Thermal regulator on DS18B20 and ATtiny2313A
THe dirty current pulse of coil can interfere in both conducted ripple and radiated EMI.
So it should be filtered separately.

conducted noise using ultra low ESR Caps or series R and low ESR Caps.

Radiated noise filtered by CM rejection using a small Loop area so forward and return paths cancel out.
Then LCD and uC can share 5V  without interference.

Series R (depends on load but low ESR e-cap must have ESR*C&lt; 10us and 0.1uF Ceramic or metal film must have lower ESR and much higher SRF (self-res. freq.)

if 100mA coil then 0.1V drop is ok with 1 Ohm R with 100uF and 0.1uF to coil with reverse diode clamp.

Low ESR e-cap is best otherwise 100uF cap is 2 Ohms vs 0.1 Ohms.


This attenuates noise by Rs (1 ohm) to ESR ratio of caps for step loads and ESR of 0.1uF for release current.



simulate this circuit &ndash; Schematic created using CircuitLab

If the power source sags that is a poor source.
if it drops 0.1V from 100mA coil and LDO maintains 5.0 for the other loads, then no conducted interference.
if loop area shown with arrows is large, then radiated interference can upset uC
Green arrow shows when relay is activated and Red Arrows when released.