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  • Author Author: jw0752
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Solder Iron for Ivan with a built in Timeout

jw0752

     Ever since I helped Grandson Ivan, age 9, with his school Science Project, See Grandson, Ivan, and the Volcano Science Project , I have continued to feed his curiosity and enthusiasm for electronics. So far, in addition to the Volcano Project, we have built a line following robot kit, a Ninja detector on a bread board, and several other small projects that he could take to school for show and tell. One day we took an old cassette deck apart. Ivan was immediately hooked on salvaging and his supportive Mom has been finding old pieces of equipment for him to tear apart. I only get to see Ivan on the weekends and sometimes it is only every other week but we really make the best of our time together. He must have some really good nerd genes as he can stay with me alert and focused on learning electronics for 5 hours straight. I think my previous record with some of his older cousins is about 45 minutes before they nod off.


     As Ivan's interest did not seem to wane I decided it was time to get him more involved in soldering. Even though he shows a lot of maturity in his ability to focus and learn I did not feel comfortable giving him a regular soldering iron to take home as I was afraid he would leave it on and forget it. (Grandpas worry about such things) So I got him involved in helping me design a modification to an old solder iron that would allow us the following functions:


Turn ON with a push button

Turn OFF with a different push button

LED to indicate when the iron is powered

Switch to choose between high and low wattage

Automatic turn off after 30 minutes.


     I told Ivan that we would make his solder iron and then we would use it to build our own small bench power supply for his own shop. We began by coming up with some plans that we could bread board and try out.


     imageimage


Here is one of our test circuits This first circuit used one section of an LM339 for a comparator of an RC timer circuit The final design will use a single  LM311MLM311M comparator The small sound module on the right side of the board is part of the discharge path of the capacitor This gives us a small beep every time the circuit shuts down The beep reminds us to turn the iron back on if we are still using it at the time One of the challenges of this project was how we were going to find a power supply for the timer small enough to fit into the case of the solder iron holder we planned to use The answer came when we decided to salvage a small 12 volt switching supply board from a wall wart power supply


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I did not want to deprive Ivan of any part of the design process but I decided that there was plenty of time for him to learn the details and I did not want to get bogged down trying to solve the problems we all know constantly crop up during the build process. At least in these early stages of his learning I wanted things to flow smoothly, more like assembly of a well designed kit. Therefore in the days between Ivan's visits I busied myself planning and preparing the parts so that the assembly would go smoothly. A new control panel fascia was made for the solder iron and the holes were positioned and drilled for the switches and indicator LED. With Ivan's help the controls were installed and the unit began to take shape.


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In the week leading up to Ivan coming today I continued to experiment with the circuit. I had originally hoped that the unit would have zero current draw in the off position but in the end the only way I could assure that we had the space for needed components was to leave the wall wart, 12 volt power supply, on and waiting for use. This would mean that the unit would draw 2 mA in the off mode just as most wall warts do when plugged in but not in use. While this doesn't sit perfectly well with my old fashioned idea that off should mean really off I guess that because so much of the appliances around me are really never truly off, I have gotten more used to the idea. When Ivan arrived today we got right down to business and began to assemble the the timer board which is the heart of the unit. Here is the schematic and circuit board layout that we used.


   imageimage


Ivan looked at the bread board with all its components and asked if all those parts were really going to fit on the small Adafruit  board we were going to use. I explained that this is why it is important to make a good plan before starting. We began to prepare for soldering by teaching and reminding Ivan about some techniques for handling the iron and cleaning the tip. In the process we managed to give him his first "Solder Iron Burn". We ran cold water on it and put a band aid on it and all he seemed concerned with was getting back to work. Little by little we assembled the board, all the while discussing the components that were being used and what they do. Here is the completed board.


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The next step was to wire the assembled board, the power supply, and the switches together and fit them into the base of the solder iron.


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     With the unit finally assembled I explained to Ivan that we would probably be disappointed and we would likely have to take the unit apart again and find a mistake or two as this is my usual procedure.

He was eager to try it out so here he is pushing the START button for the first time.


     image image

 

The power light came on, I was pleased. The Stop button worked, as did the 16W / 35W switch. Now it was just a matter of turning it on and waiting approximately 35 minutes for it to turn itself off. Ivan and I busied ourselves with cleaning the bench and putting away the tools while we waited. As the time got close I became concerned as it was almost time for Ivan to go home and I didn't want him to leave with a failure hanging over us. Finally there was the small beep of the discharging capacitor and the power light went out.

 

Here is a short video of the soldering iron doing an auto shut down:

 

 

With the soldering iron project under our belts Ivan and my next project will be a small safe power  supply for his bench so we can get him started building small circuits on his own, at home, between his visits to me. Hopefully we will have some extra time together over the Christmas Holidays.

 

John

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  • That's a neat idea; I think I want one for myself now that I'm getting old and forgetful.

