b) IC LM358-A is configured as voltage comparator to check the presence of the solder iron in its stand using a thermistor. The (-)ve input of the comparator is provided with a reference voltage of 6V using R4/R5 potential divider. The (+)ve input is also connected to a potential divider formed with R6 and the thermistor TH1.
If the solder iron is not present in its stand the thermistor would acquire the room temperature. At ambient temperature the resistance of the thermistor would be roughly 10k thus the potential divider R6/TH1 would provide 2.8V at the (+)ve input, which is less than 6V at the (-)ve input. Thus the output of LM358-A remains low and there is no change in the operation; the solder iron continues to get power thru the "NC" contacts of the relay.
c) If the solder iron is present in its stand, the increase in temperature will increase the resistance of the thermistor. As soon as it crosses 33k, the potential divider R6/TH1 provides more than 6V at the (+)ve input hence, the output of LM358-A goes HIGH. This energizes the coil of the relay via NPN transistor T1 and therefore the solder iron is disconnected from the AC mains.
The HIGH output of LM358-A also powers ON the LM358-B network, which is configured as an astable oscillator with a duty cycle of about 20%. The duty cycle is controlled through the potential divider R8/R10. The output is connected to the gate of triac BT136, via opto isolated triac MOC3020. The triac BT136 conducts and switches on the solder iron for 20% of a cycle, thus 80% of power is saved while the solder iron is at rest.
2) The triac cannot be replaced with a relay because of two main disadvantages:
a. Continuous rattling sound of the relay contacts could be annoying.
b. The continuous and swift switching of the relay contacts will cause high voltage sparks.
3) The thermistor legs should be covered with heat resistant insulation sleeves and then installed suitably on the iron stand.
4) The 12V power supply (not shown) and the circuit may be enclosed in some suitable box as shown.
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