LED UV lightbox, Inkjet stencils, SMT, PIC 16F887 and BCD code

wakibaki

Banned
2008-01-08 11:51 pm
I’m posting this in the ‘construction tips’ section, I thought about posting it in the Wiki, but there are sections that I don’t want anybody to be able to edit, particularly the code. I see there are some similar threads, but there are some details here they don't include.

I used to have a UV lightbox, I made it about 15 years ago, I haven't been able to lay my hands on it for a couple of years now. Still, I had access to one at work, so I didn't worry too much

Recently I had to quit that job, and I've been ill, so sitting at home I've got a lot of ideas unfulfilled, partly for want of a lightbox..

One of these was to construct a new lightbox.

This time I decided to go with LEDs. The previous one used fluorescent tubes installed in some fittings taken from an old camper van, I used to run it off a 12V lead-acid battery. The whole thing was constructed out of scavenged bits; a box I picked up in the street that had slots for a sliding lid that neatly accommodated a sheet of glass that I cut to fit.

This time I bought 100 UV LEDs from a seller in Nanjing, China for $10 US shipped. I had some Veroboard which I cut into 3-conductor-wide strips, this gave me 10 strips, each of which accommodated 9 LEDs.

I decided to run the LEDs in strings of 3, without a ballast resistor. The forward voltage is quoted as 3.4~3.8 volts. When I ran up the 90 LEDs to 600mA (30 strings @ 20mA) I got ~11 volts on the lab power supply, so I figured 2 rectifier diodes in series with 12V regulated from a 7812 would be about right to drop the voltage to an appropriate level. It’s not recommended to run LEDs this way, usually there’s a ballast resistor in every string or a constant current supply, but I broke from these practices first when building torches, and it turns out LEDs are more robust used like this than theory and literature would suggest.

I built a 12V power supply with a 20VA RS toroidal transformer I had lying around. The other components for the bridge and the smoothing caps were just ‘lying around’ too. I used a 78S12 regulator in conjunction with the aforementioned dropper diodes in order that the UV array wouldn’t be overdriven.

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wakibaki

Banned
2008-01-08 11:51 pm
You can see the LED array here.

[IMGDEAD]http://wakibaki.com/UVLB/UV_LED_array.jpg[/IMGDEAD]

Here it is illuminated...

[IMGDEAD]http://wakibaki.com/UVLB/UV_LEDs_lit.jpg[/IMGDEAD]

And here it is with picture frame arranged above it with a piece of printer paper to show the illumination of the target.

[IMGDEAD]http://wakibaki.com/UVLB/UV_LEDs_illum_pattern.jpg[/IMGDEAD]

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wakibaki

Banned
2008-01-08 11:51 pm
Arranging the LEDs was a problem. First they had to be seated firmly against the Veroboard to try and make the beams parallel. Then the Veroboard had to be held flat. I've got a piece of 3 mm fibreboard with 2 x 1/4 x 1/2in balsa strips adhered to it with doublesided sticky pads. Then the Veroboard is held to the balsa strips with thumbtacks, or drawing pins if that name is more familiar to you.

This has not resulted in as even illumination as I had hoped. The beams of the LEDs are quite narrow. This has the advantage that the overall beam does not spread too much, but I have had to keep the target sheet 12 inches away from the LEDs to ensure that the spots spread into one another. I’m looking for an LCD monitor to dismantle, they have some high-performance diffusion screens in them, which might fix things. I think next time I might revert to fluorescent tubes. The illumination period is 10 minutes, where the box I had previously only required about 4 minutes. It was 12 watts, this is only ~6. This was cheap, however, given that I had the Veroboard, it works, and it’s low voltage.

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wakibaki

Banned
2008-01-08 11:51 pm
Inkjet transparency stencils

When at work I used to make stencils on a laser printer. I used transparencies up to A4, and architectural drafting film up to A2. I made some pretty big PCBs, butting 3 sheets together to get up to 32 inches. Once I figured how, I made the big boards by isolation routing.

A big board:-

[IMGDEAD]http://wakibaki.com/UVLB/milled_32in.jpg[/IMGDEAD]

I was concerned that stencils made on an inkjet printer might not work. Transparency material for inkjet has a coating on one side to absorb the ink. This has tiny dots (imperfections) irregularly spaced presumably as a result of the coating drying. I thought these might show up on the PCB. I also didn't know if the contrast ratio between the inked areas and the clear would be sufficient to produce a good image in the photoresist. As it turns out, this is not a problem. Although the contrast could be better, it is still possible to produce adequate results down to 10 mil traces. This can be seen on the doublesided board shown later.

