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When working with tiny sur-

face mount devices (SMDs), the

constructor is drawn closer to the

circuit in order to get a clear view

of the soldering operation. Any-

one who does soldering will have

noticed the unwritten law that

smoke always moves towards

your face, however much you try

to avoid it. Close working with

SMDs therefore involves a much

higher risk of solder fumes

being inhaled and potential

bronchial problems.

Although the smoke absorber

described here was designed for

use with SMD soldering, it can

also be used beneficially when

soldering “ordinary” components.

It is very compact and can be

placed close to the circuit board

during population. It will remove

the dense solder fumes from the

immediate area and a built-in

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HARMFUL CHEMICALS

The harmful nature of sol-

der fumes in production envi-

ronments has been investigated

in great depth by several orga-

nizations during recent years.

The main component of com-

mon solder flux is a material

known variously as rosin or

colophony. This is derived from

a few species of pine tree and is

related to amber, which is a fos-

sil material recovered from the

sea or mined.

Rosin is a benign material

that can usually be handled and

worked without problems. It is

found in a surprisingly wide

range of industrial and domestic

products. Common applications

include paper, cosmetics and

soaps, chewing gum, tooth-

paste, varnishes, adhesives

and, of course, on violin bows.

A number of people have

contact allergy to colophony, in

paper for example, but it is very

rare. However, our concern is

the risk of inhalation. Like hard-

wood and coal, rosin is com-

pletely safe in normal usage.

But in the form of a smoke or

very fine particles, these materi-

als can be harmful, especially if

they reach our lungs.

The chemical component

responsible for the adverse re-

action, and indeed the main

component of rosin, is abietic

acid. This is a member of the

terpine family of which turpen-

tine (not the substitute) will be

familiar to do it yourself (DIY)

enthusiasts.

Significantly reduce your inhalation of soldering fumes.

charcoal filter will provide a de-

gree of filtration and absorption.

A standard mains adapter

powers the Smoke Absorber

and after power is applied the

operation is automatic. The heat

from the soldering iron triggers

the fan and it switches off after

about half a minute unless re-

triggered. This is a suitable

time delay as the average sol-

dering action is usually within

three seconds.

Automatic control is very

convenient, as the soldering

process tends to be intermittent

and remembering to switch the

unit on and off is quite a distrac-

tion. It is also safer since it re-

duces the overall current drain

from the power pack and can

therefore remain in stand-by

mode for long periods.

The Smoke Absorber in use with the assembly of a surface

mount p.c.b

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There are many other unde-

sirable components in solder

smoke, including lead deriva-

tives and, although it is not

acutely toxic, it is best to reduce

its intake.

CLOSE-UP WORK

Surface mount work in-

creases the risk of inhalation of

harmful vapors for several rea-

sons. Close-in working means

that any smoke breathed in will

not have time to disperse and

will therefore be more concen-

trated. There is a tendency to

use higher levels of flux be-

cause good solderability is so

much more important in surface

mount work.

It is also possible to use sol-

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Fig.1 Complete circuit diagram for the Smoke Absorber.

der paste for hand working with

SMDs. Solder pastes and

creams give off copious quanti-

ties of smoke and fumes when

heated to the reflow tempera-

ture. Reflow takes place just be-

low 200 C when the solder ap-

pears out of the gray paste as a

bright metal. This clean liquid

metal flows into the joint, wet-

ting both the component contact

and the printed circuit land to

complete the soldering

process. Solder paste fumes

are mildly corrosive and should

be avoided.

If the amateur envisages a lot of

work with solder paste, more

elaborate air cleaning or extrac-

tion should be considered. Simi-

larly, if working in a very confined

space for long periods a more ef-

ficient filtering system than this

smoke absorber may be needed.

This unit will help disperse

the fumes reducing the concen-

tration in the immediate working

area and a certain portion will be

absorbed. Complete removal of

finely dispersed material from air

is a complex and expensive sci-

ence. We should not be too con-

cerned about intermittent expo-

sure to traces of rosin flux. It pre-

sents a very low risk and the faint

therapeutic aroma of pine from

the soldering process has pleas-

ant relaxing associations within

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Commercial electronic pro-

duction and servicing operations

where full working day exposure

is involved take great care to

remove fumes using costly ex-

traction and filtration equipment.

