Checking Legal Regulations
Before constructing this circuit, it is advisable to verify its legality in your region, especially considering that connecting it to the public telephone network might be illegal in certain countries. Contact your local telephone manager to ensure compliance with regulations.
Telephone Network Voltage and Currents
In typical telephone networks, terminals a and b carry a direct voltage ranging from 48 to 60 V. When a call signal is received, an alternating current of 50–60 V at a frequency between 25 and 27 Hz is added to this direct voltage. When the receiver is lifted, closing the current loop, the open-circuit voltage is reduced by more than half. The current flowing in the loop typically ranges between 20-100 mA.
Functionality of the Preset Circuit
This circuit incorporates a timer that activates upon detecting the call signal. The timer triggers relay Re1, illuminating lamp La1. Following the final ring, the timer remains active briefly before reverting to its idle state, causing the lamp to turn off. However, when the receiver is lifted, the timer is permanently triggered, keeping the lamp illuminated throughout the telephone conversation and for a short period afterward. Additionally, the lamp can be manually toggled on and off by pressing switch S1.
Parallel and Series Connection with Telephone Terminals a and b
The circuit comprises two segments: one connected in parallel and the other in series with telephone terminals a and b. The parallel-connected segment, responsible for detecting the call signal, is electrically isolated from the direct voltage by capacitor C3. Consequently, there is no current flow through the LED of optoisolator 1C2, keeping the phototransistor turned off. The second optoisolator, in series with the a and b lines, also remains inactive due to the circuit being interrupted by the cradle switch. Despite this, the call signal is transmitted to IC2 through C3, causing current to flow through the LED, activating the transistor. RI limits this current, and diodes D2 and D3 truncate the peaks of both half waves to safeguard the optoisolator from potential damage.
Activation of the Timer and Relay
The phototransistor supplies enough base voltage to T1, turning on this transistor. Capacitor C4 discharges, causing the voltage at pins 2 and 6 of timer IC4a to drop to ground, triggering the timer. Its output switch state (goes high), leading to the activation of T2 and energizing the relay. The relay contact links one of the main lines (live in the UK) to lamp La1. The timer’s time constant is designed to bridge the longest intervals between call signals. If the receiver is not lifted and the last call signal has passed, the lamp remains illuminated for the duration of the time constant before the circuit returns to its idle state.
Effects of Receiver Movement on the Lamp
When the receiver is lifted, current flows through the LED in IC3, thereby activating the phototransistor in the optoisolator and subsequently turning it on. The timer remains triggered as long as the telephone conversation continues. Upon placing the receiver back in the cradle, the lamp remains illuminated for a brief period determined by the setting of P1.
Manual Operation Using the Second Timer (NE556, 1C4)
The second timer in the NE556, 1C4, is employed for the manual operation of the lamp. Initially, half of the supply voltage is present at pins 8 and 12, keeping the output (pin 9) low and preventing the charging of C7. Upon pressing S1, the trigger voltage momentarily drops below the threshold, causing the output to go high. Due to the time constant R9C7, the voltage across C7 increases gradually, ensuring the trigger input (pin 8) remains below the upper threshold. When the switch is pressed again, the full supply voltage is applied to C7, causing the trigger input to exceed the upper threshold. Consequently, the timer output goes low, turning off the lamp.
Alternative Power Supply and Safety Considerations
The power supply follows a standard design but could be substituted with a suitable 12-V mains adapter. In such a configuration, the lamp could be a 12-V car type, offering a viable and safe alternative from a safety standpoint.