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wakibaki

Banned
2008-01-08 11:51 pm
Processing

I process the PCBs on a one-shot basis, as was common with film processing in the latter days of film photography.

I use Sodium Hydroxide for preference rather than proprietary resist developer, it's cheap. I do the processing in flatbottomed plastic tubs which came free from the Chinese takeaway. I have some bigger dishes too from the old photography days.

I mix a quarter boiling water with threequarters cold tap water to half-a-pint (a mugful) and add a shallow level teaspoon of granulated hydroxide and stir immediately. This is insufficiently strong to burn my fingers so I can handle the wet board in comparative safety once the exposed resist has dissolved. A little physical manipulation is sometimes necessary to remove some stubborn resist where perhaps the illumination has been poor. Sodium hydroxide is both caustic and poisonous so take care when handling it and dispose of it carefully immediately after use.

Experience has taught me how few crystalline balls of ferric chloride will adequately process the size of board I am working on. I tip them in, followed by a small quantity of recently boiled water plus the PCB. The solution does not stay hot enough to finish the board in one go, so I zap the solution with 20 seconds in the microwave once I can feel that it has cooled down. Used ferric chloride contains dissolved copper and is highly inimical to marine life and should not be disposed of into sewage systems. Take local advice on how to dispose of it.

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wakibaki

Banned
2008-01-08 11:51 pm
A finished board

I made the board for the PSU using the new LED array driven from a lab PSU and did the timing with a stopwatch. It’s a conventional thru-hole job, single-sided.

[IMGDEAD]http://wakibaki.com/UVLB/PSU_solder_side[/IMGDEAD]

[IMGDEAD]http://wakibaki.com/UVLB/PSU_component_side[/IMGDEAD]

I may have to move the regulator off the board to get it on a heatsink.

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wakibaki

Banned
2008-01-08 11:51 pm
Double-sided, surface mount

I like surface mount components. The more surface mount you have, the less holes you have to drill. There are no problems with requiring thru-hole plating, although obviously, in the case of a double-sided board, vias are necessary. I generally use a larger head on the vias than I would for a commercially produced board. This is not ideal, but the real-estate taken up is generally not critical in DIY projects.

[IMGDEAD]http://wakibaki.com/UVLB/counter_layout.jpg[/IMGDEAD]

Double-sided boards obviously require registration between the sides. This can be accomplished by peeling the protective plastic covering from one side and sticking one stencil to the unexposed board and drilling a few holes. You will have to remove some swarf from between the stencil and board before exposure. You can use reference marks included for the purpose, or simply drill at the board corners, or through a few vias. Then the protective film is peeled from the other side and the second stencil carefully matched to the holes. An alternative method matches the stencils against each other, taping them to prevent relative movement, and inserting a piece of scrap board with an edge coincident with one of the board edges on the stencil and then taping this in place. The unexposed board can then be inserted between the stencil sheets and butted up against the scrap piece and some tape added to obviate movement between exposures.

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wakibaki

Banned
2008-01-08 11:51 pm
Vias

[IMGDEAD]http://wakibaki.com/UVLB/counter_bare_top.jpg[/IMGDEAD]

[IMGDEAD]http://wakibaki.com/UVLB/counter_bare_bot.jpg[/IMGDEAD]

Vias are all placed at the same time. Take about a metre of plated copper wire and grasp the ends in long-nose pliers, taking a couple of turns round each of the plier jaws. Hold the pliers firmly and pull them apart until the wire can be felt to give by a centimetre or so. This will put a 'set' on it, i.e. leave it straight and slightly work hardened.

Now cut sufficient 'needles' from the straightened wire to complete the board. Cut the needles with side-cutters held at a 45 degree or better angle so that a sharp pointed end is produced.

Place the board on a piece of sponge such as frequently found in kitchens or bathrooms. Place the needles to fill all the via holes, pushing them down securely into the sponge for about half their length. Once they are all placed, you can go round and solder them all. Detach the board from the sponge and trim the excess wire from the top surface. Now you can invert the board and solder the other side. Even if the board is subsequently reflowed in a toaster oven, the vias are unlikey to fall out due to surface tension, particularly if the wire is not a sloppy fit in the holes.

[IMGDEAD]http://wakibaki.com/UVLB/counter_vias.jpg[/IMGDEAD]

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wakibaki

Banned
2008-01-08 11:51 pm
[IMGDEAD]http://wakibaki.com/UVLB/counter_proc.jpg[/IMGDEAD]

The board illustrated is a timer for the lightbox. It employs a PIC 16F887 which was selected, amongst other things, for its internal oscillator.

