A random question in a picture

shaorc

[Ask if you don't understand] The picture shows an infrared detection alarm circuit that I searched for. U1 and U2 are both lm358 (only one), and the comparator on the far right is lm393. I have the following questions: [1] U2 is a comparator, and R10 and VD1 provide the reference voltage. Why use a diode instead of a resistor to provide the reference voltage? [2] R14 and C6 form a delay circuit, which is said to delay for one minute, but R14 is 470K and C6 is 47uf. According to the time constant RC calculation, 470K×47uf is not equal to one minute. How can I explain this?

qwqwqw2088

The diode is simple and practical, and it is relatively stable. When the temperature of the voltage divider resistor is high, the reference voltage provided will deviate.

topwon

The resistor voltage division method requires that the pull-up VCC voltage remains stable, and the resistance value cannot be affected by other components in the circuit, such as the input impedance of the chip. Of course, there is no insurance like using a voltage regulator.

topwon

The calculation of the RC constant generally only gives a rough value, and actual debugging is required to select the precise value, because there are other components in the circuit that will have an impact, and the cut-off voltage for charging and discharging is not necessarily VCC and 0V.