     

    I was curious about the 35 minutes for the timer. Too lazy to do the calculations, I gave it to SPICE  (Tina-TI) to do. Here's the graph for a 220u charging from a 12V supply through a 2.4M resistor. The time to the switching point defined by  R2 and R3 (10.46V) looks to be just over 18 minutes (1080 seconds). That's with a perfect resistor and a perfect capacitor.

     

    image

     

    Here's a link to TI's free spice tool:

    http://www.ti.com/tool/TINA-TI?keyMatch=tina&tisearch=Search-EN-Everything

     

    So why the difference between the calculated value and what you're seeing in practice? The first possibility is obviously the tolerance on the capacitor value. Electrolytics aren't very good in this respect: I've just looked up a few on the Farnell site and +- 20% seems to be common now, though that's much better than was once the case.

     

    But even if you were at the extreme of the tolerance range (very, very unlikely), that only gets us to 22 minutes. The resistor also has a tolerance on its value but that will be much better than the capacitor and I'm going to ignore that for the purposes of this comment. The rest will be other currents stealing from the charge current.

     

    The first to consider is the leakage of the capacitor. Once we get to 10.46V the resistor current has reduced to 0.64uA. [I calculated that using Ohms's law (12V - 10.46V) / 2400000 ohms = 0.64uA] A Panasonic electrolytic datasheet I looked at gave a leakage current of less than 3uA over the temperature range. In your case, for your particular capacitor and at room temperature, it must be much better than that or you would never have even reached the switching point.

     

    The other, perhaps less obvious, current [not really "leakage"] is the input bias current of the comparator. The (TI) datasheet gives a typical figure of 0.1uA without properly stating the conditions, but the datasheet is a bit of a mess and another of the graphs shows it varying depending on the differential input voltage [which is what you would expect] - so something in the area of 0-150nA. From the circuit given in the datasheet it looks like it flows out of the chip [base current of PNP transistor], so, curiously, it will be countering the capacitor leakage to an extent.

     

    Well, that's about as far as I can go with my musings. The point I've come to is that, whilst your circuit is absolutely fine as a one-off personal project (which is all you intended, so no criticism meant) if someone were to come to you and ask you to produce 10,000 to sell you might want to be a bit cautious. The circuit is operating in a marginal area where you would either need to do work characterising the parts yourself or take a chance (ie gamble, since money would be involved) that they were always as good as in the prototypes you had built. In practice you would probably decide to take a look at the CD4060 that D_Hersey suggests, which is a much more controlled way of timing over a period of more than a few minutes, or use the kind of small and very simple microcontroller that sells for a few 10s of cents.

  • in reply to jc2048

    Hi Jon,

    Just as I have come to expect from you, excellent analysis of the situation. I set up the prototype circuit and used empirical analysis to determine that the time in seconds for my circuit was C times R times 3 = T in seconds. The "3" being my reality constant. This is of course 3 time contants which would in theory put us closer to 11.4 volts if there were no reality to deal with. I also tried a number of values for C and R and found that the formula was fairly accurate. I picked out a good quality capacitor from my bin as I wanted to minimize leakage since I knew that this could be a factor in this marginal area of operation. This formula worked equally well with an LM 339 and a LM 311 comparator using the same resistors and capacitor. I am aware of the 4060 and even have worked with one a while back. It would have been a better choice for the project as you and have pointed out. For a more commercial application of the idea the power supply and timer could be put on the same board. I also could have designed the circuit to turn off the power supply completely when not in use with 3PDT relay and a DPST momentary pushbutton. It is enjoyable to make these "one-off personal" projects as they do not carry the stress and responsibility of doing something commercially. I have done the commercial things and they always give me high stress levels with all the side effects that come along with a large pool of users. Thanks again for taking the time to troubleshoot my design. Your input is always respected.

    John

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  • in reply to jc2048

    Hi Jon,

    Just as I have come to expect from you, excellent analysis of the situation. I set up the prototype circuit and used empirical analysis to determine that the time in seconds for my circuit was C times R times 3 = T in seconds. The "3" being my reality constant. This is of course 3 time contants which would in theory put us closer to 11.4 volts if there were no reality to deal with. I also tried a number of values for C and R and found that the formula was fairly accurate. I picked out a good quality capacitor from my bin as I wanted to minimize leakage since I knew that this could be a factor in this marginal area of operation. This formula worked equally well with an LM 339 and a LM 311 comparator using the same resistors and capacitor. I am aware of the 4060 and even have worked with one a while back. It would have been a better choice for the project as you and have pointed out. For a more commercial application of the idea the power supply and timer could be put on the same board. I also could have designed the circuit to turn off the power supply completely when not in use with 3PDT relay and a DPST momentary pushbutton. It is enjoyable to make these "one-off personal" projects as they do not carry the stress and responsibility of doing something commercially. I have done the commercial things and they always give me high stress levels with all the side effects that come along with a large pool of users. Thanks again for taking the time to troubleshoot my design. Your input is always respected.

    John

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