The Weller WTCP-series of soldering stations is (at least, in Europe) one of the types of tools that have survived the ages. I know stations of the WTCP-series that are (or at least: look) older than me. A few years ago, I got sick of using el-cheapo soldering irons, so since then I own a WTCP-50 myself.

The Weller WTCP-series work by using a principle Weller calls 'Magnastat': the point you solder with is magnetic, but loses its magnetic properties when it's heated to a certain temperature. Weller uses that magnetism to turn on a tiny switch in the soldering iron: when the point is cold, the magnetism pulls on the switch and it colses, turning on the heater. As soon as the point is hot enough, the magnetism gets too weak to hold the switch and the switch opens, opening the circuit and turning off the heating element. You can hear this happening: the soldering iron 'clicks' now and then.

A disadvantage of this mechanism is that aside from the 'clicks', you have no idea what the soldering iron is doing and if it's already hot enough. Secondly, once in a blue moon the mechanism gets 'stuck' in off-mode, which requires a firm tap with the soldering iron on something hard to get it to work again. In both cases, it'd be nice to have an indication if the Magnastat-mechanism has decided to cut the power to the heating element. So, I built a circuit to do just that.

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