deep-freeze alarm

The alarm described has been in continual use for over a year at a temperature of around -18 ¬?C, which is, of course, quite normal for a deep-freeze alarm. Its function is to indicate an accidental rise in tempera- ture. There are, of course, indicators pro- vided on the deep-freeze unit, but as these are mains-operated they are of not much use in case of mains failure! The principle of operation is quite simple: a green LED lights as long as the temperature stays within limits defined by the user, while a red LED shows when the temperature has risen above a critical level. Since operational amplifier ICI is arranged as a differentiator, two possible states ensue: (a) the output voltage is positive as long as the potential at the non-inverting input is higher than that at the inverting input, and (b) the output voltage is negative when the input levels are reversed with respect to those in (a). The voltage at the non-inverting input is derived from potential divider R2-R3-P1and...