LS1240 Electronic Two-Tone Ringer DIP-8 (LS1240 IC)
The LS1240 is a monolithic integrated circuit specifically designed to replace mechanical bells in telephone sets. Housed in an 8-pin DIP (Minidip) package, it acts as an electronic ringer when connected to an electro-acoustical converter. It can directly drive a piezoceramic converter (buzzer) or a dynamic loudspeaker. One of its standout features is the integrated bridge rectifier with Zener diodes, which provides built-in protection against line overvoltages and simplifies external circuitry.
In India, the LS1240 (and its high-current variant LS1240A) is widely used by telecommunications manufacturers and hobbyists for building extra telephone ringers, line-powered alerts, and door chimes. Because it draws very low current, multiple devices can be operated in parallel on the same telephone line without interfering with standard operation.
Key Features
- Integrated Bridge Rectifier: Includes Zener protection to handle high-voltage AC ring signals without extra components.
- Low Current Consumption: Optimized for parallel operation (up to 4 devices on one line).
- Direct Drive Capability: Drives piezobuzzers directly; can drive dynamic speakers with just a decoupling capacitor.
- Adjustable Frequencies: Both the tone frequencies ($f_1$, $f_2$) and the switching/sweep frequency ($f_{sweep}$) are externally adjustable via a resistor and capacitor.
- Built-in Hysteresis: Integrated voltage and current hysteresis ensure stable triggering and prevent noise-induced false ringing.
- No Transformer Required: Modern design eliminates the bulky and expensive transformer used in older ringer circuits.
Specifications
- IC Type: Electronic Two-Tone Ringer
- Supply Voltage ($V_S$): 26V (Maximum)
- Calling Voltage ($V_{AB}$): 120 Vrms (Continuous) / 200 Vrms (Intermittent 5s ON/10s OFF)
- Activation Voltage ($V_{ON}$): 12.2V to 13.2V (Typical)
- Sustaining Voltage ($V_{OFF}$): 8V to 9V (Typical)
- Output Current: Higher capability than standard ringers (LS1240A version provides even higher current).
- Operating Temperature: -20°C to +70°C (Industrial Grade)
- Package Type: PDIP-8 (Through-Hole)
Pinout Configuration (8-Pin DIP)
- Pin 1: Telephone Line Input (+)
- Pin 2: Ground / Line Input (-)
- Pin 3: Null / No Connection (Reserved)
- Pin 4: Buzzer Output / Ground Reference
- Pin 5: Audio Output (To Buzzer or Speaker)
- Pin 6: Output Frequency Control (External Resistor $R_1$)
- Pin 7: Sweep Rate Control (External Capacitor $C_1$)
- Pin 8: Line Rectifier Capacitor Connection
