Soldering irons are most often used for installation, repairs, and limited production work in electronics assembly. High-volume production lines use other soldering methods. You might have one on your own test bench. Often times, while working with a soldering iron, we leave it on for long times. This results in not only a smoking oxidised iron but also waste of electricity.
To solve this problem, here’s an automatic soldering iron switch circuit that automatically switches off the soldering iron after a predetermined time.
The timing here is set to 18 minutes but can be varied as required. The circuit draws no power when it is inactive. The circuit can also be used for controlling the electric iron, kitchen timer or other appliances.
Automatic Soldering Iron Switch Circuit
At the heart of the circuit is a monostable multivibrator built around timer IC 555. When the circuit is in sleep mode, to switch on the soldering iron, you should push switch S1 momentarily. The multivibrator gets triggered and its output pin 3 goes high for around 18 minutes to keep relay RL1 energised via transistor T1. At the same time, capacitor C3 charges and AC supply is provided to switch on the soldering iron via normally opened (N/O) contacts of relay RL1.
The soldering iron remains ‘on’ for the time period predetermined by resistor R1 and capacitor C2. Here, this time is set for 18 minutes. Flashing of LED1 indicates the heating progress of the soldering iron. When the predetermined time is over, relay RL1 de-energises to turn off the soldering iron and the buzzer sounds until capacitor C3 gets discharged.
For switching on the circuit, use either a bell push switch or a similar switch with appropriate current carrying capacity.
The article was first published in January 2005 and has recently been updated.