OK, let's recap everything that has been done:
Here's the current schematic
![](https://i.ibb.co/S0WWHbj/Bildschirmfoto-2020-06-07-um-15-47-38.png)
- BUL58D was replaced by a TIP122 (
if there's a better one, I'll update the schematic)
- Voltage drop at pin 6 of comparator caused by potentiometer/MIC502 (used to be pin 5 before Schmitt trigger) has been deemed negligible.
Voltage at R8/RV1 pin 1 represents the temperature of the NTC thermistor and isn't influenced by anything but it. - LM358 was replaced by an LM393 comparator (rail to rail)
- Comparator input voltages tweaked so that they fall within spec (6.8k resistor).
- Added Schmitt trigger circuit. LED shall turn on when High Threshold is reached (when NTC behaves like a 1.1k resistor, ie when temperature is around 84°C ->
10.32V) and shall turn off when voltage falls below the Low Threshold (when NTC behaves like a 1.3k resistor, ie when temperature is around 79°C ->
10.07V). Resistor values calculated using this
website (Low Threshold: 10.07V, High Threshold: 10.32V, V_Ref: 12V, Output Voltage High: 12V, Output Voltage Low: 0V). Here's the
output.
Apart from the transistor, it should be pretty much ready to go?
What do you think?