit. I didn't use that part.

Only one resistor and one capacitor would be needed to make the circuit work. But then, the circuit makes a warbling ringtone (quick switching between 2 fixed tones).

What I made is even simpler, I built this code practice oscillator (CPO), with just 4 parts, and most came out of that telephone !

Here is the simple circuit:

For the resistor, I first used a 100k trimmer, and found out that 68k would give me a nice 600 Hz tone. So I then replaced it with a fixed resistor. Even that resistor can probably be found in the phone, or make one up with several values in series. So total cost would be ... NOTHING !

Here is the test circuit on my desk, which took me less than 10 minutes to make.
The top picture shows it when mounted on a piece of wood.
The key is the "on hook" switch, and also the buzzer was found in the telephone. 

Note : adding a 100 nF capacitor from pin 3 to ground, will get you that warbling tone, which would make a good "alarm" tone in some other project.

The circuit needs 12V DC or higher to work reliably, and the supply can be as high as 26 V.
For portable use, four Li-Ion cells in series from an old laptop battery will get you a long way ... 

If you want to use an AC source of some kind, the datasheet specifies an input of MAX 120V between pins 1 and 8, but for our USA friends ... I wouldn't connect this circuit to AC mains !
But an AC output wallwart would probably work, in that case you need to add a 100µF electrolytic from pin 7 to ground. That capacitor can of course be found in the telephone ... for free!

Happy tinkering !