When building or experimenting with various things, a soldering iron is considered an essential tool. However, the actual time actually spent using a soldering iron is surprisingly small, and you are probably so distracted by other things that you often forget that the electricity is on. Also, when you go out after work, it is not uncommon to wonder whether you switched the soldering iron OFF or not.

That's why I decided to make a soldering iron timer. In my experience, the time it takes to use a soldering iron continuously is considered to be less than one hour, so I considered a timer that can be turned off in one hour.

Completed the timer for soldering iron

Structure of the timer is simple, as shown below.

The clock oscillated by the oscillator drives the counter through a gate. When the counter reaches exactly one hour, the gate is switched OFF and the counter is stopped. The counter output controls a relay through a driver to turn the soldering iron ON and OFF. To restart, push the start button to reset the counter and start a new hour.

The circuit diagram is shown below.

The counter would be unnecessary if an oscillator with a direct one-hour cycle could be made, but such an oscillator is difficult to make, so I considered combining it with a counter.

One hour is 3600 seconds. If the frequency of the oscillator that is easy to make is in the 2 kHz range, and its cycle is in the 400 μs range. The number of the counter’s pulses can be calculated from the above: 3600 seconds ÷ 400 μs = 8.4 x 10⁶. Converting this number to binary, I get approximately 2²³. Since the driver is turned ON at the leading edge of the last flip-flop in the counter, the counter should be 1/224. In practice, dividing an oscillator with a cycle of 429 μs (=2.33 kHz) by 1/2²³ gives exact 1 hour.

A logic IC is used for the oscillator. The oscillation frequency is determined by f = 1/2.2CR. If C is 10 nF, R is 19.5 kΩ, but if R2 is 22 kΩ and R5 is 180 kΩ and they are connected in parallel, the value is close to 19.5 kΩ. The combined value of 22 kΩ and 180 kΩ is 19.6 kΩ in the calculation. The timer may be slightly longer than one hour.

Since it is better to use a small number of ICs to make the counter, I decided to use two 74HC4040A ICs with a large number of counting digits. This counter has a maximum of 212, so two ICs can count up to 224. I used a one-gate logic TC7S32F for the gate that stops the input, and holds it when it counts to the desired number of counts.

The relay driver does not need a large current capacity since it only controls the relay. I used a BS170, which is a small N-channel power MOSFET that I had on hand. The relay is an old 12 V type, so I am not sure of its identity. The soldering iron we usually use is less than 100 W, so anything with a contact capacity of 1 A can be used.

The pilot indicator while the soldering iron was in operation used the collector voltage of the relay driver. A logic IC, driver FET, and 3-terminal regulator, and so on, are placed on a universal PCB. The one-gate logic IC, TC7S32F, is mounted on the back side of the board. The circuit is simple and should work without problems once assembled and wired.

Completed board with components mounted on a universal board

The PCB was installed in a 120 mm x 90 mm x 50 mm aluminum case. I had an old transformer with 100 V primary and 13.5 V secondary, so I used that. The current capacity is unknown, but 20 mA is enough, so I think it will be OK. The transformer is the largest of all the components used in this project, so the size of the transformer will determine the size of the case.

Left: Front panel                  Right: Rear panel

The frequency was slightly lower due to a slightly higher resistor value in the oscillation circuit, resulting in an operating time of about 1 hour and 2 minutes. It is quite convenient when actually used. Since the timer has a duration of 1 hour, the soldering iron does not turn OFF so often in the middle of your work, which is inconvenient. I don't have to worry about the soldering iron wasting electricity like before.

<Quote>
This article is an English translation of a Japanese article that appeared in the January 2019 issue of the monthly FB NEWS.
https://www.fbnews.jp/201901/electronics/index.html