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Fig.2. Component layout on the Smoke Absorber surface mount p.c.b.

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the cozy environment of the

electronics workshop.

was the PIS201S, which consists

of two pyroelectric elements

wired in opposition.

At a steady level of heat radi-

ation, the output from this ar-

rangement is zero. The field of

view of the device is about 45 de-

grees and the elements are ar-

ranged so that the passage of the

heat source across this aperture

heats up one element before the

other. This unbalances the sys-

tem and a momentary voltage

output develops. A built-in field-

effect transistor (FET) source fol-

lower reduces the loading on the

sensor but an output of a few mil-

livolts is all we have to work with.

The PIS201S is not the

world's fastest device with a max-

imum response frequency of a

few Hertz. The practical circuit,

shown in Fig.1, is therefore re-

quired to make this low-level sig-

nal control the filter fan.

The output from the PIS201S

(X1) develops across resistor R1,

which is the load resistor for the

internal source follower. Opamp

IC1 is configured as a low fre-

quency amplifier with its gain

rolling off above 20Hz. The val-

ues of the coupling capacitors

C1, C3, C4, and the value of

the feedback capacitor C2 opti-

mizes the gain in the 0 5Hz to

5Hz range.

COMPONENTS

Resistors

R1 to R3 47k (3 off)

R4, R5, R8 1M2 (3 off)

R6 4k7

R7 68k

R9 390 ohms

R10, R11 1k8 (2 off)

Rx zero ohm link (see text)

All resistors SMD 1206 package

CIRCUIT DETAILS

The full circuit diagram for

the Smoke Absorber is shown in

Fig.1. A 12V 500mA regulated

mains adapter is needed to

power the circuit.

The circuit is required to

switch on the fan for a period of

about 30 seconds and then shut

it down unless it is re-triggered.

Since the soldering iron is to be

used in close proximity to the

smoke absorber, it is logical to

use the infrared (IR) radiation

from the hot iron to start the

timing cycle. The elements in

the circuit are therefore infrared

sensor, amplifier/filter, pulse

shaper, timer, and motor driver.

Detection of the heat from

the soldering requires a long

wave infrared detector. Semi-

conductor IR diodes and transis-

tors are very unresponsive to

radiation above about one mi-

cron wavelength. The useful

heat radiation from the iron cov-

ers the wavelengths longer than

about five microns and, there-

fore, a different type of detector

is required.

Fortunately, the passive in-

frared sensors used in security

systems work in the range of

five microns to about 20 mi-

crons and are ideal for the pur-

pose of soldering iron heat de-

tection. The device selected

Capacitors

C1 1uF tantalum, 16V, SMD

C2 82n ceramic, SMD,

1206 package

C3 to C6 22u tantalum, 22V

SMD (4 off)

Semiconductors

D1 BAS32 signal diode, SMD

SOD80 package

D2 Red LED 5mm

D3 Green LED 5mm

D4 BZX84 Zener diode, 6.2V,

SMD SOT23 package

TR1 BCF32

npn

transistor,

SMD SOT23 package

TR2 ZVN2106 MOSFET,

SMD SOT223 package

IC1 LF351 opamp, SMD SO8

package

IC2 NE555 timer, SMD SO8

package

X1 PIS201S IR detector

Miscellaneous

Printed circuit board available

from the

EPE Online store

, code

7000223 (www.epemag.com); fan,

60mm 12V DC brushless; power

socket, 2.5mm chassis mounting,

24mm fixing centers; LED mounting

clip, 5mm (2 off);plastic case type

PB1, 114mm x 76mm x 38mm;

speaker mesh, Sentinel, galvanized,

22g, 6mm x 6mm holes, cut to

10 holes x 10 holes; filter, charcoal

filled; finger guard, 60mm square,

chromed steel (2 off); M3 nuts and

bolts, 2cm long (rear finger guard

and fan mounting) (4 off); M4 nuts

and bolts, 6mm long (front finger

guard) (4 off); mains power adapter,

12V 500mA, regulated; connecting

wire; solder, etc.

