PCB Mount Push Button Identification Help

For reliability reasons I wouldn't go for an open-gate design and rely on the 50Hz noise. It probably might work, but your idea of using interleaved fingers should work quite well.

The resistance of dry skin is somewhere between 1k and 100k, so you could think of an NMOS transistor (find one with ESD protection) and a large pull down of e.g. 1M ohm. Then you can use the finger as a pull-up resistor to turn on the mosfet.

^{simulate this circuit – Schematic created using CircuitLab}

You could also use a bipolar (or darlington) transistor, they are less vulnerable to ESD defects, but cannot supply a large current at the output if needed so you would need to buffer the output.

A capacitive sensor would be an alternative solution, but requires a more complicated circuit.

Draw the schematic, and use your multimeter or oscilloscope, to see what it does, as you vary Vin, and as the gun heats up.

Likely functions might be:

• low voltage cutoff
• chopper to keep average Vout fairly constant as battery discharges
• timer

As far as controlling it goes: I understand that they often use PTC heating elements for glue guns. The heating element itself is self-regulating, moreso if the supply voltage is constant.

This also means that you can sense its temperature by measuring its resistance - you do not need to add an external sensor. You do however need to work out its R-T curve.

If you measure V/I of the glue gun in-circuit, you have R. An ADC will do division for you, so you can arrange the signals to get the ADC N to be I/V or V/I, without any further computation.

^{simulate this circuit – Schematic created using CircuitLab}

(n.b. In this circuit Fet1 must have low R as it forms part of measured R1 i.e it is an error term)

One thing to note about PTC heaters, is that they are good to temperature control to the design temperature, because they will have the correct R and I. So electronic control can easily make them more accurate.

However you can't make them much hotter - the resistance just goes up, and current stops flowing.

You can have trouble making them much colder, because the R goes down, and the current goes up - you end up with low duty cycle big current pulses. Also at some temperatures (e.g. 50C) the R-T curve can be NTC or flat. Control is impossible in this area.

This is probably irrelevant to a heat gun, which is only useful in the working zone anyway.

But, when sizing your FET you need to watch what the max current during warmup is. The lowest R may be at (say) 50C not at cold or hot, so max I over whole temp cycle needs to be measured.