This circuit is from an 80's documentation, firing when the input voltage exceeds 5 V. The SCR can be the TIC106 need not have a heatsink.
This photo-relay uses a Darlington stage to increase sensivity. Any photo-transistor can be used....
Texas Instruments TMP116 and TMP116N digital temperature sensors can be obtained from Mouser...
Microsemi Le87536 Dual Differential Amplifier Microsemi / Microchip is making available through Mouser...