Energy Saver Solder Iron Station

In this post we learn how to build a energy efficient soldering iron station circuit for achieving maximum power saving from the unit, by ensuring that it is automatically switched OFF when inot being used for sometime.


To design a circuit for solder iron which would not only save the energy but also avoid the over-heating of solder iron tip.


a) Switch ON and warm-up the solder iron for about 1 minute.

b) Check if the solder iron is present in stand or not.

c) If not present, the solder iron gets 100% power, directly from AC mains.

d) If present, the solder iron gets 20% power thru regulated circuit.

e) Go to procedure (b).

Circuit Set up and Schematic


a) A 555 timer is configured to delay power on for about minute. During this period the solder iron is connected to AC mains thru the "NC" contacts of the relay.

The red LED would indicate the initial warm-up of 1 minute after which it goes off and the green LED would light up to indicate that the solder iron is ready to use.

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 R5/R6 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 R4/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 R4/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 thru the potential divider R8/R10. The output is connected to the gate of triac BT136, which 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.


1) Since the triac (operating AC mains) is directly connected to the rest of the circuit via R12, care should be taken and the circuit should not be touched when powered on. For protection, opto-isolator like MOC3020 can be incorporated.

2) Any value of thermistor may be used but, the value of the R4 should be selected accordingly such that R4/TH1 should provide about 3V at normal temperature. Moreover, the increase in temperature of the spiral steel wire sleeve due to the presence of solder iron should also be taken into account.

3) 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.

4) The thermistor legs should be covered with heat resistant insulation sleeves and then installed suitably on the iron stand.

5) The 12V DC supply (not shown) may be obtained from AC mains using a step-down 12V transformer, 4 x 1N4007 diodes and a filter capacitor. For details, read this article

Written and Submitted By: Abu-Hafss

The above explained circuit of an energy saver soldering iron is appropriately modified and corrected in the following diagram. Please refer to the comments for a detailed info regarding this modification:

Need Help? Please send your queries through Comments for quick replies!


Norie Coquilla said…
I like to build this project mr. Swagatam. Hopefuly this will help me save energy consumption..
Swagatam said…
sure Mr.Norie, I appreciate your interest.
chinmoy1955 said…
There is a problem with this circuit. The trigger voltage given to the triac is not referenced to either terminal 1 or terminal 2 of the triac, in other words the gate is isolated. This way there will be no current flowing in or out of the gate and the device will never get triggered. Correct me if I am wrong in my observation.
Kind regards
Abu-Hafss said…

You are correct! Thanks for pointing it out.
An opto coupler would be definitely solve the issue.

I would ask Swagatam to update the revised schematic as below

Kind regards to you as well :)
Swagatam said…
Thanks Abu-Hafss,

However I don't think your new idea would solve the issue either, because in your diagram there's no way the triac would be able to get a positive trigger from the MOC IC.

A better idea would be to use another relay driver stage with the R12 end.
Abu-Hafss said…
Hi Swagatam

According to your idea, the neutral of the mains should be grounded. And that has to be done manually using a multi-tester, which is not a good idea. I am sure MOC would do the job, and I might need your help to get it done :)
Swagatam said…
Hi Abu-Hafss,

My idea was to replace the triac MT1, MT2 leads with relay contacts (NO) of a second relay which would be triggered by R12 opamp via a transistor stage

actually i have not studied the circuit yet, I am just trying to solve it by looking at the diagram....and I believe that the triac is supposed to be triggered by the lower opamp and override the first relay according to me this could be done by using another relay.
Abu-Hafss said…
Hmm, yes it reveals that you have not studied the circuit and my description. I have clearly discussed in note no. 3(a) and 3(b) that the relay cannot be used.

I have assembled it already without the opto coupler. Please spare a few moments to sort out the issue because frankly I have no practical experience of triacs.
Abu-Hafss said…
The following para would definitely give the core idea of saving energy:

"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 thru the potential divider R8/R10. The output is connected to the gate of triac BT136, which 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."
Swagatam said…
OK, thanks Abu-Hfass,

I have updated the new diagram using the MOC opto-coupler, please check it out.
edwin siy said…
Hey,awesome circuit,may I ask what is the voltage of the 0.01uf cap in series with the 39 ohms resistor at the AC side?How about the 39 ohms resistor,whats the wattage?Thank you
Swagatam said…
thanks edwin, the capacitor is a regular 50V cap, and the resistor can be an ordinary 1/4 watt resistor
edwin siy said…
Thank you very much for your immediate response.
Abu-Hafss said…

Thanks for pointing out the error. Swagatam, would you please change the labels in both the diagrams:

R4 ====> R6
R6 ====> R4

For thermistor, yes I did not mentioned NTC or PTC but the statement "the increase in temperature will increase the resistance of the thermistor" in para (C) clearly depicts that it is PTC.
Swagatam said…
Hi Abu-Hafss,

changing the text instead of the image will be easier...I'll do it shortly...
Abu-Hafss said…
Thanks Swagatam.

But there are two more to be changed to "R4/TH1"
One in para (c) and the other in NOTE (2).
Swagatam said…
Thanks Abu-Haffs, I think it's all done now, please confirm.....
Abu-Hafss said…
All is well !!!
francis said…
Good Job guys!!! Well done this circuit works pretty well. May God give you extra knowledge.
Swag said…
Glad it worked for you...keep up the good work.

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