This application is a useful addition for the bedroom telephone stand. When the telephone rings, or when the handset is lifted, the night light is turned on and remains on while the conversation takes place. When the handset is replaced in the cradle, the light remains on for about 11 s. This circuit is illustrated in Figure 7-38. Operationally the circuit may be divided into four sections.
- U1 – TCM1520 Ring Detector
- U2 – TLC555 Timer
- U3 – TLC271 End-of-Call Detector
- U4 – MOC3011 Light Switch
During standby conditions, the -48 V de bias on the phone line maintains the output of U3 in a high state.
Also the timer discharge transistor is on, preventing the charging of timer capacitor C1. When the ac ring signal is applied to the phone line, it is processed by the ring detector U1, producing a ‘negative output pulse at pin 2 for each ring. These pulses are applied to the trigger input (pin 2 of U2) and trigger U2 causing its output to go high and the discharge transistor to turn off. The high output of U2 activates opto-isolator U4 which turns on the night light. The discharge transistor, being off, allows C1 to begin charging. Each ring retriggers the timer and discharges C1 preventing it from reaching 2/3 VDD threshold level. Thus the night light will remain on while the phone is ringing and for about 11 s after the last ring. After 11 s C1 will be charged to the U2 threshold level (2/3 VDD) resulting in the U2 output returning to a low level and its discharge output turning on discharging C1. At this time the lamp will turn off if the phone is not answered.
When the phone is answered, a 1 kΩ load is placed across the phone line (tip and ring in Figure 7-38). This removes the differential input to operational amplifier U3, causing its output to go low and capacitor C1 starts discharging through R1. As long as the voltage across Cl remains low, timer U2 cannot start its cycle and the lamp will remain on.
When the phone is hung up, the low impedance is removed from the phone line and the differential voltage across the line causes the U3 output to go high. This allows Cl to start charging, initiating the timing that will turn off the night light.