Touch switch off the soldering iron. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Ham Radio Technologies
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When working with field-effect transistors and microcircuits of the CMOS structure, there is a danger of their damage when soldering with a network soldering iron (the appearance of a high potential on its sting). This is due to the fact that field-effect transistors and microcircuits built using CMOS technology are very sensitive to electric fields. The most dangerous are the electric fields that arise as a result of mains pickups when the soldering iron is powered by an alternating voltage of 220 V. Under the influence of such fields, the element can fail even if its output is briefly touched with the soldering iron's tip.
In amateur radio literature, it is usually recommended to reduce the soldering iron supply voltage to 12 ... 42 V by redoing the soldering iron. This is a rather laborious operation, which still does not give a full guarantee against the breakdown of a transistor or microcircuit. However, the problem can be solved in another way. Indeed, in order to completely eliminate the influence of high-voltage pickups, it is enough to disconnect the soldering iron from the network for the time of soldering the output of the element. According to the technical requirements, the soldering time for the leads of semiconductor devices should be minimal (no more than 3 s), therefore, during the soldering of even several leads, the soldering iron tip simply does not have time to cool down enough to make high-quality soldering impossible. Naturally, when the soldering iron is placed on a stand, you must not forget to turn it on.
A simple device allows you to automate the process of turning the soldering iron on and off. Switching of the heating element is carried out using the contacts of the electromagnetic relay K1. The relay is controlled by touch contacts B1-B2. In the initial state, the inputs of the logic element DD1 have a high level, respectively, the output is low. Transistor VT1 is locked, the relay winding is de-energized, and the soldering iron is connected to the network. If you touch the electrodes B1 and B2 with your finger, then the resistance between them will decrease, and a low level potential will appear at the input of the inverter DD1, and a high one at the base of the transistor. The transistor opens, the relay turns on, and its contacts open the circuit of the soldering iron heating element. Capacitor C1 and resistor R2 form a filter that eliminates the influence of network interference.
As a sensor, you can use two pieces of tinned copper wire with a diameter of about 0,5 mm and a length of 5 ... 7 mm, which are mounted on the soldering iron handle in a convenient place, at a distance of about 3 mm from each other. The method of fastening is arbitrary and depends on the design of the handle. The device board is placed separately, in a plastic case, the sensor is connected to the board using a shielded wire. In this case, the braid of the wire should be connected to the common wire of the circuit. So that the wire does not interfere with soldering, it can be wrapped with a "snake" around the handle, securing it with electrical tape at its end.
As an electromagnetic relay, any relay can be used that reliably operates at a voltage of 9 V, for example, REC37. Instead of the K176LA7 chip, you can use any other CMOS chip, on the basis of which you can build an inverter.
Author: O.Borisenko, Stavropol
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