Determining Time Intervals with Capacitor C1
Basic timer: In this circuit, the crucial element for determining time intervals is capacitor C1. It is charged through a current source relying on T1, resulting in the capacitor voltage forming a ramp. Comparing this ramp with a preset voltage provides a relatively precise time indication. In the given configuration, the time duration falls within the range of 1 to 10 minutes.
Comparing Ramp and Reference Voltage
IC1a and IC1b, both featuring open-collector outputs, are responsible for comparing the ramp and the reference voltage. Disregarding IC1a momentarily, if the set voltage surpasses the potential across C1, the output transistor of IC1b remains off. However, once the potential across C1 exceeds the reference voltage, the output transistor in IC1b activates, causing T2 to turn off. In practical terms, this setup implies that the buzzer will sound until the set voltage level is surpassed, which is not the desired outcome. To ensure the buzzer only briefly activates when the set level is exceeded, the presence of IC1a is essential.
Determining Comparator Offset for Buzzer Activation
In this setup, the comparator evaluates the reference voltage using a potential slightly higher than that across C1. This offset is created by R4-P1-D3. Consequently, IC1a responds differently from IC1b: the output transistor in IC1a remains active when the potential across C1, along with the voltage at the wiper of P1, is lower than the reference voltage. It switches off only when the reference voltage is exceeded. This behavior ensures that IC1a triggers the buzzer shortly before the set time elapses, sustaining the activation for an extended period.
Summarizing the Operation
To summarize the process: one output initiates the buzzer after a specific time duration, while the other output deactivates the buzzer slightly later. As a result, the buzzer only sounds during the brief overlap between these two periods. The duration of this overlap is determined by P1. P2 is responsible for setting the time interval before the buzzer is triggered. The timer begins its operation when S1 is pressed.