[IMGDEAD]http://wakibaki.com/UVLB/counter_side.jpg[/IMGDEAD]

[IMGDEAD]http://wakibaki.com/UVLB/counter_illum.jpg[/IMGDEAD]

The board provides a simple timer function with a relay to control the UV light. The circuit is shown here.

[IMGDEAD]http://wakibaki.com/UVLB/counter_circuit.jpg[/IMGDEAD]

When switched on the timer displays 000. There are 3 control button inputs, 2 for setting the period.. The first causes the display to count upwards. The count proceeds at second intervals when the button is held down until the count reaches 5, then the delay period is reduced and the count increases at a faster rate, this means that setting relatively long delays can be accomplished in a relatively short time.

The second button functions in a similar fashion to the first, but the count is down.

The third button turns on the UV light and starts the counter counting down at 1 second intervals. When the count reaches zero the UV is turned off and the display reverts to the number originally set. Subsequent exposures can be initiated by pressing the start button again.

A number of the components used are mounted unconventionally. The voltage regulator, electrolytic caps and diodes and relay are all conventional thru-hole types applied to one side of the board and ‘glued’ there with solder. In some cases I have used a custom foil pattern, and bent the component legs to mate. The 7-segment displays are common-anode types, each segment is ballasted with a 470R resistor. They are mounted in a 30-pin DIL socket which is surface-mounted to the PCB, this provides some stand-off and means that the components can be replaced or re-used if so desired. Since 30-pin sockets are not generally available, one can be made up from 2 smaller or cut down from a larger one (just cut off 2 pins leaving the plastic intact). Past experience with the MPU and displays (the LED forward voltage is >3V) have shown that the chip will easily support the dissipations encountered with 470R ballasts.

Where possible I like to use solder paste and reflow in a toaster over. A good source for solder paste is DealExtreme: Cool Gadgets at the Right Price - Site-Wide Free Shipping - DX, although there is a considerable delay in postage. Obviously solder paste can be obtained from sources closer at hand, but many suppliers like to ship fairly large quantities in an insulated package, to keep the temperature of the contents low. This can result in high prices and the quantities involved are greater than an amateur could reasonably use before it deteriorated. Solder paste is commonly used in commercial applications. If the paste fails to reflow suitably thousands of dollars worth of components can be affected in a production run. This can result in litigation in the worst cases. While this is understandable, the insistence on shipping large quantities in controlled circumstances is inconvenient to say the least, and the availability of small quantities from sites such as dealextreme is a way round this.

The MCU itself is a TQFP44. Soldering these by hand only requires the correct technique, which bears repeating here. Carefully apply solder to a corner pad of the PCB foil pattern. Slide the IC into place and secure the one pin. Adjust the positioning until you are completely happy with it. Now solder a single pin at the opposite corner. Starting with one of the unsecured edges, solder thickly and indiscriminately all the pins along that edge, pulling the solder bead from one end to another, making sure all pins on that edge are wetted. Now solder the remaining pins in the same manner. Remove the excess solder with a solder-sucker, working quickly to use as little heat as possible. If a string or sphere between 2 pins is difficult to remove, try re-applying some solder in that area and try again with the sucker.

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wakibaki

Banned
2008-01-08 11:51 pm
Software

A couple of years ago I looked around for a BCD routine written under Mpasm, the free PIC assembler. I couldn’t find anything. I wanted to display the contents of the 10-bit PIC A/D register on 7-segment displays, effectively to build a digital meter. The routine here does this and can be used to build a meter with some analog circuitry added to process e.g. AC current to voltage or WHY.

The board is programmed with a Pickit2 USB programmer via an ISP header which is simply a 6-pin 0.1 pitch SMT pin-header, connected to the appropriate pins on the PIC. The Pickit2 is available for a comparatively low price from regular suppliers, and even cheaper as a clone from ebay. It is very simple to knock up a board with a socket and header which will permit the programming of a wide variety of conventional thru-hole PICs using the Pickit2. Although it is possible to build programmers running off the PC’s serial or parallel ports, the cost and effort mean that this is hardly worthwhile, and the Pickit2 is extremely versatile. There is now a Pickit3 available with (I believe) better debug facilities, but I have always found the PK2 to be adequate. Anyone who likes to be able to add a few digital features to a design should have one.

