The IC LS1240A from SGS Thompson is specifically designed for generating a 2 tone ringtone generator circuit. Thus the IC specifically becomes suitable for landline telephones for producing the ringtones when a call is received.
However with little modifications the IC can be easily utilized for normal applications, in circuits where a double tone alarm may be required.
Technical Specifications
Basically the IC is a 8-pin DIL chip resembling the IC 555 and has the following specs:
- Very low current consumption yet good audible response at the output.
- Internal built-in rectifier stage for rectifying an AC input supply, a filter capacitor is also a built-in feature of the IC.
- Hardly any external components required for setting up the circuit.
- The tone of the output musical audio can be adjusted externally through a resistor.
- If the IC os used inside a landline telephone then the supply to the IC is derived from the line voltage via a 1uF capacitor.
- The IC is able to drive a piezo electric transducer directly without the need of inductors or amplifiers.
- The IC is also capable of driving a small loudspeaker directly for generating the musical output.
- However the speaker must be rated at minimum of 50 Ohms.
- The following 2 tone ringtone generator circuit shows how to configure the IC LS1240 as a two tone musical signal generator circuit.
Pinout Details of the IC LS1240 dual tone generator IC
Circuit Diagram of two tone music generator using IC LS1240
Two Tone Generator using piezo element
Courtesy:
Dear Swagatam,
I am trying to find a circuit with which to build an accurate stable two frequency AC signal generator.
The two frequencies I require are 528Hz and 1/13th harmonic of 40.6Hz which I then need to push through an amplifier circuit to achieve a minimum of 200 ohms.
The voltage input would ideally be 15volt DC, which I wish to switch via an existing frequency generator with a Photomos gate.
This would hopefully give me a 50:50 528Hz/40.6Hz AC signal output to an amp. switched via the Photomos gate also opening and closing at 528Hz.
The original circuit is accurate to 3 decimal places.
Can you help, please?
Thank you so much,
Dear Kit, I am sorry, I have no idea how this can be implemented?
Dear Swagatham
I assembled this circuit. There is no sound, from speaker/ ceramic transducer.
My experiments :-
1) I tried two LS1240A ICs removed from two landphones.
2) I tried speaker ( 2x30ohms speakers in series.
3) I used same piezo ceramic transducer (without inbuilt oscillator) from the landphone.
4) I tested with 9vdc to 17vdc.
5) I shorted pin 1 (LINE) to pin 2 (Negative).
Why there is no output….? Does this circuit need AC voltage at pin1 and pin 8….? If yes what should be the minimum and maximum AC voltage range….?
Regards
Dear Annil,
the IC has an internal rectifier circuit so it does not matter whether the input is an AC or DC, it should work with both.
However the IC has only one negative pin at pin#1, no other point in the circuit is connected with the negative supply, so I hope you have connected this correctly.
I think your IC may be faulty, you can further confirm the circuit by using a headphone instead of the speaker.
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