Basic timer: The time-determining element in the present circuit is capacitor C1, which is charged via a current source based on T1. The voltage across the capacitor is, therefore, a ramp. A comparison of this ramp with a preset voltage gives a fairly accurate time indication. With values shown in the present circuit, that time lies between 1 and 10 minutes.
The ramp and the reference voltage are compared by IC1a and ICib, both open-collector output types. Ignoring IC1a for a moment, the output transistor of ICib will be off as long as the set voltage is higher than the potential across C1. if that potential becomes higher than the reference voltage, the output transistor in ICib switches on and T2 goes off.In practice, this would mean that the buzzer sounds until the set voltage level is exceeded: not exactly ideal. Since the intention is that the buzzer sounds only briefly when the set level is exceeded, IC1a is needed. This comparator evaluates the reference voltage with a potential that is slightly higher than that across C1. This offset is provided by R4-P1-D3. The result is that IC1a reacts in a different way from IC lb: the output transistor in IC1a is on when the potential across C1 plus that at the wiper of P1 is lower than the reference voltage and is switched off when the reference voltage is exceeded.Thus, this comparator starts the buzzer sounding for an infinitely long period just before the set time has elapsed.
summing up the action: one output actuates the buzzer after a certain period of time has elapsed, while the other output switches the buzzer off again after a slightly longer period of time has elapsed. Thus, the buzzer sounds only during the short overlap of these two periods. The length of this overlap is set with P1. The time period before the buzzer is actuated is set with P2. The timer is started when S1 is pressed.