Since the display runs 000-999, a ten-bit buffer is required to contain the count. Arranging for this to overflow correctly is comparatively straightforward. The 3 input switches select amongst up- and down-counting routines while the program continuously cycles in one big loop. The loop processes the count in the buffer into 3 secondary Binary Corrected Decimal (hundreds, tens and units) buffers which are in turn processed into 7-segment display buffers. The process is comparatively straightforward, if tedious.

Each bit in the count buffer is examined and, if set, a corresponding number is added to the BCD registers, i.e. if BUF_HI, bit 1 is set, 5 is added to the hundreds count, 1 to the tens count and 2 to the units count, and so on, down to BUF_LO, bit 0, which is worth 1. When all the bits have been examined the BCD units buffer is reprocessed to move any accumulated tens to the BCD tens buffer by repeated subtraction of tens from the units buffer until a borrow occurs, when the units count is restored by adding ten. Similarly the BCD tens buffer is stripped of any accumulated hundreds. The size of the numbers involved is small enough that there is no risk of unintended overflows.

A simple delay routine at the end of the loop pads the time taken to execute to ~1 second. A second, shorter delay is switched in during the up and down setting routines after the buttons have been held down for a count of 5. This means that setting up a high count e.g. 600 does not become too tedious. The RC clock in the MPU is not guaranteed to keep accurate time, but the seconds delay can be trimmed to fair accuracy by timing the count over a minute or so and adding or subtracting a few cycles from the delay routine. On my board the accuracy is within 1 second per minute, which is plain lucky with such an uncomplicated delay routine.

The seconds delay means that, depending on where the routine is in the loop, there may be a brief wait for the system to respond to a button press, but I haven’t bothered to complicate the routine by eliminating that.

The relay can easily be used with a minor modification to the circuit to switch mains voltage if mains-driven fluorescents are used. In that case only a 5V supply would be required.

The accompanying circuit diagram shows the proper correspondence between the PIC port bits and the common-anode display pins. The displays used are 0.5in character height which are readily available at a reasonable price on ebay.

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wakibaki

Banned
2008-01-08 11:51 pm
The code

; To display 0-999 on 3 * 7-seg displays

#include <p16F887.inc>
__CONFIG _CONFIG1, _LVP_OFF & _FCMEN_OFF & _IESO_OFF & _BOR_OFF & _CPD_OFF & _CP_OFF & _MCLRE_OFF & _PWRTE_ON & _WDT_OFF & _INTRC_OSC_NOCLKOUT
__CONFIG _CONFIG2, _WRT_OFF & _BOR21V
cblock 0x20
BUF_LO
BUF_HI
BUF2HI
BUF2LO
BUF3HI
BUF3LO
THOUS
HUNS
TENS
UNITS
TH_DISP
HU_DISP
TE_DISP
UN_DISP
CTR
CTR2
CTR3
FLAGS
S_FLAG
C_FLAG
F_FLAG
endc

ZERO equ B'10001000'
ONE equ B'11101011'
TWO equ B'01001100'
THREE equ B'01001001'
FOUR equ B'00101011'
FIVE equ B'00011001'
SIX equ B'00011000'
SEVEN equ B'11001011'
EIGHT equ B'00001000'
NINE equ B'00001011'
DECIMAL equ B'00000011'

org 0
start:
banksel TRISE
clrf TRISE ; Make PortE all output
bcf TRISE,0 ; Set RE0 to output
bsf TRISE,1 ; Set RE1 to input
bsf TRISE,2 ; Set RE2 to input

bcf STATUS,RP0 ; select Register Page 0
bcf STATUS,RP1 ; select Register Page 0
clrf PORTA
bsf STATUS,RP0 ; select Register Page 1
clrf TRISA ; Make PortA all output
clrf TRISB ; Make PortB all output
clrf TRISC ; Make PortC all output
clrf TRISD ; Make PortD all output
movlw 0x80 ; right justify, Vdd and Vss as Vref
movwf ADCON1
bsf TRISA,0 ; Set RA0 to input
bsf STATUS,RP1 ; select Register Page 3
clrf ANSEL
bcf STATUS,RP1 ; select Register Page 1
bcf STATUS,RP0 ; select Register Page 0
movlw 0xFF
movwf PORTB ; turn off all outputs
movwf PORTC
movwf PORTD
clrf BUF_LO
clrf BUF_HI
clrf BUF2LO
clrf BUF2HI
clrf BUF3LO
clrf BUF3HI
clrf FLAGS
clrf S_FLAG