Transistor TR1 provides the

final voltage amplification to a

level suitable to trigger the type

555 timer, IC2. Whilst the trigger

input to IC2 pin 2 is held high by

See also the

SHOP TALK Page!

Approx. Cost

Guidance Only

$65

Enlarged view of the assembled p.c.b

Maxfield & Montrose Interactive Inc

resistor R7, the timer output on

pin 3 remains low. To start the

timer, a negative-going signal is

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fan assembly

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Internal layout of the Smoke Absorber

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avoid feedback instability in this

high gain circuit. A 6V supply is

adequate for the low power cir-

cuits and, with a stabilized

mains power pack supplying

12V, there is sufficient head-

room to use a 6V2 Zener diode

(D4) as an effective stabilizer.

The fan specified is a

brushless DC type. These de-

vices are very quiet and electri-

cally clean in operation.

larger devices, in this case the

tantalum capacitors, plus IC1

and IC2.

There are no tight spots in

the layout and no particularly

sensitive devices. All the resis-

tors can be soldered in place

followed by the capacitors and

finally the active devices.

A normal 12W or 15W sol-

dering iron can be used for

SMD placement. Select the

finest tip available. Standard

non-corrosive rosin-cored solder

will suffice for component place-

ment, but finer 26 s.w.g. silver-

loaded solder wire is preferred

by the author. A pair of non-

magnetized tweezers is essen-

tial for initial placement of

the chips.

To solder a chip resistor in

place, the most basic approach

is to “fix one end first”. Place

the selected chip on its PCB

lands and hold it in position by

pressing it gently with the tip of

the tweezers. Now touch the

end of the solder wire with the

iron tip to collect a small drop of

solder. Apply this to one end of

the component to solder it in

place. Speed is important here

as the protective flux is rapidly

evaporating from the molten

solder drop. (Purists who ob-

serve the dictum “never carry

solder to the joint” can

carefully

remake the solder joint later if

they wish!)

As soon as the flux evapo-

rates, oxidation of the solder

surface will begin. Some

residue of flux is also required

for proper tinning of the compo-

nent terminal and the copper

pad. Pre-coating of the

copper with flux is useful for

this reason.

Having fixed one end, the

second end can now be sol-

dered in the normal two-handed

manner where the solder and

CONSTRUCTION

This is a surface mount pro-

ject and it is based on readily

available SMDs. The printed

circuit board (PCB) component

layout and track pattern is

shown in Fig.2 and Fig.3, re-

spectively. This board is avail-

able from the

EPE Online Store

(code 7000223) at

www.epemag.com

If you choose to make your

own board, by any of the tech-

niques as used for “ordinary”

through-hole circuit boards, note

that a clean surface is very im-

portant for SM work. If the PCB

is less than bright and shiny,

then a few moments spent

cleaning it with a little mild

household abrasive will greatly

improve solderability.

After polishing, rinse off the

copper surface with water or

PCB cleaning solvent. A light

coat of rework flux from an

aerosol will protect the clean

surface. When this has dried

the board is ready for assembly.

Although the type 0805 and

smaller size chip resistors are

popular in commercial produc-

tion, the slightly larger 1206

sized devices are best for hand

working of SM circuits, at least

until some experience is gained.

The overall size of the PCB is

determined by the size of the

required to take pin 2 below

one-third of the supply voltage.

Diode D1 fully discharges

C4 on positive-going signals so

that a well-defined trigger

pulse approaching 0V reaches

the timer.

The timer will remain on for

as long as the input on pin 2 is

low. The time constant of the

C4/R7 combination is therefore

kept short to give a clear timing

period. With the component val-

ues suggested for R8 and C5,

the timer remains on for about

30 seconds.

The output of the timer, on

pin 3, is high during the timing

period and it can be connected

directly to the motor driver TR2.

This is MOSFET device, type

ZNV2106, in a surface mount

SOT223 package. TR2 has a

very low channel on-resistance

and can easily handle the

120mA required by the fan with

minimal heating.

Switching the motor on and

off results in a considerable rip-

ple in the supply voltage. This

must be removed in order to

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