The post discusses a simple automatic soldering iron power shut off timer circuit which ensures that the iron is always switched OFF even if the user forgets to do the same during the course of this routine electronic assembly job work. The idea was requested by Mr. Amir

Technical Specifications


My name is amir of Argentina ... and I am repairing technician but I have a problem I always forget the soldering iron on, ested can help me with a circuit for self disconnection time, my idea is ...

after a while the low power soldering iron in half ...

and sounds a beep beep until you press a button and set the counter to zero, but if not pressed after once off.

from already thank you very much.

The Design


Initially when the circuit is powered via mains AC, it stays switched OFF due to REL1 contacts being in a deactivated state.As soon as S1 is pressed the IC 4060 momentarily gets powered via TR1, bridge network activating T2.

T2 instantly energizes REL1 coil at its collector which in turn activates the N/O contacts of REL1 wired across S1.

The above activation bypasses S1 and latches the circuit so that now releasing S1 keeps REL1 activated.

This also switches ON the connected soldering iron via REL1 and N/C of REL2.
Now IC 4060 which is wired as a timer being powered begins counting the timing period set by adjusting P1 as per the requirements.

Suppose P1 is set for 10 minutes, pin3 of the IC is set for becoming high after 10 minutes interval.
However this also means pin2 of the IC would go high after 5 minutes interval.

With pin2 switching ON first after 5 minutes triggers REL2 which now shifts its contacts from N/C to N/O. Here N/O can be seen connected to iron via a high watt resistor, meaning now the iron gets switched to receive less current making its heat lower than the optimal range.

In the above condition T1 being switched ON, the buzzer at pin7 gets the required ground supply via T1 and starts beeping at some frequency indicating the iron being shifted to low heat position.

Now if the user prefers to restore the iron to its original condition could press S2 resetting IC timing back to zero.

Conversely if the user is inattentive, the condition persists for another 5 minutes (total 10 minutes) until pin3 of the IC also goes high switching OFF T1,/REL1 such the whole circuit now shuts down.

Circuit Diagram




Parts List for the proposed automatic soldering iron power saver circuit

R1 = 100K
R2, R3, R4 = 10K
P1 = 1M
C1 = 1uF NON POLAR
C2 = 0.1uF
C3 = 1000uF/25V
R5 = 20 OHMS 10 WATT
ALL DIODES = 1N4007
IC PIN12 RESISTOR = 1M
T1 = BC547
T2 = BC557
REL1, REL2 = RELAY 12V/400 OHMS
TR1 = 12V/500MA TRANSFORMER
S1/S2 = PUSH TO ON SWITCHES
BUZZER = ANY 12V PIEZO BUZZER UNIT

A redrawn version of the above diagram can be seen below, it was suitably improved by Mr. Mike for helping easier understanding of the wiring details.

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