mainloop:
btfsc C_FLAG,0 ; test the count flag
goto main2 ; execute count
btfsc PORTE,2 ; else test down bit
goto sw2
btfsc PORTE,1 ; test up bit
goto sw1
btfsc PORTA,0 ; test start bit
goto sw0
clrf FLAGS ; if no button pressed
clrf S_FLAG ; clear the short delay flag and counter
goto main2_done
sw2:
btfsc FLAGS,0 ; if short count flag set
goto main1 ; else count down
incf S_FLAG ; else count to 5
btfss S_FLAG,2 ; check for 4
goto main1 ; else count down
btfss S_FLAG,0 ; 4+1=5, so skip to set short count flag
goto main1 ; else count down
bsf FLAGS,0 ; set short count flag
goto main1 ; and count down
sw1:
btfsc FLAGS,0 ; same as down count, but up
goto main0
incf S_FLAG
btfss S_FLAG,2
goto main0
btfss S_FLAG,0
goto main0
bsf FLAGS,0
goto main0 ; and count up
sw0:
bsf C_FLAG,0 ; continuous count
bsf PORTE,0 ; turn on relay
movfw BUF2HI ; store count for future reference
movwf BUF3HI
movfw BUF2LO
movwf BUF3LO
goto main2 ; start count
main0:

movlw 0x01 ; upcount under button control
addwf BUF2LO,1 ; increment buffer memoryy
btfsc STATUS,Z
incf BUF2HI,1
btfss BUF2HI,1 ; check for 1000
goto main2_done
btfss BUF2HI,0
goto main2_done
btfss BUF2LO,7
goto main2_done
btfss BUF2LO,6
goto main2_done
btfss BUF2LO,5
goto main2_done
btfss BUF2LO,3 ; not 1000, continue
goto main2_done
clrf BUF2LO ; else set buffer to 0
clrf BUF2HI
goto main2_done
main1:
movlw 0x01 ; downcount under button control without switching on relay
subwf BUF2LO,1 ; decrement buffer memory
btfsc STATUS,C ; carry bit is UNSET on borrow when decrementing
goto main2_done
subwf BUF2HI,1
btfsc STATUS,C ; check for 000
goto main2_done
movlw 0x03
movwf BUF2HI
movlw 0xE7
movwf BUF2LO
goto main2_done
main2:
movlw 0x01 ; downcount continuously with relay turned on
subwf BUF2LO,1 ; decrement buffer memory
btfsc STATUS,C ; carry bit is UNSET on borrow when decrementing
goto main2_done
subwf BUF2HI,1
btfsc STATUS,C ; check for 000
goto main2_done
movfw BUF3HI
movwf BUF2HI
movfw BUF3LO
movwf BUF2LO
clrf C_FLAG ; discontinue count
bcf PORTE,0 ; turn off relay
main2_done:
movfw BUF2HI ; transfer counting buffer to main register
movwf BUF_HI
movfw BUF2LO
movwf BUF_LO

clrf TH_DISP ; clear display registers
clrf HU_DISP
clrf TE_DISP
clrf UN_DISP
clrf THOUS ; clear BCD registers
clrf HUNS
clrf TENS
clrf UNITS
btfss BUF_HI,1 ; test buffer bits and distribute values to BCD registers
goto bitnine
movlw 5
addwf HUNS,f
movlw 1
addwf TENS,f
movlw 2
addwf UNITS,f
bitnine:
btfss BUF_HI,0
goto biteight
movlw 2
addwf HUNS,f
movlw 5
addwf TENS,f
movlw 6
addwf UNITS,f
biteight:
btfss BUF_LO,7
goto bitseven
movlw 1
addwf HUNS,f
movlw 2
addwf TENS,f
movlw 8
addwf UNITS,f
bitseven:
btfss BUF_LO,6
goto bitsix
movlw 6
addwf TENS,f
movlw 4
addwf UNITS,f
bitsix:
btfss BUF_LO,5
goto bitfive
movlw 3
addwf TENS,f
movlw 2
addwf UNITS,f
bitfive:
btfss BUF_LO,4
goto bitfour
movlw 1
addwf TENS,f
movlw 6
addwf UNITS,f
bitfour:
btfss BUF_LO,3
goto bitthree
movlw 8
addwf UNITS,f
bitthree:
btfss BUF_LO,2
goto bittwo
movlw 4
addwf UNITS,f
bittwo:
btfss BUF_LO,1
goto bitone
movlw 2
addwf UNITS,f
bitone:
btfss BUF_LO,0
goto bitzero
movlw 1
addwf UNITS,f
bitzero:
movlw D'10' ; move any tens to tens register
subwf UNITS,f
btfsc STATUS,Z
goto units_zero
btfss STATUS,C
goto units_carry
incf TENS,f
goto bitzero
units_carry:
movlw D'10'
addwf UNITS,f
goto units_done
units_zero:
incf TENS,f
units_done:
movlw D'10' ; move any hundredss to hundreds register
subwf TENS,f
btfsc STATUS,Z
goto tens_zero
btfss STATUS,C
goto tens_carry
incf HUNS,f
goto units_done
tens_carry:
movlw D'10'
addwf TENS,f
goto tens_done
tens_zero:
incf HUNS,f
tens_done:
movlw D'10'
subwf HUNS,f
btfsc STATUS,Z
goto huns_zero
btfss STATUS,C
goto huns_carry
incf THOUS,f
goto tens_done
huns_carry:
movlw D'10' ; move any thous to thous register
addwf HUNS,f
goto huns_done
huns_zero:
incf THOUS,f
huns_done:

movlw 1 ; test value in BCD units and set display registers
subwf UNITS,f
btfsc STATUS,C
goto units_1
movlw ZERO ; move '0' to UN_DISP
movwf UN_DISP
goto tens_0
units_1:
movlw 1
subwf UNITS,f
btfsc STATUS,C
goto units_2
movlw ONE ; move '1' to UN_DISP
movwf UN_DISP
goto tens_0
units_2:
movlw 1
subwf UNITS,f
btfsc STATUS,C
goto units_3
movlw TWO ; move '2' to UN_DISP
movwf UN_DISP
goto tens_0
units_3:
movlw 1
subwf UNITS,f
btfsc STATUS,C
goto units_4
movlw THREE ; move '3' to UN_DISP
movwf UN_DISP
goto tens_0
units_4:
movlw 1
subwf UNITS,f
btfsc STATUS,C
goto units_5
movlw FOUR ; move '4' to UN_DISP
movwf UN_DISP
goto tens_0
units_5:
movlw 1
subwf UNITS,f
btfsc STATUS,C
goto units_6
movlw FIVE ; move '5' to UN_DISP
movwf UN_DISP
goto tens_0
units_6:
movlw 1
subwf UNITS,f
btfsc STATUS,C
goto units_7
movlw SIX ; move '6' to UN_DISP
movwf UN_DISP
goto tens_0
units_7:
movlw 1
subwf UNITS,f
btfsc STATUS,C
goto units_8
movlw SEVEN ; move '7' to UN_DISP
movwf UN_DISP
goto tens_0
units_8:
movlw 1
subwf UNITS,f
btfsc STATUS,C
goto units_9
movlw EIGHT ; move '8' to UN_DISP
movwf UN_DISP
goto tens_0
units_9:
movlw 1
subwf UNITS,f
btfsc STATUS,C
goto tens_0
movlw NINE ; move '9' to UN_DISP
movwf UN_DISP
tens_0:
movlw 1
subwf TENS,f
btfsc STATUS,C
goto tens_1
movlw ZERO ; move '0' to TE_DISP
movwf TE_DISP
goto huns_0
tens_1:
movlw 1
subwf TENS,f
btfsc STATUS,C
goto tens_2
movlw ONE ; move '1' to TE_DISP
movwf TE_DISP
goto huns_0
tens_2:
movlw 1
subwf TENS,f
btfsc STATUS,C
goto tens_3
movlw TWO ; move '2' to TE_DISP
movwf TE_DISP
goto huns_0
tens_3:
movlw 1
subwf TENS,f
btfsc STATUS,C
goto tens_4
movlw THREE ; move '3' to TE_DISP
movwf TE_DISP
goto huns_0
tens_4:
movlw 1
subwf TENS,f
btfsc STATUS,C
goto tens_5
movlw FOUR ; move 4 to TE_DISP
movwf TE_DISP
goto huns_0
tens_5:
movlw 1
subwf TENS,f
btfsc STATUS,C
goto tens_6
movlw FIVE ; move '5' to TE_DISP
movwf TE_DISP
goto huns_0
tens_6:
movlw 1
subwf TENS,f
btfsc STATUS,C
goto tens_7
movlw SIX ; move 6 to TE_DISP
movwf TE_DISP
goto huns_0
tens_7:
movlw 1
subwf TENS,f
btfsc STATUS,C
goto tens_8
movlw SEVEN ; move '7' to TE_DISP
movwf TE_DISP
goto huns_0
tens_8:
movlw 1
subwf TENS,f
btfsc STATUS,C
goto tens_9
movlw EIGHT ; move 8 to TE_DISP
movwf TE_DISP
goto huns_0
tens_9:
movlw 1
subwf TENS,f
btfsc STATUS,C
goto huns_0
movlw NINE ; move '9' to TE_DISP
movwf TE_DISP
huns_0:
movlw 1
subwf HUNS,f
btfsc STATUS,C
goto huns_1
movlw ZERO ; move '0' to HU_DISP
movwf HU_DISP
goto thous_0
huns_1:
movlw 1
subwf HUNS,f
btfsc STATUS,C
goto huns_2
movlw ONE ; move '1' to HU_DISP
movwf HU_DISP
goto thous_0
huns_2:
movlw 1
subwf HUNS,f
btfsc STATUS,C
goto huns_3
movlw TWO ; move '2' to HU_DISP
movwf HU_DISP
goto thous_0
huns_3:
movlw 1
subwf HUNS,f
btfsc STATUS,C
goto huns_4
movlw THREE ; move '3' to HU_DISP
movwf HU_DISP
goto thous_0
huns_4:
movlw 1
subwf HUNS,f
btfsc STATUS,C
goto huns_5
movlw FOUR ; move '4' to HU_DISP
movwf HU_DISP
goto thous_0
huns_5:
movlw 1
subwf HUNS,f
btfsc STATUS,C
goto huns_6
movlw FIVE ; move '5' to HU_DISP
movwf HU_DISP
goto thous_0
huns_6:
movlw 1
subwf HUNS,f
btfsc STATUS,C
goto huns_7
movlw SIX ; move '6' to HU_DISP
movwf HU_DISP
goto thous_0
huns_7:
movlw 1
subwf HUNS,f
btfsc STATUS,C
goto huns_8
movlw SEVEN ; move '7' to HU_DISP
movwf HU_DISP
goto thous_0
huns_8:
movlw 1
subwf HUNS,f
btfsc STATUS,C
goto huns_9
movlw EIGHT ; move '8' to HU_DISP
movwf HU_DISP
goto thous_0
huns_9:
movlw 1
subwf HUNS,f
btfsc STATUS,C
goto thous_0
movlw NINE ; move '9' to HU_DISP
movwf HU_DISP
thous_0:
movlw 1
subwf THOUS,f
btfsc STATUS,C
goto thous_1
movlw ZERO ; move '0' to TH_DISP
movwf TH_DISP
goto thous_2
thous_1:
movlw 1
subwf THOUS,f
btfsc STATUS,C
goto thous_2
movlw ONE ; move '1' to TH_DISP
movwf TH_DISP
thous_2:
movf UN_DISP,0 ; paint display registers onto 7-segment displays
movwf PORTC
movf TE_DISP,0
movwf PORTD
movf HU_DISP,0
movwf PORTB
btfsc FLAGS,0 ; fast/slow count switch
goto short2delay
call secdelay
goto mainloop
short2delay:
call shortdelay
goto mainloop
secdelay:
movlw 0x05
movwf CTR3
decfsz CTR,1
goto $-1
decfsz CTR2,1
goto $-3
decfsz CTR3,1
goto $-5
return
shortdelay:
movlw 0x3F
movwf CTR2
decfsz CTR,1
goto $-1
decfsz CTR2,1
goto $-3
return
end

Damn, the forum has truncated the spacing for the comments, it makes it a bit harder to read.

This code is tested and works on the hardware circuit shown previously.

w
 
Last edited:

wakibaki

Banned
2008-01-08 11:51 pm
Bill of Materials

Resistors

24 R2-R25 470R
1 R27 1k
2 R28,R29,R30 10k

Capacitors

3 C2-C4 100n
2 C9,C10 2.2uF

Integrated Circuits

1 U2 PIC16F887A-PT
3 U3-U5 7SEGBLUE
1 U7 78S05

Transistors

1 Q1 BC108

Diodes

3 D5-D7 DIODE 1N4001
90 D8-D97 LED UV

Miscellaneous

1 TR1 TRANS-12V-20VA
1 J1 PIC_ICSP_HDR
1 J2 CONN-H4
1 J5 TBLOCK-I2
1 RL1 G2R-14-DC12
1 Large Veroboard

w
 
thanks for the thread mate!! really handy for me, as I have been tyhinking on buying my first round of materials for building some prototype PCBs before I get the full small run done. I also cannot do anything with the current components I have chosen for my regulator PCB without having a PCB to place them on (very small DFN package linear tech bipolar LDOs) so this thread will come in very handy I think. do you have any recommendations for oven 'profiles' for toaster oven reflow/soldering work? I read of using a marker of sorts (crayon or something from memory) to give a visual indication of when to turn the oven off, do you have a technique for this? how do you adhere the parts to the PCB while placing before baking?

thanks very much
 

wakibaki

Banned
2008-01-08 11:51 pm
I don't have a toaster oven at home, I used one at work. The time taken to heat to reflow point was about 5 minutes. I believe it was an 800W oven, it had a radiant bar (like out of a radiant fire, silica tube outside) top and bottom. I used both simultaneously.

Since I only used the oven infrequently, it was generally at room temperature (70F) to start with. I didn't have a 'profile', I put the boards in the oven, spread out to pretty much fill the area, but avoiding the edges and corners by about 2cm. These were maybe 9 off 2*1.5in boards. I found pretty quickly that just over 5 minutes by the built-in mechanical timer was generally required. The oven had a glass panel in the door, so it was pretty easy to see when the solder had gone off, you can see it change from grey to bright silver, and all the components 'sit down' and align to the pads. The heating was not uniform, but from start to all joints done only took maybe 60 seconds, less in most cases. This was enough to melt DIL sockets though, but not seriously deform them, they were used as surface mount anyway, which meant that they were closer to the heating element, and more exposed.

It's mostly about skill and experimentation, rather than prescribed solutions. You need to be able to do a few trials without caring about whether you destroy a chip, or a board for that matter.

I generally only reflowed 1 side and hand-soldered the other. The hand-soldered side was designed to be less problematic than the reflow side. Obviously, if you are arranging a PCB then you know the production facilities available to you and this influences the physical layout. If I had to pick off a 44-pin chip or suchlike I'd heat it up with a heat-gun and get a scalpel tip in between the legs and pick or flick it off. Just a regular De Walt DIY heat-gun with plenty of power and a big blast of air available. I didn't use adhesive, just a laser-cut mylar stencil to apply the solder, then hand-place the components, handle the boards with care and reflow them before they could get disturbed. The solder paste is sticky, it certainly sticks to anything you don't want it to, you can push the components down into it like bricks into mortar, in fact this is the one time you'd prefer it stickier, but it works out.

One thing worth knowing is that the green photo-resist on professional supplies of board from is solder-through. You are better not to process hand-etched boards beyond the etching point. The unexposed resist will keep the copper clean and solderable. I used to re-expose and redevelop boards, or strip the etch with emery paper, but this is unnecessary unless the boards are inadvertently heated without tinning taking place, in which case they can become resistant and need scouring.

The solder paste I used had noticeably more aggressive flux than regular multicore wire. I've got some new stuff here, but I haven't bought an oven yet, so I haven't had a chance to evaluate it.

The problem with some of the very small DFN packages with all the contacts underneath is that the pads are so small that they are approaching the maximum resolution usefully produced with regular software and a laser printer.

w
 
thanks for the reply. yeah I wont be working with a profile at the beginning either, maybe never, but there are a few reflow controller boards that can be built, so perhaps i'll get around to it some day. I plan to do some tests with some cheaper TO52 regs and 0805 resistors etc first to get my head around the use of the oven before I tackle anything I actually need to survive. after watching plenty of youtube vids last night it seems I underestimated the stickiness of the solder paste, so yeah I cant see me needing the adhesive either. I hadnt realized (hadnt really thought about it) that the surface tension would be so friendly and align it so neatly :cool:

I enjoy hand soldering and at this stage i'm not planning any large runs or anything, just finding a way to stuff decent size builds and also a way of soldering these components at home, because pick and place for only a couple of boards is crazy expensive.

I could have redesigned with a more diy friendly reg chip, but this is the only package this reg comes in and I dont know that there is another chip like it low noise small very low dropout bipolar linear regs are pretty thin on the ground and it works with my charge pump perfectly too, can be shutdown by the charge monitor when VCC/2 hits 3v

I have a heat gun, but its not really hot enough

luckily WRT the laser printer resolution. luckily in a past life I was a graphic designer and as is my mother. so I personally have a xerox 1200dpi colour laser printer and have access to a docucolour at 3 x that and software isnt an issue, it isnt an overly complex PCB, if needed I could edit the PDF in illustrator and draw in the correct landing patterns. I think i'll be fine though in that regard.

thanks again